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International audience; Steroid-resistant nephrotic syndrome (SRNS) is characterized by high-range proteinuria and most often focal and segmental glomerulosclerosis (FSGS). Identification of mutations in genes causing SRNS has improved our understanding of disease mechanisms and highlighted defects in the podocyte, a highly specialized glomerular epithelial cell, as major factors in disease pathogenesis. By exome sequencing, we identified missense mutations in TBC1D8B in two families with an X-linked early-onset SRNS with FSGS. TBC1D8B is an uncharacterized Rab-GTPase-activating protein likely involved in endocytic and recycling pathways. Immunofluorescence studies revealed TBC1D8B presence in human glomeruli, and affected individual podocytes displayed architectural changes associated with migration defects commonly found in FSGS. In zebrafish we demonstrated that both knockdown and knockout of the unique TBC1D8B ortholog-induced proteinuria and that this phenotype was rescued by human TBC1D8B mRNA injection, but not by either of the two mutated mRNAs. We also showed an interaction between TBC1D8B and Rab11b, a key protein in vesicular recycling in cells. Interestingly, both internalization and recycling processes were dramatically decreased in affected individuals' podocytes and fibroblasts, confirming the crucial role of TBC1D8B in the cellular recycling processes, probably as a Rab11b GTPase-activating protein. Altogether, these results confirmed that pathogenic variations in TBC1D8B are involved in X-linked podocytopathy and points to alterations in recycling processes as a mechanism of SRNS.

Radial glial (RG) cells are the neural stem cells of the developing neocortex. Apical RG (aRG) cells can delaminate to generate basal RG (bRG) cells, a cell type associated with human brain expansion. Here, we report that this delamination is regulated by the post-Golgi secretory pathway. Using in situ subcellular live imaging, we show that post-Golgi transport of RAB6+ vesicles occurs toward the minus ends of microtubules and depends on dynein. We demonstrate that the apical determinant Crumbs3 (CRB3) is also transported by dynein. Double knockout of RAB6A/A’ and RAB6B impairs apical localization of CRB3, and induces a retraction of aRG cell apical process, leading to delamination and ectopic division. These defects are phenocopied by knock-out of the dynein activator LIS1. Overall, our results identify a RAB6-dynein-LIS1 complex for Golgi to apical surface transport in aRG cells, and highlights the role of this pathway in the maintenance of neuroepithelial integrity.

The Golgi-associated RAB GTPases, RAB6A and RAB6A′, regulate anterograde and retrograde transport pathways from and to the Golgi. In vitro, RAB6A/A′ control several cellular functions including cell division, migration, adhesion and polarity. However, their role remains poorly described in vivo. Here, we generated BlgCre; Rab6aF/F mice presenting a specific deletion of Rab6a in the mammary luminal secretory lineage during gestation and lactation. Rab6a loss severely impaired the differentiation, maturation and maintenance of the secretory tissue, compromising lactation. The mutant epithelium displayed a decreased activation of STAT5, a key regulator of the lactogenic process primarily governed by prolactin. Data obtained with a mammary epithelial cell line suggested that defective STAT5 activation might originate from a perturbed transport of the prolactin receptor, altering its membrane expression and signaling cascade. Despite the major functional defects observed upon Rab6a deletion, the polarized organization of the mammary epithelial bilayer was preserved. Altogether, our data reveal a crucial role for RAB6A/A′ in the lactogenic function of the mammary gland and suggest that the trafficking pathways controlled by RAB6A/A′ depend on cell-type specialization and tissue context., Summary: This study reveals a role for the Golgi-associated RAB GTPases RAB6A/A′ in the lactogenic function of the mammary gland.

