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The posttranslational regulation of transferrin receptor (TfR1) is largely unknown. We investigated whether iron availability affects TfR1 endocytic cycle and protein stability in HepG2 hepatoma cells exposed to ferric ammonium citrate (FAC). NH

The developmental and epileptic encephalopathies (DEEs) are heterogeneous disorders with a strong genetic contribution, but the underlying genetic etiology remains unknown in a significant proportion of individuals. To explore whether statistical support for genetic etiologies can be generated on the basis of phenotypic features, we analyzed whole-exome sequencing data and phenotypic similarities by using Human Phenotype Ontology (HPO) in 314 individuals with DEEs. We identified a de novo c.508C>T (p.Arg170Trp) variant in AP2M1 in two individuals with a phenotypic similarity that was higher than expected by chance (p = 0.003) and a phenotype related to epilepsy with myoclonic-atonic seizures. We subsequently found the same de novo variant in two individuals with neurodevelopmental disorders and generalized epilepsy in a cohort of 2,310 individuals who underwent diagnostic whole-exome sequencing. AP2M1 encodes the mu-subunit of the adaptor protein complex 2 (AP-2), which is involved in clathrin-mediated endocytosis (CME) and synaptic vesicle recycling. Modeling of protein dynamics indicated that the p.Arg170Trp variant impairs the conformational activation and thermodynamic entropy of the AP-2 complex. Functional complementation of both the mu-subunit carrying the p.Arg170Trp variant in human cells and astrocytes derived from AP-2 mu conditional knockout mice revealed a significant impairment of CME of transferrin. In contrast, stability, expression levels, membrane recruitment, and localization were not impaired, suggesting a functional alteration of the AP-2 complex as the underlying disease mechanism. We establish a recurrent pathogenic variant in AP2M1 as a cause of DEEs with distinct phenotypic features, and we implicate dysfunction of the early steps of endocytosis as a disease mechanism in epilepsy.

Despite multitudes of reports on cancer remedies available, we are far from being able to declare that we have arrived at that defining anti-cancer therapy. In recent decades, researchers have been looking into the possibility of enhancing cell death-related signaling pathways in cancer cells using pro-apoptotic proteins. Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and Mu-2/AP1M2 domain containing, death-inducing (MUDENG, MuD) have been established for their ability to bring about cell death specifically in cancer cells. Targeted cell death is a very attractive term when it comes to cancer, since most therapies also affect normal cells. In this direction TRAIL has made noteworthy progress. This review briefly sums up what has been done using TRAIL in cancer therapeutics. The importance of MuD and what has been achieved thus far through MuD and the need to widen and concentrate on applicational aspects of MuD has been highlighted. This has been suggested as the future perspective of MuD towards prospective progress in cancer research.

There is a growing need for the discovery of new prognostic factors for cases where the scoring and staging system of hepatocellular carcinoma (HCC) does not result in a clear definition. We analyzed whether AP-2 complex subunit mu (AP2M1) expression could be a new prognostic marker for HCC based on the roles of AP2M1 in influencing hepatocyte growth factor (HGF) promoter regulation and hepatitis C virus (HCV) assembly. Patient data were extracted from cohorts of the Gene Expression Omnibus (GSE10186), International Cancer Genome Consortium (ICGC) and The Cancer Genome Atlas (TCGA). Differential expression value between matched cancer and normal liver was identified using ICGC cohort. Subsequently, we compared AP2M1 expression as a prognostic gene with other well-known prognostic genes for HCC, using the time-dependent area under the curve (AUC) of the Uno's C-index, the AUC value of the receiver operating characteristics at 5 years, Kaplan-Meier survival curve, and multivariate analysis. Particularly, TCGA and GSE10186 patients were divided into subgroups based on alcohol intake, hepatitis B, and C viral infections, and analyzed in the same methods. The AP2M1 expression values in patients with cancer were much higher than matched normal liver. The AP2M1 level showed excellent prognosis predictions in comparison with existing markers in the three independent cohorts (n = 647). In particular, it was more predictive of prognosis than other markers in alcohol intake and HCV infections. In conclusion, we were confident that AP2M1 provides sufficient value as a new prognostic marker for HCC especially patients with HCV infection and/or alcohol intake.

Chitosan has wide-spectrum antimicrobial activity but knowledge of its antifungal mechanism is still incomplete. In this study, transcriptome of Penicillium expansum upon chitosan treatment was analyzed by RNA-Seq. KEGG enrichment analysis revealed that endocytosis as well as other physiological pathways was regulated by chitosan treatment. Clathrin adaptor protein mu-subunit (PeCAM) gene, which encodes a protein associated with clathrin-dependent endocytosis, was up-regulated after chitosan treatment. Deletion of PeCAM resulted in changes of conidial, hyphal and colonial morphology. Confocal microscopy images of the distribution of fluorescein isothiocyanate-labeled chitosan confirmed cellular internalization of chitosan. However, deletion of PeCAM almost completely blocked uptake of chitosan into fungal cells and ΔPeCAM mutant exhibited less sensitivity to chitosan compared with wild type, suggesting that chitosan uptake is mediated by clathrin-dependent endocytosis and internalized chitosan also plays an important role in its antifungal activity. Collectively, our results provide a new insight into the antifungal mechanism of chitosan.

Wnts are glycosylated proteins secreted from various cell types including mesenchymal, hematopoietic and epithelial cells. Directional secretion of Wnts in polarized epithelial cells is unique; Wnt11 is secreted apically, whereas Wnt5a and Wnt3a are secreted basolaterally. Here, we found that Wnt1 is equivalently secreted both apically and basolaterally in MDCK cells. Wnt1 was modified with a complex- or hybrid-type glycan at Asn29 and Asn359 and the high-mannose- or hybrid-type glycan at Asn316. Although glycosylation of Wnt11 at the N-terminal site was shown to be essential for its apical secretion, glycosylation of Asn29 of Wnt1 was not required. Instead, the apical secretion of Wnt1 was inhibited by knockdown of Sec6 and Sec8, suggesting that Wnt1 is secreted apically via exocyst-mediated transport. Basolateral secretion of Wnt1 was mediated by clathrin and AP-1, in mechanism similar to that used by Wnt5a and Wnt3a. Although Wingless was reported to be transcytosed to the basolateral region in the Drosophila wing disc, transcytosis was not involved in the basolateral secretion of Wnt1. Thus, the polarized secretion of Wnt1 is regulated by different mechanisms than other Wnts.

Transient receptor potential vanilloid 1 (TRPV1), a nonselective, ligand-gated cation channel, responds to multiple noxious stimuli and is targeted by many kinases that influence its trafficking and activity. Studies on the internalization of TRPV1 have mainly focused on that induced by capsaicin or other agonists. Here, we report that constitutive internalization of TRPV1 occurred in a manner dependent on clathrin, dynamin, and adaptor protein complex 2 (AP2). The μ2 subunit of AP2 (AP2μ2) interacted directly with TRPV1 and was required for its constitutive internalization. Cyclin-dependent kinase 5 (CDK5) phosphorylated AP2μ2 at Ser45, which reduced the interaction between TRPV1 and AP2μ2, leading to decreased TRPV1 internalization. Intrathecal delivery of a cell-penetrating fusion peptide corresponding to the Cdk5 phosphorylation site in AP2μ2, which competed with AP2μ2 for phosphorylation by Cdk5, increased the abundance of TRPV1 on the surface of dorsal root ganglion neurons and reduced complete Freund's adjuvant (CFA)-induced inflammatory thermal hyperalgesia in rats. In addition to describing a mechanism of TRPV1 constitutive internalization and its inhibition by CDK5, these findings demonstrate that CDK5 promotes inflammatory thermal hyperalgesia by reducing TRPV1 internalization, providing previously unidentified insights into the search for drug targets to treat pain.

