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The spike (S) protein of SARS-CoV-2 is delivered to the virion assembly site in the ER-Golgi Intermediate Compartment (ERGIC) from both the ER and cis-Golgi in infected cells. However, the relevance and modulatory mechanism of this bidirectional trafficking are unclear. Here, using structure-function analyses, we show that S incorporation into virus-like particles (VLP) and VLP fusogenicity are determined by coatomer-dependent S delivery from the cis-Golgi and restricted by S-coatomer dissociation. Although S mimicry of the host coatomer-binding dibasic motif ensures retrograde trafficking to the ERGIC, avoidance of the host-like C-terminal acidic residue is critical for S-coatomer dissociation and therefore incorporation into virions or export for cell-cell fusion. Because this C-terminal residue is the key determinant of SARS-CoV-2 assembly and fusogenicity, our work provides a framework for the export of S protein encoded in genetic vaccines for surface display and immune activation.

Cytochrome P450 is a hemoprotein that has been linked to the oxidative metabolism of biologically occurring compounds such as fatty acids and steroids. To investigate further the basis for the increased activity of C2pmBalCl in COS-1 cells, the cytochrome P450 hemoprotein was expressed in insect cells and E. coli. Results showed that mutations introduced in the C-terminal region of P450 helped to facilitate protein assembly and partially reversed the effect of N-terminal mutations.

The glycosyltransferases of the mammalian Golgi complex must recycle between the stacked cisternae of that organelle to maintain their proper steady-state localization. This trafficking is mediated by COPI-coated vesicles, but how the glycosyltransferases are incorporated into these transport vesicles is poorly understood. Here we show that the N-terminal cytoplasmic tails (N-tails) of a number of cis Golgi glycosyltransferases which share a ϕ-(K/R)-X-L-X-(K/R) sequence bind directly to the δ- and ζ-subunits of COPI. Mutations of this N-tail motif impair binding to the COPI subunits, leading to mislocalization of the transferases to lysosomes. The physiological importance of these interactions is illustrated by mucolipidosis III patients with missense mutations in the N-tail of GlcNAc-1-phosphotransferase that cause the transferase to be rapidly degraded in lysosomes. These studies establish that direct binding of the N-tails of mammalian cis Golgi glycosyltransferases with COPI subunits is essential for recycling within the Golgi. [ABSTRACT FROM AUTHOR]

The UDP-Glc NAc:lysosomal enzyme, N-acetylglucosamine-1-phosphotransferase (Glc NAc-1- PT), is an α2β2γ2 hexamer that mediates the initial step in the formation of the mannose 6-phosphate targeting signal on newly synthesized lysosomal acid hydrolases. The GNPTAB gene encodes the 1256 amino acid long α/β precursor which is normally cleaved at K928 in the early Golgi by Site-1 protease (S1P). Here, we show that removal of the so-called 'spacer-1′ domain (residues 86-322) results in cleavage almost exclusively at a second S1P consensus sequence located upstream of K928. In addition, Glc NAc-1- PT lacking spacer-1 exhibits enhanced phosphorylation of several non-lysosomal glycoproteins, while the phosphorylation of lysosomal acid hydrolases is not altered. In view of these effects on the maturation and function of Glc NAc-1- PT, we suggest renaming `spacer-1′ the `regulatory-1′ domain. [ABSTRACT FROM AUTHOR]

Shows that the VHS domain of the GGA2 protein family binds to the acidic cluster-dileucine motif in the cytoplasmic tail of the cation-independent mannose 6-phosphate receptor (CI-MPR). Defects of receptors with mutations in the motif in lysosomal enzyme sorting; Hinge domain of GGA2 bound clathrin; Importance for lysosomal enzyme targeting.

In vertebrates, newly synthesized lysosomal enzymes traffick to lysosomes through the mannose-6-phosphate (M6P) pathway. The Golgi membrane protein TMEM251 was recently discovered to regulate lysosome biogenesis by controlling the level of GlcNAc-1-phosphotransferase (GNPT). However, its precise function remained unclear. In this study, we demonstrate that TMEM251 is a two-transmembrane protein indispensable for GNPT stability, cleavage by Site-1-Protease (S1P), and enzymatic activity. We reconcile conflicting models by showing that TMEM251 enhances GNPT cleavage and prevents its mislocalization to lysosomes for degradation. We further establish that TMEM251 achieves this by interacting with GOLPH3 and retromer complexes to anchor the TMEM251-GNPT complex at the Golgi. Alanine mutagenesis identified F 4 XXR 7 motif in TMEM251's N-tail for GOLPH3 binding. Together, our findings uncover TMEM251's multi-faceted role in stabilizing GNPT, retaining it at the Golgi, and ensuring the fidelity of the M6P pathway, thereby providing insights into its molecular function., Competing Interests: Competing interests S.K. is co-founder of M6P Therapeutics and holds stock options in the company. The rest of the authors declare no competing interests.