The Golgi-associated RAB GTPases, RAB6A and RAB6A’, regulate anterograde and retrograde transport pathways from and to the Golgi.In vitro, RAB6A/A’ have been reported to be involved in several cellular functions, including, in addition to transport, cell division, migration, adhesion and polarity. However, their role remains poorly describedin vivo, in particular in epithelial tissues. Here, we generated BlgCre;Rab6aF/Fmouse presenting a specific deletion ofRab6ain the mammary luminal secretory lineage during gestation and lactation.Rab6aloss severely impaired the differentiation, maturation and maintenance of the secretory tissue, compromising lactation. It led to a decreased activation of STAT5, a key regulator of the lactogenic process primarily governed by prolactin. Data obtained with a human mammary epithelial cell line suggested that defective STAT5 activation might originate from a perturbed transport of the prolactin receptor, altering its membrane expression and signaling cascade. Despite the major functional defects observed uponRab6adeletion, the polarized organization of the mammary epithelial bilayer was preserved. Altogether, our data reveal a crucial role for RAB6A/A’ in the lactogenic function of the mammary gland. They also suggest that the trafficking pathways controlled by RAB6A/A’ depend on cell type specialization and tissue context.SUMMARY STATEMENTThis study reveals a role for the Golgi-associated RAB GTPases, RAB6A/A’, in the lactogenic function of the mammary gland.

Fourriere et al. demonstrate the existence of secretion hotspots juxtaposed to focal adhesions. Post-Golgi transport carriers use a subset of microtubules to reach focal adhesions. RAB6 acts as a general regulator of post-Golgi secretion and, together with ELKS, restricts protein secretion at focal adhesions., To ensure their homeostasis and sustain differentiated functions, cells continuously transport diverse cargos to various cell compartments and in particular to the cell surface. Secreted proteins are transported along intracellular routes from the endoplasmic reticulum through the Golgi complex before reaching the plasma membrane along microtubule tracks. Using a synchronized secretion assay, we report here that exocytosis does not occur randomly at the cell surface but on localized hotspots juxtaposed to focal adhesions. Although microtubules are involved, the RAB6-dependent machinery plays an essential role. We observed that, irrespective of the transported cargos, most post-Golgi carriers are positive for RAB6 and that its inactivation leads to a broad reduction of protein secretion. RAB6 may thus be a general regulator of post-Golgi secretion.

International audience; TRPM8 is the main ion channel responsible for cold transduction in the somatosensory system. Nerve terminal availability of TRPM8 determines cold sensitivity, but how axonal secretory organelles control channel delivery remains poorly understood. Here we examine the distribution of TRPM8 and trafficking organelles in cold-sensitive peripheral axons and disrupt trafficking by targeting the ARF-GEF GBF1 pharmacologically or the small GTPase RAB6 by optogenetics. In axons of the sciatic nerve, inhibition of GBF1 interrupts TRPM8 trafficking and increases association with the trans-Golgi network, LAMP1, and Golgi satellites, which distribute profusely along the axonal shaft. Accordingly, both TRPM8-dependent ongoing activity and cold-evoked responses reversibly decline upon GBF1 inhibition in nerve endings of corneal cold thermoreceptors. Inhibition of RAB6, which also associates to Golgi satellites, decreases cold-induced responses in vivo. Our results support a non-conventional axonal trafficking mechanism controlling the availability of TRPM8 in axons and cold sensitivity in the peripheral nervous system.

One challenge in current cell biology is to decipher the biophysical mechanisms governing protein enrichment on curved membranes and the resulting membrane deformation. The ERM protein ezrin is abundant and associated with cellular membranes that are flat or with positive or negative curvatures. Using in vitro and cell biology approaches, we assess mechanisms of ezrin’s enrichment on curved membranes. We evidence that ezrin (ezrinWT) and its phosphomimetic mutant T567D (ezrinTD) do not deform membranes but self-assemble anti-parallelly, zipping adjacent membranes. EzrinTD’s specific conformation reduces intermolecular ezrin interactions, allows binding to actin filaments, and promotes ezrin binding to positively curved membranes. While neither ezrinTD nor ezrinWT senses negative membrane curvature alone, we demonstrate that interacting with curvature sensors I-BAR-domain proteins facilitates ezrin enrichment in negatively curved membrane protrusions. Overall, our work reveals new mechanisms, specific conformation or binding to a curvature sensor partner, for targeting curvature insensitive proteins to curved membranes.