Proper sorting of exocytosed synaptic vesicle (SV) proteins into individual SVs during endocytosis is of the utmost importance for the fidelity of subsequent neurotransmission. Recent studies suggest that each SV protein is sorted into individual SVs by its own dedicated adaptors as well as by association between SV proteins. The SH3-containing GRB2-like protein 3-interacting protein 1 (SGIP1), an ortholog of Fer/Cip4 homology domain-only (FCHo) proteins, contains a μ-homology domain (μHD) and binds AP-2 and Eps15, thus functioning as an endocytic regulator of clathrin-mediated endocytosis (CME). Its longest isoform SGIP1α is predominantly expressed in the brain but the functional significance of SGIP1 in SV recycling remains unknown. Here, we found that SGIP1α, a brain-specific long isoform of SGIP1 binds synaptotagmin1 (Syt1) via its μHD and promotes the internalization of Syt1 on the neuronal surface. The small hairpin RNA (shRNA)-mediated knockdown (KD) of SGIP1α caused selective impairment of Syt1 internalization at hippocampal synapses and it was fully rescued by coexpression of the shRNA-resistant form of SGIP1α in KD neurons. We further found that the μHD of SGIP1α is structurally similar to those of AP-2 and stonin2, and mutations at Trp771 and Lys781, which correspond to Syt1-recognition motifs of AP-2 and stonin2, to Ala bound less efficiently to Syt1 and failed to rescue the endocytic defect of Syt1 caused by KD. Our results indicate that SGIP1α is an endocytic adaptor dedicated to the retrieval of surface-stranded Syt1. Since endocytic sorting of Syt1 is also mediated by the overlapping activities of synaptic vesicle glycoprotein 2A/B (SV2A/B) and stonin2, our results suggest that complementary fail-safe mechanism by these proteins ensures high fidelity of Syt1 retrieval.

SummarySynaptic scaling is a form of homeostatic plasticity driven by transcription-dependent changes in AMPA-type glutamate receptor (AMPAR) trafficking. To uncover the pathways involved, we performed a cell-type-specific screen for transcripts persistently altered during scaling, which identified the μ subunit (μ3A) of the adaptor protein complex AP-3A. Synaptic scaling increased μ3A (but not other AP-3 subunits) in pyramidal neurons and redistributed dendritic μ3A and AMPAR to recycling endosomes (REs). Knockdown of μ3A prevented synaptic scaling and this redistribution, while overexpression (OE) of full-length μ3A or a truncated μ3A that cannot interact with the AP-3A complex was sufficient to drive AMPAR to REs. Finally, OE of μ3A acted synergistically with GRIP1 to recruit AMPAR to the dendritic membrane. These data suggest that excess μ3A acts independently of the AP-3A complex to reroute AMPAR to RE, generating a reservoir of receptors essential for the regulated recruitment to the synaptic membrane during scaling up.

In contrast with the wealth of recent reports about the function of μ-adaptins and clathrin adaptor protein (AP) complexes, there is very little information about the motifs that determine the sorting of membrane proteins within clathrin-coated vesicles in plants. Here, we investigated putative sorting signals in the large cytosolic loop of the Arabidopsis (Arabidopsis thaliana) PIN-FORMED1 (PIN1) auxin transporter, which are involved in binding μ-adaptins and thus in PIN1 trafficking and localization. We found that Phe-165 and Tyr-280, Tyr-328, and Tyr-394 are involved in the binding of different μ-adaptins in vitro. However, only Phe-165, which binds μA(μ2)- and μD(μ3)-adaptin, was found to be essential for PIN1 trafficking and localization in vivo. The PIN1:GFP-F165A mutant showed reduced endocytosis but also localized to intracellular structures containing several layers of membranes and endoplasmic reticulum (ER) markers, suggesting that they correspond to ER or ER-derived membranes. While PIN1:GFP localized normally in a μA (μ2)-adaptin mutant, it accumulated in big intracellular structures containing LysoTracker in a μD (μ3)-adaptin mutant, consistent with previous results obtained with mutants of other subunits of the AP-3 complex. Our data suggest that Phe-165, through the binding of μA (μ2)- and μD (μ3)-adaptin, is important for PIN1 endocytosis and for PIN1 trafficking along the secretory pathway, respectively.

Adaptor complexes are important for cargo sorting in clathrin-coated vesicles. The µ adaptor subunits Apm1 and Apm2 create functionally distinct versions of the yeast AP-1 complex. A novel regulatory protein is identified that selectively binds Apm2-containing complexes and contributes to their membrane recruitment., Heterotetrameric adaptor protein complexes are important mediators of cargo protein sorting in clathrin-coated vesicles. The cell type–specific expression of alternate μ chains creates distinct forms of AP-1 with altered cargo sorting, but how these subunits confer differential function is unclear. Whereas some studies suggest the μ subunits specify localization to different cellular compartments, others find that the two forms of AP-1 are present in the same vesicle but recognize different cargo. Yeast have two forms of AP-1, which differ only in the μ chain. Here we show that the variant μ chain Apm2 confers distinct cargo-sorting functions. Loss of Apm2, but not of Apm1, increases cell surface levels of the v-SNARE Snc1. However, Apm2 is unable to replace Apm1 in sorting Chs3, which requires a dileucine motif recognized by the γ/σ subunits common to both complexes. Apm2 and Apm1 colocalize at Golgi/early endosomes, suggesting that they do not associate with distinct compartments. We identified a novel, conserved regulatory protein that is required for Apm2-dependent sorting events. Mil1 is a predicted lipase that binds Apm2 but not Apm1 and contributes to its membrane recruitment. Interactions with specific regulatory factors may provide a general mechanism to diversify the functional repertoire of clathrin adaptor complexes.

The adaptor AP2 is required for initiation of clathrin-mediated endocytosis. Kadlecova et al. delineate the functional hierarchy of AP2 interactions with phosphatidylinositol lipids and cargo and their relationship to distinct steps in clathrin-coated pit nucleation and maturation in living cells., The critical initiation phase of clathrin-mediated endocytosis (CME) determines where and when endocytosis occurs. Heterotetrameric adaptor protein 2 (AP2) complexes, which initiate clathrin-coated pit (CCP) assembly, are activated by conformational changes in response to phosphatidylinositol-4,5-bisphosphate (PIP2) and cargo binding at multiple sites. However, the functional hierarchy of interactions and how these conformational changes relate to distinct steps in CCP formation in living cells remains unknown. We used quantitative live-cell analyses to measure discrete early stages of CME and show how sequential, allosterically regulated conformational changes activate AP2 to drive both nucleation and subsequent stabilization of nascent CCPs. Our data establish that cargoes containing Yxxφ motif, but not dileucine motif, play a critical role in the earliest stages of AP2 activation and CCP nucleation. Interestingly, these cargo and PIP2 interactions are not conserved in yeast. Thus, we speculate that AP2 has evolved as a key regulatory node to coordinate CCP formation and cargo sorting and ensure high spatial and temporal regulation of CME.