GABAergic interneuron deficits have been implicated in the epileptogenesis of multiple neurological diseases. While epileptic seizures are a key clinical hallmark of CLN2 disease, a childhood-onset neurodegenerative lysosomal storage disorder caused by a deficiency of tripeptidyl peptidase 1 (TPP1), the etiology of these seizures remains elusive. Given that Cln2 R207X/R207X mice display fatal spontaneous seizures and an early loss of several cortical interneuron populations, we hypothesized that those two events might be causally related. To address this hypothesis, we first generated an inducible transgenic mouse expressing lysosomal membrane-tethered TPP1 (TPP1LAMP1) on the Cln2 R207X/R207X genetic background to study the cell-autonomous effects of cell-type-specific TPP1 deficiency. We crossed the TPP1LAMP1 mice with Vgat-Cre mice to introduce interneuron-specific TPP1 deficiency. Vgat-Cre ; TPP1LAMP1 mice displayed storage material accumulation in several interneuron populations both in cortex and striatum, and increased susceptibility to die after PTZ-induced seizures. Secondly, to test the role of GABAergic interneuron activity in seizure progression, we selectively activated these cells in Cln2 R207X/R207X mice using Designer Receptor Exclusively Activated by Designer Drugs (DREADDs) in in Vgat-Cre : Cln2 R207X/R207X mice. EEG monitoring revealed that DREADD-mediated activation of interneurons via chronic deschloroclozapine administration accelerated the onset of spontaneous seizures and seizure-associated death in Vgat-Cre : Cln2 R207X/R207X mice, suggesting that modulating interneuron activity can exert influence over epileptiform abnormalities in CLN2 disease. Taken together, these results provide new mechanistic insights into the underlying etiology of seizures and premature death that characterize CLN2 disease.

The functional redundancy of the three mammalian Golgi-localized, γ-ear-containing, ADP-ribosylation factor-binding proteins (GGAs) was addressed in a previous study. Using insertional mutagenesis, we found that Gga1 or Gga3 homozygous knockout mice were for the most part normal, whereas mice homozygous for two different Gga2 gene-trap alleles exhibited either embryonic or neonatal lethality in the C57BL/6 background, depending on the source of the vector utilized (Byg vs. Tigm, respectively). We now show that the Byg strain harbors a disrupted Gga2 allele that is hypomorphic, indicating that the Byg lethality is attributable to a mechanism independent of GGA2. This is in contrast to the Tigm Gga2 allele, which is a true knockout and establishes a role for GGA2 during the neonatal period. Placement of the Tigm Gga2 allele into the C57BL6/Ola129Sv mixed background results in a lower incidence of neonatal lethality, showing the importance of genetic background in determining the requirement for GGA2 during this period. The Gga2(-/-) mice that survive have reduced body weight at birth and this runted phenotype is maintained through adulthood., (Copyright © 2014 Doray et al.)

The clathrin adaptors AP-1 and AP-2 bind cargo proteins via two types of motifs: tyrosine-based Yxx phi and dileucine-based [DE]XXXL[LI]. Although it is well established that Yxx phi motifs bind to the mu subunits of AP-1 or AP-2, dileucine motifs have been reported to bind to either the mu or beta subunits of these adaptors as well as the gamma/sigma1 hemicomplex of AP-1. To clarify this controversy, the various subunits of AP-1 and AP-2 were expressed individually and in hemicomplex form in insect cells, and they were used in glutathione S-transferase pull-down assays to determine their binding properties. We report that the gamma/sigma1 or alpha/sigma2 hemicomplexes bound the dileucine-based motifs of several proteins quite strongly, whereas binding by the beta1/mu1 and beta2/mu2 hemicomplexes, and the individual beta or mu subunits, was extremely weak or undetectable. The gamma/sigma1 and alpha/sigma2 hemicomplexes displayed substantial differences in their preference for particular dileucine-based motifs. Most strikingly, an aspartate at position -4 compromised binding to the gamma/sigma1 hemicomplex, whereas minimally affecting binding to alpha/sigma2. There was an excellent correlation between binding to the alpha/sigma2 hemicomplex and in vivo internalization mediated by the dileucine-based sorting signals. These findings provide new insights into the trafficking mechanisms of D/EXXXL[LI]-mediated sorting signals.

Background: Estrogens and their receptors are important in human development, physiology and disease. In this study, we utilized an integrated genome-wide molecular and computational approach to characterize the interaction between the activated estrogen receptor (ER) and the regulatory elements of candidate target genes., Results: Of around 19,000 genes surveyed in this study, we observed 137 ER-regulated genes in T-47D cells, of which only 89 were direct target genes. Meta-analysis of heterogeneous in vitro and in vivo datasets showed that the expression profiles in T-47D and MCF-7 cells are remarkably similar and overlap with genes differentially expressed between ER-positive and ER-negative tumors. Computational analysis revealed a significant enrichment of putative estrogen response elements (EREs) in the cis-regulatory regions of direct target genes. Chromatin immunoprecipitation confirmed ligand-dependent ER binding at the computationally predicted EREs in our highest ranked ER direct target genes, NRIP1, GREB1 and ABCA3. Wider examination of the cis-regulatory regions flanking the transcriptional start sites showed species conservation in mouse-human comparisons in only 6% of predicted EREs., Conclusions: Only a small core set of human genes, validated across experimental systems and closely associated with ER status in breast tumors, appear to be sufficient to induce ER effects in breast cancer cells. That cis-regulatory regions of these core ER target genes are poorly conserved suggests that different evolutionary mechanisms are operative at transcriptional control elements than at coding regions. These results predict that certain biological effects of estrogen signaling will differ between mouse and human to a larger extent than previously thought.