To ensure their homeostasis and sustain differentiated functions, cells continuously transport diverse cargos to various cell compartments and in particular to the cell surface. Secreted proteins are transported along intracellular routes from the endoplasmic reticulum through the Golgi complex before reaching the plasma membrane along microtubule tracks. Using a synchronized secretion assay, we report here that exocytosis does not occur randomly at the cell surface but on localized hotspots juxtaposed to focal adhesions. Although microtubules are involved, the RAB6-dependent machinery plays an essential role. We observed that, irrespective of the transported cargos, most post-Golgi carriers are positive for RAB6 and that its inactivation leads to a broad reduction of protein secretion. RAB6 may thus be a general regulator of post-Golgi secretion.

Background Information Rab6 is one of the most conserved Rab GTPaes throughout evolution and the most abundant Rab protein associated with the Golgi complex. The two ubiquitous Rab isoforms, Rab6A and Rab6A′, that are generated by alternative splicing of the RAB6A gene, regulate several transport steps at the Golgi level, including retrograde transport between endosomes and Golgi, anterograde transport between Golgi and the plasma membrane, and intra-Golgi and Golgi to endoplasmic reticulum transport. Results We have generated mice with a conditional null allele of RAB6A. Mice homozygous for the RAB6A null allele died at an early stage of embryonic development. Mouse embryonic fibroblasts (MEFs) were isolated from RAB6AloxP/loxPRosa26-CreERT2 and incubated with 4-hydroxy tamoxifen, resulting in the efficient depletion of Rab6A and Rab6A′. We show that Rab6 depletion affects cell growth, alters Golgi morphology and decreases the Golgi-associated levels of some known Rab6 effectors such as Bicaudal-D and myosin II. We also show that Rab6 depletion protects MEFs against ricin toxin and delays VSV-G secretion. Conclusions Our study shows that RAB6 is an essential gene required for normal embryonic development. We confirm in MEF cells most of the functions previously attributed to the two ubiquitous Rab6 isoforms.

GTPases of the RAB family are key regulators of multiple steps of membrane trafficking. Several members of the RAB GTPase family have been implicated in mitotic progression. In this review, we will first focus on the function of endosome-associated RAB GTPases reported in early steps of mitosis, spindle pole maturation, and during cytokinesis. Second, we will discuss the role of Golgi-associated RAB GTPases at the metaphase/anaphase transition and during cytokinesis. Fil: Miserey lenkei, Stéphanie. Institute Curie; Francia Fil: Colombo, Maria Isabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto de Histología y Embriología de Mendoza Dr. Mario H. Burgos. Universidad Nacional de Cuyo. Facultad de Cs.médicas. Instituto de Histología y Embriología de Mendoza Dr. Mario H. Burgos; Argentina

Myosins are actin-based motor proteins that are involved in a wide variety of cellular processes such as membrane transport, muscle contraction, and cell division. Humans have over 40 myosins that can be placed into 18 classes, the malfunctioning of a number of which can lead to disease. There are three members of the human class V myosin family, myosins Va, Vb, and Vc. People lacking functional myosin Va suffer from a rare autosomal recessive disease called Griscelli's Syndrome type I (GS1) that is characterized by severe neurological defects and partial albinism. Mutations in the myosin Vb gene lead to an epithelial disorder called microvillus inclusion disease (MVID) that is often fatal in infants. The class V myosins have been implicated in the transport of diverse cargoes such as melanosomes in pigment cells, synaptic vesicles in neurons, RNA transcripts in a variety of cell types, and organelles such as the endoplasmic reticulum. The Rab GTPases play a critical role in recruiting class V myosins to their cargo. We recently published a study in which we used the yeast two-hybrid system to systematically test myosin Va for its ability to interact with each member of the human Rab GTPase family. We present here a detailed description of this yeast two-hybrid "living chip" assay. Furthermore, we present a protocol for validating positive interactions obtained from this screen by coimmunoprecipitation.