ORF9p (homologous to herpes simplex virus 1 [HSV-1] VP22) is a varicella-zoster virus (VZV) tegument protein essential for viral replication. Even though its precise functions are far from being fully described, a role in the secondary envelopment of the virus has long been suggested. We performed a yeast two-hybrid screen to identify cellular proteins interacting with ORF9p that might be important for this function. We found 31 ORF9p interaction partners, among which was AP1M1, the μ subunit of the adaptor protein complex 1 (AP-1). AP-1 is a heterotetramer involved in intracellular vesicle-mediated transport and regulates the shuttling of cargo proteins between endosomes and the trans-Golgi network via clathrin-coated vesicles. We confirmed that AP-1 interacts with ORF9p in infected cells and mapped potential interaction motifs within ORF9p. We generated VZV mutants in which each of these motifs was individually impaired and identified leucine 231 in ORF9p to be critical for the interaction with AP-1. Disrupting ORF9p binding to AP-1 by mutating leucine 231 to alanine in ORF9p strongly impaired viral growth, most likely by preventing efficient secondary envelopment of the virus. Leucine 231 is part of a dileucine motif conserved among alphaherpesviruses, and we showed that VP22 of Marek's disease virus and HSV-2 also interacts with AP-1. This indicates that the function of this interaction in secondary envelopment might be conserved as well.IMPORTANCE Herpesviruses are responsible for infections that, especially in immunocompromised patients, can lead to severe complications, including neurological symptoms and strokes. The constant emergence of viral strains resistant to classical antivirals (mainly acyclovir and its derivatives) pleads for the identification of new targets for future antiviral treatments. Cellular adaptor protein (AP) complexes have been implicated in the correct addressing of herpesvirus glycoproteins in infected cells, and the discovery that a major constituent of the varicella-zoster virus tegument interacts with AP-1 reveals a previously unsuspected role of this tegument protein. Unraveling the complex mechanisms leading to virion production will certainly be an important step in the discovery of future therapeutic targets. ispartof: JOURNAL OF VIROLOGY vol:92 issue:15 ispartof: location:United States status: published

Extensive genetic and genomic studies of the relationship between alcohol drinking preference and withdrawal severity have been performed using animal models. Data from multiple such publications and public data resources have been incorporated in the GeneWeaver database with >60,000 gene sets including 285 alcohol withdrawal and preference-related gene sets. Among these are evidence for positional candidates regulating these behaviors in overlapping quantitative trait loci (QTL) mapped in distinct mouse populations. Combinatorial integration of functional genomics experimental results revealed a single QTL positional candidate gene in one of the loci common to both preference and withdrawal. Functional validation studies in Ap3m2 knockout mice confirmed these relationships. Genetic validation involves confirming the existence of segregating polymorphisms that could account for the phenotypic effect. By exploiting recent advances in mouse genotyping, sequence, epigenetics, and phylogeny resources, we confirmed that Ap3m2 resides in an appropriately segregating genomic region. We have demonstrated genetic and alcohol-induced regulation of Ap3m2 expression. Although sequence analysis revealed no polymorphisms in the Ap3m2-coding region that could account for all phenotypic differences, there are several upstream SNPs that could. We have identified one of these to be an H3K4me3 site that exhibits strain differences in methylation. Thus, by making cross-species functional genomics readily computable we identified a common QTL candidate for two related bio-behavioral processes via functional evidence and demonstrate sufficiency of the genetic locus as a source of variation underlying two traits.

L-selectin regulates leukocyte adhesion and rolling along the endothelium. Proteins binding to the cytoplasmic tail of L-selectin regulate L-selectin functions. We used L-selectin cytoplasmic tail peptide pulldown assays combined with high sensitivity liquid chromatography/mass spectrometry to identify novel L-selectin tail-binding proteins. Incubation of the L-selectin tail with cell extracts from phorbol 12-myristate 13-acetate-stimulated Raw 264.7 macrophages resulted in the binding of μ1A of the clathrin-coated vesicle AP-1 complex. Furthermore, full-length GST-μ1A and the GST-μ1A C-terminal domain, but not the GST-μ1A N-terminal domain, bind to L-selectin tail peptide, and the intracellular pool of L-selectin colocalizes with AP-1 at the trans-Golgi network. We identified a novel basic protein motif consisting of a cluster of three dibasic residues (356RR357, 359KK360, and 362KK363) in the membrane-proximal domain of the L-selectin tail as well as a doublet of aspartic acid residues (369DD370) in the membrane-distal end of the L-selectin tail involved in μ1A binding. Stimulation of Raw 264.7 macrophages with PMA augmented the amount of μ1A associated with anti-L-selectin immunoprecipitates. However, full-length GST-μ1A did not bind to the phospho-L-selectin tail or phospho-mimetic S364D L-selectin tail. Accordingly, we propose that phosphorylation of μ1A is required for interaction with the L-selectin tail and that L-selectin tail phosphorylation may regulate this interaction in vivo. Molecular docking of the L-selectin tail to μ1A was used to identify the μ1A surface domain binding the L-selectin tail and to explain how phosphorylation of the L-selectin tail abrogates μ1A interaction. Our findings indicate that L-selectin is transported constitutively by the AP-1 complex, leading to the formation of a trans-Golgi network reserve pool and that phosphorylation of the L-selectin tail blocks AP-1-dependent retrograde transport of L-selectin.

SummaryAn outstanding question in protein sorting is why polarized epithelial cells express two isoforms of the μ1 subunit of the AP-1 clathrin adaptor complex: the ubiquitous μ1A and the epithelial-specific μ1B. Previous studies led to the notion that μ1A and μ1B mediate basolateral sorting predominantly from the trans-Golgi network (TGN) and recycling endosomes, respectively. Using improved analytical tools, however, we find that μ1A and μ1B largely colocalize with each other. They also colocalize to similar extents with TGN and recycling endosome markers, as well as with basolateral cargoes transiting biosynthetic and endocytic-recycling routes. Instead, the two isoforms differ in their signal-recognition specificity. In particular, μ1B preferentially binds a subset of signals from cargoes that are sorted basolaterally in a μ1B-dependent manner. We conclude that expression of distinct μ1 isoforms in epithelial cells expands the repertoire of signals recognized by AP-1 for sorting of a broader range of cargoes to the basolateral surface.

Adaptor protein (AP) complexes are the predominant coat proteins of membrane vesicles in post-Golgi trafficking of mammalian cells. Each AP complex contains a specific medium subunit, μ-adaptin, that selects cargo proteins bearing sequence-specific sorting motifs. Much less is known about the AP complexes and their μ subunits in plants. Because of uncertain homology, the μ-adaptins of Arabidopsis have been designated muA through muD [Happel et al. (2004) Plant J 37(5):678–693]. Furthermore, only muD has been assigned to a specific AP complex, AP-3, involved in Golgi-vacuolar trafficking [Niihama et al. (2009) Plant Cell Physiol 50(12):2057–2068, Zwiewka et al. (2011) Cell Res 21(12):1711–1722, and Wolfenstetter et al. (2012) Plant Cell 24(1):215–232]. In contrast, the μ subunit of neither the post-Golgi trafficking AP-1 complex nor the endocytic AP-2 complex has been identified. Here, we report the functional analysis of redundant AP-1 μ-adaptins AP1M1 (also known as muB1) and AP1M2 (also known as muB2). Coimmunoprecipitation revealed that both AP1M2 and its less strongly expressed isoform AP1M1 are complexed with the large subunit γ-adaptin of AP-1. In addition, AP1M2 was localized at or near the trans -Golgi network. Knockout mutations of AP1M2 impaired pollen function and arrested plant growth whereas the ap1m1 ap1m2 double mutant was nearly pollen-lethal. At the cellular level, the absence of AP1M2 entailed inhibition of multiple trafficking pathways from the trans -Golgi network to the vacuole and to the plasma membrane in interphase and to the plane of cell division in cytokinesis. Thus, AP-1 is crucial in post-Golgi trafficking in plant cells and required for cell division and plant growth.

Tyrosine-based signals fitting the YXXO motif mediate sorting of transmembrane proteins to endosomes, lysosomes, the basolateral plasma membrane of polarized epithelial cells, and the somatodendritic domain of neurons through interactions with the homologous μ1, μ2, μ3, and μ4 subunits of the corresponding AP-1, AP-2, AP-3, and AP-4 complexes. Previous x-ray crystallographic analyses identified distinct binding sites for YXXO signals on μ2 and μ4, which were located on opposite faces of the proteins. To elucidate the mode of recognition of YXXO signals by other members of the μ family, we solved the crystal structure at 1.85 A resolution of the C-terminal domain of the μ3 subunit of AP-3 (isoform A) in complex with a peptide encoding a YXXO signal (SDYQRL) from the trans-Golgi network protein TGN38. The μ3A C-terminal domain consists of an immunoglobulin-like β-sandwich organized into two subdomains, A and B. The YXXO signal binds in an extended conformation to a site on μ3A subdomain A, at a location similar to the YXXO-binding site on μ2 but not μ4. The binding sites on μ3A and μ2 exhibit similarities and differences that account for the ability of both proteins to bind distinct sets of YXXO signals. Biochemical analyses confirm the identification of the μ3A site and show that this protein binds YXXO signals with 14–19 μm affinity. The surface electrostatic potential of μ3A is less basic than that of μ2, in part explaining the association of AP-3 with intracellular membranes having less acidic phosphoinositides.

Circuit formation in the brain requires neurite outgrowth throughout development to establish synaptic contacts with target cells. Active endocytosis of several adhesion molecules facilitates the dynamic exchange of these molecules at the surface and promotes neurite outgrowth in developing neurons. The endocytosis of N-cadherin, a calcium-dependent adhesion molecule, has been implicated in the regulation of neurite outgrowth, but the mechanism remains unclear. Here, we identified that a fraction of N-cadherin internalizes through clathrin-mediated endocytosis (CME). Two tyrosine-based motifs in the cytoplasmic domain of N-cadherin recognized by the μ2 subunit of the AP-2 adaptor complex are responsible for CME of N-cadherin. Moreover, β-catenin, a core component of the N-cadherin adhesion complex, inhibits N-cadherin endocytosis by masking the 2 tyrosine-based motifs. Removal of β-catenin facilitates μ2 binding to N-cadherin, thereby increasing clathrin-mediated N-cadherin endocytosis and neurite outgrowth without affecting the steady-state level of surface N-cadherin. These results identify and characterize the mechanism controlling N-cadherin endocytosis through β-catenin-regulated μ2 binding to modulate neurite outgrowth.

Acidic clusters act as sorting signals for packaging cargo into clathrin-coated vesicles. Navarro Negredo et al. show that the clathrin adaptor AP-1 subunit µ1 interacts with endogenous acidic cluster cargo as well as with the HIV-1 acidic cluster protein Nef, and they tease out the contribution of this interaction to acidic cluster protein trafficking., Acidic clusters act as sorting signals for packaging cargo into clathrin-coated vesicles (CCVs), and also facilitate down-regulation of MHC-I by HIV-1 Nef. To find acidic cluster sorting machinery, we performed a gene-trap screen and identified the medium subunit (µ1) of the clathrin adaptor AP-1 as a top hit. In µ1 knockout cells, intracellular CCVs still form, but acidic cluster proteins are depleted, although several other CCV components were either unaffected or increased, indicating that cells can compensate for long-term loss of AP-1. In vitro experiments showed that the basic patch on µ1 that interacts with the Nef acidic cluster also contributes to the binding of endogenous acidic cluster proteins. Surprisingly, µ1 mutant proteins lacking the basic patch and/or the tyrosine-based motif binding pocket could rescue the µ1 knockout phenotype completely. In contrast, these mutants failed to rescue Nef-induced down-regulation of MHC class I, suggesting a possible mechanism for attacking the virus while sparing the host cell.

Thrombotic diseases are among the leading causes of morbidity and mortality in the world. To add insights into the genetic regulation of thrombotic disease, we conducted a genome-wide association study (GWAS) of 6135 self-reported blood clots events and 252 827 controls of European ancestry belonging to the 23andMe cohort of research participants. Eight loci exceeded genome-wide significance. Among the genome-wide significant results, our study replicated previously known venous thromboembolism (VTE) loci near the F5, FGA-FGG, F11, F2, PROCR and ABO genes, and the more recently discovered locus near SLC44A2. In addition, our study reports for the first time a genome-wide significant association between rs114209171, located upstream of the F8 structural gene, and thrombosis risk. Analyses of expression profiles and expression quantitative trait loci across different tissues suggested SLC44A2, ILF3 and AP1M2 as the three most plausible candidate genes for the chromosome 19 locus, our only genome-wide significant thrombosis-related locus that does not harbor likely coagulation-related genes. In addition, we present data showing that this locus also acts as a novel risk factor for stroke and coronary artery disease (CAD). In conclusion, our study reveals novel common genetic risk factors for VTE, stroke and CAD and provides evidence that self-reported data on blood clots used in a GWAS yield results that are comparable with those obtained using clinically diagnosed VTE. This observation opens up the potential for larger meta-analyses, which will enable elucidation of the genetics of thrombotic diseases, and serves as an example for the genetic study of other diseases.

SummaryPlasma membranes of the somatodendritic and axonal domains of neurons are known to have different protein compositions, but the molecular mechanisms that determine this polarized protein distribution remain poorly understood. Herein we show that somatodendritic sorting of various transmembrane receptors in rat hippocampal neurons is mediated by recognition of signals within the cytosolic domains of the proteins by the μ1A subunit of the adaptor protein-1 (AP-1) complex. This complex, in conjunction with clathrin, functions in the neuronal soma to exclude somatodendritic proteins from axonal transport carriers. Perturbation of this process affects dendritic spine morphology and decreases the number of synapses. These findings highlight the primary recognition event that underlies somatodendritic sorting and contribute to the evolving view of AP-1 as a global regulator of cell polarity.

The assembly of clathrin-coated vesicles is important for numerous cellular processes, including nutrient uptake and membrane organization. Important contributors to clathrin assembly are four tetrameric assembly proteins, also called adaptor proteins (APs), each of which contains a β subunit. We identified a single β subunit, named β1/2, that contributes to both the AP1 and AP2 complexes of Dictyostelium. Disruption of the gene encoding β1/2 resulted in severe defects in growth, cytokinesis and development. Additionally, cells lacking β1/2 displayed profound osmoregulatory defects including the absence of contractile vacuoles and mislocalization of contractile vacuole markers. The phenotypes of β1/2 null cells were most similar to previously described phenotypes of clathrin and AP1 mutants, supporting a particularly important contribution of AP1 to clathrin pathways in Dictyostelium cells. The absence of β1/2 in cells led to significant reductions in the protein amounts of the medium-sized subunits of the AP1 and AP2 complexes, establishing a role for the β subunit in the stability of the medium subunits. Dictyostelium β1/2 could resemble a common ancestor of the more specialized β1 and β2 subunits of the vertebrate AP complexes. Our results support the essential contribution of a single β subunit to the stability and function of AP1 and AP2 in a simple eukaryote.