The closely related Rab6 isoforms, Rab6A and Rab6A’, have been shown to regulate vesicular trafficking within the Golgi and post-Golgi compartments, but studies using dominant active or negative mutant suggested conflicting models. Here, we report that reduction in the expression of Rab6 isoform using specific small interfering RNA reveals noticeable differences in the Rab6A and Rab6A’ biological functions. Surprisingly, Rab6A seems to be largely dispensable in membrane trafficking events, whereas knocking down the expression of Rab6A’ hampers the intracellular transport of the retrograde cargo marker, the Shiga Toxin B-subunit along the endocytic pathway, and causes defects in Golgiassociated protein recycling through the endoplasmic reticulum. We also showed that Rab6A’ is required for cell cycle progression through mitosis and identify Ile 62 as a key residue for uncoupling Rab6A’ functions in mitosis and retrograde trafficking. Thus, our work shows that Rab6A and Rab6A’ perform different functions within the cell and suggests a novel role for Rab6A’ as the major Rab6 isoform regulating previously described Rab6dependent transport pathways.

The two isoforms of the Rab6 GTPase, Rab6A and Rab6A', regulate a retrograde transport route connecting early endosomes and the endoplasmic reticulum via the Golgi complex in interphasic cells. Here we report that when Rab6A' function is altered cells are unable to progress normally through mitosis. Such cells are blocked in metaphase, despite displaying a normal Golgi fragmentation and with the Mad2-spindle checkpoint activated. Furthermore, the Rab6 effector p150(Glued), a subunit of the dynein/dynactin complex, remains associated with some kinetochores. A similar phenotype was observed when GAPCenA, a GTPase-activating protein of Rab6, was depleted from cells. Our results suggest that Rab6A' likely regulates the dynamics of the dynein/dynactin complex at the kinetochores and consequently the inactivation of the Mad2-spindle checkpoint. Rab6A', through its interaction with p150(Glued) and GAPCenA, may thus participate in a pathway involved in the metaphase/anaphase transition.

Myosins are actin-based motor proteins that are involved in a wide variety of cellular processes such as membrane transport, muscle contraction, and cell division. Humans have over 40 myosins that can be placed into 18 classes, the malfunctioning of a number of which can lead to disease. There are three members of the human class V myosin family, myosins Va, Vb, and Vc. People lacking functional myosin Va suffer from a rare autosomal recessive disease called Griscelli's Syndrome type I (GS1) that is characterized by severe neurological defects and partial albinism. Mutations in the myosin Vb gene lead to an epithelial disorder called microvillus inclusion disease (MVID) that is often fatal in infants. The class V myosins have been implicated in the transport of diverse cargoes such as melanosomes in pigment cells, synaptic vesicles in neurons, RNA transcripts in a variety of cell types, and organelles such as the endoplasmic reticulum. The Rab GTPases play a critical role in recruiting class V myosins to their cargo. We recently published a study in which we used the yeast two-hybrid system to systematically test myosin Va for its ability to interact with each member of the human Rab GTPase family. We present here a detailed description of this yeast two-hybrid "living chip" assay. Furthermore, we present a protocol for validating positive interactions obtained from this screen by coimmunoprecipitation.