Background & Aims Epithelial cells that cover the intestinal mucosal surface maintain immune homeostasis and tolerance in the gastrointestinal tract. However, little is known about the molecular mechanisms that regulate epithelial immune functions. Epithelial cells are distinct in that they are highly polarized; this polarity is, at least in part, established by the epithelium-specific polarized sorting factor adaptor protein (AP)-1B. We investigated the role of AP-1B–mediated protein sorting in the maintenance of gastrointestinal immune homeostasis. Methods The role of AP-1B in intestinal immunity was examined in AP-1B–deficient mice ( Ap1m2 −/− ) by monitoring their phenotypes, intestinal morphology, and epithelial barrier functions. AP-1B–mediated protein sorting was examined in polarized epithelial cells from AP-1B knockdown and Ap1m2 −/− mice. Results Ap1m2 −/− mice developed spontaneous chronic colitis, characterized by accumulation of interleukin-17A–producing, T-helper 17 cells. Deficiency of AP-1B caused epithelial immune dysfunction, such as reduced expression of antimicrobial proteins and impaired secretion of immunoglobulin A. These defects promoted intestinal dysbiosis and increased bacterial translocation within the mucosa. Importantly, AP-1B deficiency led to mistargeting of a subset of basolateral cytokine receptors to the apical plasma membrane in a polarized epithelial cell line and in colonic epithelial cells from mice. AP1M2 expression was reduced significantly in colonic epithelium samples from patients with Crohn's disease. Conclusions AP-1B is required for proper localization of a subset of cytokine receptors in polarized epithelial cells, which allows them to respond to cytokine signals from underlying lamina propria cells. The AP-1B–mediated protein sorting machinery is required for maintenance of immune homeostasis and prevention of excessive inflammation.

SummaryWnt association with its receptor, Frizzled (Fz), and recruitment by the latter of an adaptor, Dishevelled (Dvl), initiates signaling through at least two distinct pathways (“canonical” and “noncanonical”). Endocytosis and compartmentalization help determine the signaling outcome. Our previous work has shown that Dvl2 links at least one Frizzled family member (Fz4) to clathrin-mediated endocytosis by interacting with the μ2 subunit of the AP-2 clathrin adaptor, through both a classical endocytic tyrosine motif and a so-called “DEP domain.” We report here the crystal structure of a chimeric protein that mimics the Dvl2-μ2 complex. The DEP domain binds at one end of the elongated, C-terminal domain of μ2. This domain:domain interface shows that parts of the μ2 surface distinct from the tyrosine-motif site can help recruit specific receptors or adaptors into a clathrin coated pit. Mutation of residues at the DEP-μ2 contact or in the tyrosine motif reduce affinity of Dvl2 for μ2 and block efficient internalization of Fz4 in response to ligation by Wnt5a. The crystal structure has thus allowed us to identify the specific interaction that leads to Frizzled uptake and to downstream, noncanonical signaling events.PaperClip

Dictyostelium discoideum amoebae have been used extensively to study the structure and dynamics of the endocytic pathway. Here, we show that while the general structure of the endocytic pathway is maintained in starved cells, its dynamics rapidly slow down. In addition, analysis of apm3 and lvsB mutants reveals that the functional organization of the endocytic pathway is profoundly modified upon starvation. Indeed, in these mutant cells, some of the defects observed in rich medium persist in starved cells, notably an abnormally slow transfer of endocytosed material between endocytic compartments. Other parameters, such as endocytosis of the fluid phase or the rate of fusion of postlysosomes to the cell surface, vary dramatically upon starvation. Studying the endocytic pathway in starved cells can provide a different perspective, allowing the primary (invariant) defects resulting from specific mutations to be distinguished from their secondary (conditional) consequences.

The authors show that PI(3,4,5)P3 in recycling endosomes is needed for AP-1B recruitment and AP-1Bdependent sorting to the basolateral membrane. Interestingly, AP-1B expression itself triggers the formation of PI(3,4,5)P3 in this location., Polarized epithelial cells coexpress two almost identical AP-1 clathrin adaptor complexes: the ubiquitously expressed AP-1A and the epithelial cell–specific AP-1B. The only difference between the two complexes is the incorporation of the respective medium subunits μ1A or μ1B, which are responsible for the different functions of AP-1A and AP-1B in TGN to endosome or endosome to basolateral membrane targeting, respectively. Here we demonstrate that the C-terminus of μ1B is important for AP-1B recruitment onto recycling endosomes. We define a patch of three amino acid residues in μ1B that are necessary for recruitment of AP-1B onto recycling endosomes containing phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3]. We found this lipid enriched in recycling endosomes of epithelial cells only when AP-1B is expressed. Interfering with PI(3,4,5)P3 formation leads to displacement of AP-1B from recycling endosomes and missorting of AP-1B–dependent cargo to the apical plasma membrane. In conclusion, PI(3,4,5)P3 formation in recycling endosomes is essential for AP-1B function.

Adaptins are subunits of the heterotetrameric (beta/mu/gamma/sigma) adaptor protein (AP) complexes that are involved in clathrin-mediated membrane trafficking. Here, we show that in Schizosaccharomyces pombe the deletion strains of each individual subunit of the AP-1 complex [Apl2 (beta), Apl4 (gamma), Apm1 (mu) and Aps1 (sigma)] caused distinct phenotypes on growth sensitivity to temperature or drugs. We also show that the Deltaapm1 and Deltaapl2 mutants displayed similar but more severe phenotypes than those of Deltaaps1 or Deltaapl4 mutants. Furthermore, the Deltaapl2Deltaaps1 and Deltaapl2Deltaapl4 double mutants displayed synthetic growth defects, whereas the Deltaaps1Deltaapl4 and Deltaapl2Deltaapm1 double mutants did not. In pull-down assay, Apm1 binds Apl2 even in the absence of Aps1 and Apl4, and Apl4 binds Aps1 even in the absence of Apm1 and Apl2. Consistently, the deletion of any subunit generally caused the disassociation of the heterotetrameric complex from endosomes, although some subunits weakly localized to endosomes. In addition, the deletion of individual subunits caused similar endosomal accumulation of v-SNARE synaptobrevin Syb1. Altogether, results suggest that the four subunits are all essential for the heterotetrameric complex formation and for the AP-1 function in exit transport from endosomes.

Coronin-7 (Crn7) is a ubiquitous mammalian WD40-repeat protein that localizes to the Golgi complex, interacts with AP-1 adaptor complex via binding of a tyrosine-288-based sorting signal to the mu1-subunit of AP-1, and participates in the maintenance of the Golgi structure and function. Here, we define the requirements for the recruitment of Crn7 from the cytosol to the Golgi. We establish that Src activity is indispensable for the interaction of Crn7 with Golgi membranes. Crn7 binds Src in vivo and can be phosphorylated by recombinant Src in vitro. We demonstrate that tyrosine-758 is the major Src phosphorylation site. Further, to be targeted to membranes Crn7 requires the presence of cargo in the Golgi complex. Finally, downregulation of the mu1-subunit of AP-1 leads to the dispersal of Crn7 from the Golgi membranes. We propose a mechanism whereby sequential events of protein interaction and posttranslational modification result in the membrane targeting of Crn7.

The adaptor protein complex AP-1 mediates vesicular protein sorting between the trans Golgi network and endosomes. AP-1 recycles between membranes and the cytoplasm together with clathrin during transport vesicle formation and vesicle uncoating. AP-1 recycles independent of clathrin, indicating binding to unproductive membrane domains and premature termination of vesicle budding. Membrane recruitment requires ADP ribosylation factor-1-GTP, a transmembrane protein containing an AP-1-binding motif and phosphatidyl-inositol phosphate (PI-4-P). Little is known about the regulation of AP-1 membrane-cytoplasm recycling. We identified the N-terminal domain of micro1A-adaptin as being involved in the regulation of AP-1 membrane-cytoplasm recycling by constructing chimeras of micro1A and its homologue micro2. The AP-1* complex containing this mu2-micro1A chimera had slowed down recycling kinetics, resulting in missorting of mannose 6-phosphate receptors. The N-terminal domain is only accessible from the cytoplasmic AP-1 surface. None of the proteins known to influence AP-1 membrane recycling bound to this micro1A domain, indicating the regulation of AP-1 membrane-cytoplasm recycling by an yet unidentified cytoplasmic protein.

The mechanisms underlying the engulfment of apoptotic corpses, which is involved in development, cellular homeostasis, and autoimmunity, remain largely unknown in mammals. MEGF10 is a mammalian ortholog of nematode CED-1, a transmembrane protein involved in engulfment of apoptotic corpses. MEGF10-expressing cells display an irregular, mosaic-like pattern of MEGF10, causing cells to tightly adhere to coated glass dishes. This restricted cell motility caused cells to adopt a flat appearance. In the present study, we observed that these cells formed unusually large vacuoles, the formation of which we linked to the cytoplasmic domain of MEGF10. While investigating the signaling pathway and trafficking of MEGF10, we identified an interaction between MEGF10 and clathrin assembly protein complex 2 medium chain (AP50), a component of clathrin-coated pits. In cells co-expressing MEGF10 and AP50, MEGF10 and AP50 colocalized and mirrored the adhesion pattern of MEGF10. LC-MS/MS and immunoblot analyses revealed that the MEGF10 associated with AP2 alpha and beta subunits in addition to associating with AP50 and beta-actin, and that MEGF10 was ubiquitinated and tyrosine phosphorylated. Moreover, we observed that MEGF10 mRNA expression is primarily restricted to the brain, with robust expression in the stellate cells of the cerebellum. Elucidating the trafficking and regulatory machinery of MEGF10 will guide us in having a deeper understanding of the mechanisms involved in clearing apoptotic cells.

Evidence that some types of epilepsies show strong genetic predisposition has been well documented. AP3M2 is considered to be an epileptogenic gene because AP3M2 knockout mice exhibit symptoms of spontaneous epileptic seizures. In order to investigate whether the AP3M2 gene causes susceptibility to epilepsy, we performed mutation screening of the genomic DNA of 190 patients with six epilepsy types; this screening involved all the 9 exons and the relevant exon-intron boundaries of AP3M2. Although neither missense nor nonsense mutations were detected, we identified 21 sequence variations, of which 16 variations were novel. Of the 21 variations, 11 were detected in 5' and 3' UTRs, while the remaining variations were detected in introns. Although the present study failed to identify the possible AP3M2 mutations that may cause epilepsy, our results suggest that some AP3M2 mutations still remain candidates for unmapped disorders including epilepsy, febrile seizure, and other neuronal developmental disorders associated with functional abnormalities of GABAergic transmission.

The plasma membranes of epithelial cells plasma membranes contain distinct apical and basolateral domains that are critical for their polarized functions. However, both domains are continuously internalized, with proteins and lipids from each intermixing in supranuclear recycling endosomes (REs). To maintain polarity, REs must faithfully recycle membrane proteins back to the correct plasma membrane domains. We examined sorting within REs and found that apical and basolateral proteins were laterally segregated into subdomains of individual REs. Subdomains were absent in unpolarized cells and developed along with polarization. Subdomains were formed by an active sorting process within REs, which precedes the formation of AP-1B–dependent basolateral transport vesicles. Both the formation of subdomains and the fidelity of basolateral trafficking were dependent on PI3 kinase activity. This suggests that subdomain and transport vesicle formation occur as separate sorting steps and that both processes may contribute to sorting fidelity.

Apical membrane sodium hydrogen exchanger 3 (NHE3), a major pathway for non-nutrient-dependent intestinal Na+absorption, is tightly regulated by second messenger systems that affect its functional activity and membrane trafficking. However, the events and components involved in NHE3 regulation are only partially understood. We report that the adaptor protein synaptotagmin I (Syt I) plays a pivotal role in cAMP- and Ca2+-induced cargo recognition of NHE3 and initiation of its endocytosis. Both mouse small intestine (jejunum) and Caco-2BBe Syt I coimmunoprecipitated with NHE3, particularly following increases in cellular cAMP or Ca2+. Following short interfering RNA (siRNA) suppression of Syt I expression, cAMP- and Ca2+-induced inhibition of NHE3 activity were still observed but NHE3 endocytosis was blocked, as assessed by22Na influx and apical membrane biotin labeling, respectively. Similar effects on NHE3 inhibition and endocytosis were observed by siRNA suppression of either the μ-subunit of the adaptor protein 2 (AP2) complex or the heavy chain of clathrin. Coimmunoprecipitation analyses of NHE3 with these adaptor proteins revealed that cAMP- and Ca2+-induced NHE3-Syt I interaction preceded and was required for recruitment of AP2 and the clathrin complex. Confocal microscopy confirmed both the time sequence and protein associations of these events. We conclude that Syt I plays a pivotal role in mediating cAMP- and Ca2+-induced endocytosis of NHE3 (but not in inhibition of activity) through cargo recognition of NHE3 and subsequent recruitment of AP2-clathrin assembly required for membrane endocytosis.

α-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors undergo constitutive and ligand-induced internalization that requires dynamin and the clathrin adaptor complex AP-2. We report here that an atypical basic motif within the cytoplasmic tails of AMPA-type glutamate receptors directly associates with μ2-adaptin by a mechanism similar to the recognition of the presynaptic vesicle protein synaptotagmin 1 by AP-2. A synaptotagmin 1-derived AP-2 binding peptide competes the interaction of the AMPA receptor subunit GluR2 with AP-2μ and increases the number of surface active glutamate receptors in living neurons. Moreover, fusion of the GluR2-derived tail peptide with a synaptotagmin 1 truncation mutant restores clathrin/AP-2-dependent internalization of the chimeric reporter protein. These data suggest that common mechanisms regulate AP-2-dependent internalization of pre- and postsynaptic membrane proteins.