Mitotic kinesins are essential regulators of cancer cell replication and migration. The mitotic kinesin MKLP2/KIF20A, a member of the kinesin-6 family, plays an essential role during cytokinesis and was identified as a potential new target for cancer chemotherapy1. We have previously identified Paprotrain, a new synthetic compound, as the first selective MKLP2 inhibitor2. Recently, we obtained Paprotrain analogues with higher potency on MKLP23. Herein we describe the identification and characterization of BKS0349, a new potent analogue of Paprotrain. BKS0349 compound is 10 times more potent than paprotain on MKLP2 inhibition and has shown an even more restricted specificity profile when tested on a large set of kinases. In-vitro this compound is highly cytotoxic on a wide panel of human cancer cell lines (IC50 ranged 10-70 nM, which corresponds to 1000 fold improvement of paprotrain potency) while no toxicity is observed on human normal cells such as peripheral blood mononuclear cells (PBMC) and primary hepatocytes (IC50 >50µM). BKS0349 compound is well tolerated in mice using repeated intravenous administrations (200 mg/kg, twice a week for 4 weeks). In xenografted nude mice, in-vivo treatment with BKS0349 compound demonstrates a high antitumor activity against various human cancer cell models, either sensitive or resistant to some standard-of-care treatments. In addition, human cancer cells treated in-vitro as well as in-vivo with BSK0349 compound display Golgi scattering and a mitotic arrest leading to cell death, as hallmarks of BKS0349’ mode of action. These findings show that MKLP2 is a potential new target for cancer chemotherapy and BKS0349 a good candidate to be developed for cancer treatment. 1Paclitaxel targets FOXM1 to regulate KIF20A in mitotic catastrophe and breast cancer paclitaxel resistance. P. Khongkow et al. Oncogene, 2015, 1-13. 2Relocation of Aurora B and Survivin from centromeres to the central spindle impaired by a kinesin-specific MKLP-2 inhibitor. S. Tcherniuk et al. Angew. Chem. Int. Ed., 2010, 49:8228-8231. 3New MKLP-2 inhibitors in the paprotrain series: design, synthesis and biological evaluations. C. Labrière et al. Bioorg and Med Chem, 2016, 24:731-734. Note: This abstract was not presented at the meeting. Citation Format: Yves Collette, Stephanie Miserey-Lenkei, Catherine Guillou, Denis Carniato, Bernard Pau, Bruno Goud, Norbert Vey, Cecile E. Bougeret. Identification of a selective MKLP2/KIF20A inhibitor with high in-vivo antitumor activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4140. doi:10.1158/1538-7445.AM2017-4140

Rab6 is one of the most conserved Rab GTPaes throughout evolution and the most abundant Rab protein associated with the Golgi complex. The two ubiquitous Rab isoforms, Rab6A and Rab6A', that are generated by alternative splicing of the RAB6A gene, regulate several transport steps at the Golgi level, including retrograde transport between endosomes and Golgi, anterograde transport between Golgi and the plasma membrane, and intra-Golgi and Golgi to endoplasmic reticulum transport.We have generated mice with a conditional null allele of RAB6A. Mice homozygous for the RAB6A null allele died at an early stage of embryonic development. Mouse embryonic fibroblasts (MEFs) were isolated from RAB6A(loxP/loxP) Rosa26-CreERT2 and incubated with 4-hydroxy tamoxifen, resulting in the efficient depletion of Rab6A and Rab6A'. We show that Rab6 depletion affects cell growth, alters Golgi morphology and decreases the Golgi-associated levels of some known Rab6 effectors such as Bicaudal-D and myosin II. We also show that Rab6 depletion protects MEFs against ricin toxin and delays VSV-G secretion.Our study shows that RAB6 is an essential gene required for normal embryonic development. We confirm in MEF cells most of the functions previously attributed to the two ubiquitous Rab6 isoforms.

SummaryEarly endosomes consist of vacuolar sorting and tubular recycling domains that segregate components fated for degradation in lysosomes or reuse by recycling to the plasma membrane or Golgi. The tubular transport intermediates that constitute recycling endosomes function in cell polarity, migration, and cytokinesis. Endosomal tubulation and fission require both actin and intact microtubules, but although factors that stabilize recycling endosomal tubules have been identified, those required for tubule generation from vacuolar sorting endosomes (SEs) remain unknown. We show that the microtubule motor KIF13A associates with recycling endosome tubules and controls their morphogenesis. Interfering with KIF13A function impairs the formation of endosomal tubules from SEs with consequent defects in endosome homeostasis and cargo recycling. Moreover, KIF13A interacts and cooperates with RAB11 to generate endosomal tubules. Our data illustrate how a microtubule motor couples early endosome morphogenesis to its motility and function.