Assembly of E-cadherin-based adherens junctions (AJ) is obligatory for establishment of polarized epithelia and plays a key role in repressing the invasiveness of many carcinomas. Here we show that type Igamma phosphatidylinositol phosphate kinase (PIPKIgamma) directly binds to E-cadherin and modulates E-cadherin trafficking. PIPKIgamma also interacts with the mu subunits of clathrin adaptor protein (AP) complexes and acts as a signalling scaffold that links AP complexes to E-cadherin. Depletion of PIPKIgamma or disruption of PIPKIgamma binding to either E-cadherin or AP complexes results in defects in E-cadherin transport and blocks AJ assembly. An E-cadherin germline mutation that loses PIPKIgamma binding and shows disrupted basolateral membrane targeting no longer forms AJs and leads to hereditary gastric cancers. These combined results reveal a novel mechanism where PIPKIgamma serves as both a scaffold, which links E-cadherin to AP complexes and the trafficking machinery, and a regulator of trafficking events via the spatial generation of phosphatidylinositol-4,5-bisphosphate.

Upon activation by Wnt, the Frizzled receptor is internalized in a process that requires the recruitment of Dishevelled. We describe a novel interaction between Dishevelled2 (Dvl2) and micro2-adaptin, a subunit of the clathrin adaptor AP-2; this interaction is required to engage activated Frizzled4 with the endocytic machinery and for its internalization. The interaction of Dvl2 with AP-2 requires simultaneous association of the DEP domain and a peptide YHEL motif within Dvl2 with the C terminus of micro2. Dvl2 mutants in the YHEL motif fail to associate with micro2 and AP-2, and prevent Frizzled4 internalization. Corresponding Xenopus Dishevelled mutants show compromised ability to interfere with gastrulation mediated by the planar cell polarity (PCP) pathway. Conversely, a Dvl2 mutant in its DEP domain impaired in PCP signaling exhibits defective AP-2 interaction and prevents the internalization of Frizzled4. We suggest that the direct interaction of Dvl2 with AP-2 is important for Frizzled internalization and Frizzled/PCP signaling.

Alzheimer amyloid precursor protein (APP) is the precursor for the Abeta peptide involved in pathogenesis of Alzheimer's disease. The soluble ectodomain fragment of APP (sAPP) functions as a growth factor for epithelial cells, suggesting an important function for APP outside neuronal tissue. Previous studies have shown that in polarized epithelial cells, APP is targeted to the basolateral domain. Tyr653 within the cytoplasmic tail of APP mediates the basolateral targeting of APP, but the sorting machinery that binds to this residue has largely remained unknown. In this study, we analyzed the role of adaptor complexes in the polarized sorting of APP. We show that the medium subunit mu1B of the epithelia-specific adaptor protein (AP)-1B binds onto the cytoplasmic tail of APP in a Tyr653-dependent way. Moreover, ectopic expression of mu1B in cells lacking AP-1B resulted in correction of apical missorting of wild-type but not Tyr653Ala APP. Basolateral secretion of sAPP was found to be independent of Tyr653. We propose a model for polarized targeting of APP according to which sorting of APP to basolateral domain is dependent on binding of AP-1B on Tyr653 in basolateral endosomes. This model is in accordance with the current understanding of sorting mechanisms mediating polarized targeting of membrane proteins.

Purpose: To compare gene expression profiles of chromophobe renal cell carcinoma (RCC) and benign oncocytoma, aiming at identifying differentially expressed genes. Experimental Design: Nine cases each of chromophobe RCC and oncocytoma were analyzed by oligonucleotide microarray. Candidate genes that showed consistent differential expression were validated by reverse transcription-PCR using 25 fresh-frozen and 15 formalin-fixed, paraffin-embedded tumor samples. Immunohistochemical analysis was also done for two selected gene products, claudin 8 and MAL2. Results: Unsupervised hierarchical clustering separated the chromophobe RCC and oncocytoma into two distinct groups. By a combination of data analysis approaches, we identified 11 candidate genes showing consistent differential expression between chromophobe RCC and oncocytoma. Five of these genes, AP1M2, MAL2, PROM2, PRSS8, and FLJ20171, were shown to effectively separate these two tumor groups by quantitative reverse transcription-PCR using fresh tissue samples, with similar trends seen on formalin-fixed tissues. Immunohistochemical analysis revealed selective expression of MAL2 and claudin 8 in distal renal tubules, with MAL2 antibody showing differential expression between chromophobe RCC and oncocytoma. Functional analyses suggest that genes encoding tight junction proteins and vesicular membrane trafficking proteins, normally expressed in distal nephrons, are retained in chromophobe RCC and lost or consistently down-regulated in oncocytoma, indicating that these two tumor types, believed to be both derived from distal tubules, are likely distinctive in their histogenesis. Conclusions: We showed that chromophobe RCC and oncocytoma are distinguishable by mRNA expression profiles and a panel of gene products potentially useful as diagnostic markers were identified.

The heterotetrameric adaptor protein (AP) complexes AP-1, AP-2, AP-3, and AP-4 play key roles in transport vesicle formation and cargo sorting in post-Golgi trafficking pathways. Studies on cultured mammalian cells have shown that AP-2 mediates rapid endocytosis of a subset of plasma membrane receptors. To determine whether this function is essential in the context of a whole mammalian organism, we carried out targeted disruption of the gene encoding the mu2 subunit of AP-2 in the mouse. We found that mu2 heterozygous mutant mice were viable and had an apparently normal phenotype. In contrast, no mu2 homozygous mutant embryos were identified among blastocysts from intercrossed heterozygotes, indicating that mu2-deficient embryos die before day 3.5 postcoitus (E3.5). These results indicate that AP-2 is indispensable for early embryonic development, which might be due to its requirement for cell viability.

The short cytoplasmic tail of mouse CD1d (mCD1d) is required for its endosomal localization, for the presentation of some glycolipid Ags, and for the development of Vα14i NKT cells. This tail has a four-amino acid Tyr-containing motif, Tyr-Gln-Asp-Ile (YQDI), similar to those sequences known to be important for the interaction with adaptor protein complexes (AP) that mediate the endosomal localization of many different proteins. In fact, mCD1d has been shown previously to interact with the AP-3 adaptor complex. In the present study, we mutated each amino acid in the YQDI motif to determine the importance of the entire motif sequence in influencing mCD1d trafficking, its interaction with adaptors, and its intracellular localization. The results indicate that the Y, D, and I amino acids are significant functionally because mutations at each of these positions altered the intracellular distribution of mCD1d and reduced its ability to present glycosphingolipids to NKT cells. However, the three amino acids are not all acting in the same way because they differ with regard to how they influence the intracellular distribution of CD1d, its rate of internalization, and its ability to interact with the μ subunit of AP-3. Our results emphasize that multiple steps, including interactions with the adaptors AP-2 and AP-3, are required for normal trafficking of mCD1d and that these different steps are mediated by only a few cytoplasmic amino acids.