The angiotensin II (Ang II) AT1A receptor was tagged at its C terminus with the enhanced green fluorescent protein (EGFP), and the corresponding chimeric cDNA was expressed in HEK-293 cells. This tagged receptor presents wild-type pharmacological and signaling properties and can be immunodetected by Western blotting and immunoprecipitation using EGFP antibodies. Therefore, this EGFP-tagged AT1A receptor is the perfect tool for analyzing in parallel the subcellular distributions of the receptor and its interacting G protein and their trafficking using confocal microscopy. Morphological observation of both the fluorescent receptor and its cognate Gαq/11 protein, identified by indirect immunofluorescence, and the development of a specific software for digital image analysis together allow examination and quantification of the cellular distribution of these proteins before and after the binding of different agonist or antagonist ligands. These observations result in several conclusions: 1) Expression of incre...

Mammalian cells show a large diversity in shape and are both shape-changing and mobile when cultured on conventional uniform substrates. The use of micropatterning techniques limits the number of variable parameters, by imposing shape and standardized adhesive areas on the cells, which facilitates analysis. By changing size or shape of the micropattern, for example, forcing a polar axis on the cell, it is possible to study how these parameters impact organelle organization, distribution, and dynamics inside the cell. To study the mitochondrial network, which is composed of dynamic tubular organelles dependent on the microtubule cytoskeleton for its distribution, it is important to be able to distinguish between distinct mitochondria. Here, we present a practical method with which we spread the cells on large patterns created with deep UV technique, which not only makes the cells uniform in size and shape as well as immobile, and therefore easier to compare and analyze, but also expands the mitochondrial network and allows for an easier tracking of appropriately labeled individual mitochondria.

Although essential for their neuronal function, the molecular mechanisms underlying the dendritic targeting of serotonin G-protein-coupled receptors are poorly understood. Here, we characterized a Yif1B-dependent vesicular scaffolding complex mediating the intracellular traffic of the rat 5-HT1Areceptor (5-HT1AR) toward dendrites. By combining directed mutagenesis, GST-pull down, and surface plasmon resonance, we identified a tribasic motif in the C-tail of the 5-HT1AR on which Yif1B binds directly with high affinity (KD≈ 37 nm). Moreover, we identified Yip1A, Rab6, and Kif5B as new partners of the 5-HT1AR/Yif1B complex, and showed that their expression in neurons is also crucial for the dendritic targeting of the 5-HT1AR. Live videomicroscopy revealed that 5-HT1AR, Yif1B, Yip1A, and Rab6 traffic in vesicles exiting the soma toward the dendritic tree, and also exhibit bidirectional motions, sustaining their role in 5-HT1AR dendritic targeting. Hence, we propose a new trafficking pathway model in which Yif1B is the scaffold protein recruiting the 5-HT1AR in a complex including Yip1A and Rab6, with Kif5B and dynein as two opposite molecular motors coordinating the traffic of vesicles along dendritic microtubules. This targeting pathway opens new insights for G-protein-coupled receptors trafficking in neurons.

Familial hyperkalemic hypertension (FHHt) is a Mendelian form of arterial hypertension that is partially explained by mutations in WNK1 and WNK4 that lead to increased activity of the Na(+)-Cl(-) cotransporter (NCC) in the distal nephron. Using combined linkage analysis and whole-exome sequencing in two families, we identified KLHL3 as a third gene responsible for FHHt. Direct sequencing of 43 other affected individuals revealed 11 additional missense mutations that were associated with heterogeneous phenotypes and diverse modes of inheritance. Polymorphisms at KLHL3 were not associated with blood pressure. The KLHL3 protein belongs to the BTB-BACK-kelch family of actin-binding proteins that recruit substrates for Cullin3-based ubiquitin ligase complexes. KLHL3 is coexpressed with NCC and downregulates NCC expression at the cell surface. Our study establishes a role for KLHL3 as a new member of the complex signaling pathway regulating ion homeostasis in the distal nephron and indirectly blood pressure.