The human immunodeficiency virus type 1 virulence protein Nef interacts with the endosomal sorting machinery via a leucine-based motif. Similar sequences within the cytoplasmic domains of cellular transmembrane proteins bind to the adaptor protein (AP) complexes of coated vesicles to modulate protein traffic, but the molecular basis of the interactions between these motifs and the heterotetrameric complexes is controversial. To identify the target of the Nef leucine motif, the native sequence was replaced with either leucine- or tyrosine-based AP-binding sequences from cellular proteins, and the interactions with AP subunits were correlated with function. Tyrosine motifs predictably modulated the interactions between Nef and the μ subunits of AP-1, AP-2, and AP-3; heterologous leucine motifs caused little change in these interactions. Conversely, leucine motifs mediated a ternary interaction between Nef and hemicomplexes containing the σ1 plus γ subunits of AP-1 or the σ3 plus δ subunits of AP-3, whereas tyrosine motifs did not. Similarly, only leucine motifs supported the Nef-mediated association of AP-1 and AP-3 with endosomal membranes in cells treated with brefeldin A. Functionally, Nef proteins containing leucine motifs down-regulated CD4 from the cell surface and enhanced viral replication, whereas those containing tyrosine motifs were inactive. Apparently, the interaction of Nef with the μ subunits of AP complexes is insufficient for function. A leucine-specific mode of interaction that likely involves AP hemicomplexes is further required for Nef activity. The μ and hemicomplex interactions may cooperate to yield high avidity binding of AP complexes to Nef. This binding likely underlies the unusual ability of Nef to induce the stabilization of these complexes on endosomal membranes, an activity that correlates with enhancement of viral replication.

Reversible phosphorylation has long been an attractive mechanism to control cycles of coat assembly and disassembly during clathrin-mediated endocytosis. Many of the coat proteins are phosphorylated in vivo and in vitro. Our work has focused on the role of phosphorylation of the $#x03BC;2 subunit of AP-2 (adaptor protein 2), which appears to be necessary for efficient cargo recruitment. Studies to probe the regulation of $#x03BC;2 phosphorylation demonstrated that clathrin is a specific activator of the $#x03BC;2 kinase, and, in permeabilized cells, cargo sequestration, driven by exogenously added clathrin, results in elevated levels of $#x03BC;2 phosphorylation. Furthermore, phosphorylated $#x03BC;2 is mainly associated with assembled clathrin in vivo and its steady-state level is strongly reduced in cells depleted of clathrin heavy chain. Our results imply a central role for clathrin in the regulation of cargo selection via modulation of phospho-$#x03BC;2 levels. This is therefore a novel regulatory role for clathrin that is independent of its structural role and that provides elegant spatial control of AP-2 and cargo interactions, ensuring that AP-2 is only activated at the correct cellular location and in the correct functional context. Ongoing studies are exploring further the roles of reversible phosphorylation in the coated vesicle cycle.

Multi-parameter flow cytometric techniques coupled with dual colour fluorescent staining were used to study the physical and metabolic consequences of inclusion body formation in batch cultures of the recombinant Escherichia coli strain MSD3735. This strain contains a plasmid coding for the isopropylthiogalactopyranoside-inducible model eukaryotic protein AP50. It is known that the synthesis of foreign proteins at high concentrations can exert a severe metabolic stress on the host cell and that morphological changes can occur. In this work, using various points of induction, it was shown that inclusion body formation is followed immediately by measurable changes in the characteristic intrinsic light scatter patterns for the individual cell (forward scatter, 90 degrees side scatter) and a concomitant progressive change in the individual cell physiological state with respect to both cytoplasmic membrane polarisation and permeability. This work establishes flow cytometry as a potentially valuable tool for monitoring recombinant fermentation processes, providing important information for scale-up. Further, we discuss the possibility of optimising inclusion body formation by manipulating the fermentation conditions based on these rapid "real-time" measurements.

Calcineurin is a highly conserved regulator of Ca2+signaling in eukaryotes. In fission yeast, calcineurin is not essential for viability but is required for cytokinesis and Cl-homeostasis. In a genetic screen for mutations that are synthetically lethal with calcineurin deletion, we isolated a mutant, cis1-1/apm1-1, an allele of the apm1+gene that encodes a homolog of the mammalian μ1A subunit of the clathrin-associated adaptor protein-1 (AP-1) complex. The cis1-1/apm1-1 mutant as well as the apm1-deleted (Δapm1) cells showed distinct phenotypes: temperature sensitivity; tacrolimus (FK506) sensitivity; and pleiotropic defects in cytokinesis, cell integrity, and vacuole fusion. Electron micrographs revealed that Δapm1 cells showed large vesicular structures associated with Golgi stacks and accumulated post-Golgi secretory vesicles. Δapm1 cells also showed the massive accumulation of the exocytic v-SNARE Syb1 in the Golgi/endosomes and a reduced secretion of acid phosphatase. These phenotypes observed in apm1 mutations were accentuated upon temperature up-shift and FK506 treatment. Notably, Apm1-GFP localized to the Golgi/endosomes, the spindle pole bodies, and the medial region. These findings suggest a role for Apm1 associated with the Golgi/endosome function, thereby affecting various cellular processes, including secretion, cytokinesis, vacuole fusion, and cell integrity and also suggest that calcineurin is involved in these events.

In receptor-mediated transport pathways in mammalian cells, clathrin-coated vesicle (CCV) mu-adaptins are the main binding partners for the tyrosine sorting/internalization motif (YXXØ). We have analyzed the function of the mu A-adaptin, one of the five mu-adaptins from Arabidopsis thaliana, by pull-down assays and plasmon resonance measurements using its receptor-binding domain (RBD) fused to a histidine tag. We show that this adaptin is able to bind the consensus tyrosine motif YXXØ from the pea vacuolar sorting receptor (VSR)-PS1, as well as from the mammalian trans-Golgi network (TGN)38 protein. Moreover, the tyrosine residue was revealed to be crucial for binding of the complete cytoplasmic tail of VSR-PS1 to the plant mu A-adaptin. The trans-Golgi localization of the mu A-adaptin strongly suggests its involvement in Golgi- to vacuole-trafficking events.

Endocytic cargo such as the transferrin receptor is incorporated into clathrin-coated pits by associating, via tyrosine-based motifs, with the AP2 complex. Cargo–AP2 interactions occur via the μ2 subunit of AP2, which needs to be phosphorylated for endocytosis to occur. The most likely role for μ2 phosphorylation is in cargo recruitment because μ2 phosphorylation enhances its binding to internalization motifs. Here, we investigate the control of μ2 phosphorylation. We identify clathrin as a specific activator of the μ2 kinase and, in permeabilized cells, we show that ligand sequestration, driven by exogenous clathrin, results in elevated levels of μ2 phosphorylation. Furthermore, we show that AP2 containing phospho-μ2 is mainly associated with assembled clathrin in vivo, and that the level of phospho-μ2 is strongly reduced in a chicken B cell line depleted of clathrin heavy chain. Our results imply a central role for clathrin in the regulation of cargo selection via the modulation of phospho-μ2 levels.

The sorting of membrane-bound proteins from the trans-Golgi network to lysosomal/endosomal compartments is achieved by preferential inclusion into clathrin-coated vesicles. Contained within the cytoplasmic domains of such proteins, specific sequence motifs have been identified (tyrosine-based and/or di-leucine-based) that are essential for targeting and are recognized by a family of heterotetrameric adaptor complexes, which then recruit clathrin. These cytosolic protein complexes, which have been found in a wide variety of higher eukaryotic organisms, are essential for the development of multicellular organisms. In trypanosomatids, the adaptin-mediated sorting of proteins is largely uncharacterized. In order to identify components of the adaptor-complex machinery, this study reports the cloning and characterization of sigma 1- and mu 1-adaptin gene homologues from the eukaryotic protozoan parasite, Leishmania mexicana. Generation of sigma 1- and mu 1-adaptin gene deletion mutants shows that these promastigote parasites are viable in culture, but are unable to establish infection of macrophages or mice, indicating that adaptin function is crucial for pathogenesis in these unicellular organisms.