Trafficking between membrane compartments is a characteristic of eukaryotic cells and relies on transport carriers that bud and fission from a donor membrane, before being transported and fusing with the correct acceptor compartment. Rab GTPases ensure specificity and directionality of trafficking steps by regulating the movement of transport carriers along cytoskeletal tracks, and the recruitment of tethering factors required for the docking and fusion processes. Here we show that Rab6, a Golgi-associated Rab, forms a complex with myosin II, contributes to its localization at the Golgi complex and, unexpectedly, controls the fission of Rab6 vesicles. Inhibition of either Rab6 or myosin II function impairs both the fission of Rab6 transport carriers from Golgi membranes and the trafficking of anterograde and retrograde cargo from the Golgi. These effects are consistent with myosin II being an effector of Rab6 in these processes. Our results provide evidence that the actomyosin system is required in vesicle biogenesis at the Golgi, and uncover a function for Rab GTPases in vesicle fission.

Rab11 and Rab6 guanosine triphosphatases are associated with membranes of the recycling endosomes (REs) and Golgi complex, respectively. Evidence indicates that they sequentially regulate a retrograde transport pathway between these two compartments, suggesting the existence of proteins that must co-ordinate their functions. Here, we report the characterization of two isoforms of a protein, Rab6-interacting protein 1 (R6IP1), originally identified as a Rab6-binding protein. R6IP1 also binds to Rab11A in its GTP-bound conformation. In interphase cells, R6IP1 is targeted to the Golgi in a Rab6-dependent manner but can associate with Rab11-positive compartments when the level of Rab11A is increased within the cells. Fluorescence resonance energy transfer analysis using fluorescence lifetime imaging shows that the overexpression of R6IP1 promotes an interaction between Rab11A and Rab6 in living cells. Accordingly, the REs marked by Rab11 and transferrin receptor are depleted from the cell periphery and accumulate in the pericentriolar area. However, endosomal and Golgi membranes do not appear to fuse with each other. We also show that R6IP1 function is required during metaphase and cytokinesis, two mitotic steps in which a role of Rab6 and Rab11 has been previously documented. We propose that R6IP1 may couple Rab6 and Rab11 function throughout the cell cycle.

The closely related Rab6 isoforms, Rab6A and Rab6A', have been shown to regulate vesicular trafficking within the Golgi and post-Golgi compartments, but studies using dominant active or negative mutant suggested conflicting models. Here, we report that reduction in the expression of Rab6 isoform using specific small interfering RNA reveals noticeable differences in the Rab6A and Rab6A' biological functions. Surprisingly, Rab6A seems to be largely dispensable in membrane trafficking events, whereas knocking down the expression of Rab6A' hampers the intracellular transport of the retrograde cargo marker, the Shiga Toxin B-subunit along the endocytic pathway, and causes defects in Golgi- associated protein recycling through the endoplasmic reticulum. We also showed that Rab6A' is required for cell cycle progression through mitosis and identify Ile(62) as a key residue for uncoupling Rab6A' functions in mitosis and retrograde trafficking. Thus, our work shows that Rab6A and Rab6A' perform different functions within the cell and suggests a novel role for Rab6A' as the major Rab6 isoform regulating previously described Rab6-dependent transport pathways.

As constitutively active mutants (CAMs) mimic an active conformation, they can be used to characterize the process of G protein-coupled receptor activation. Here, we used CAMs to study the link between activation and internalization of the angiotensin II AT(1A) receptor. The cellular localization of fluorescently tagged N111A, I245T, and L305Q mutants was determined by confocal microscopy. In the absence of ligand, CAMs were mostly located in intracellular vesicles, whereas the wild-type AT(1A) was found at the cell surface. After 2 h incubation with inverse agonist, losartan, CAMs were translocated to the plasma membrane. Similar observations were made in H295, a human adrenocortical cell line which expresses physiologically the AT(1) receptor. This phenomenon, which was not dependent on protein synthesis and the pharmacology and kinetics of which were similar to the recycling of the wild-type receptor, was called "externalization". After externalization and losartan removal, the L305Q CAM underwent rapid ligand-independent endocytosis, with the same kinetics and temperature sensitivity as the angiotensin II-induced internalization of the wild-type AT(1A). Moreover, the addition of a second mutation known to block internalization (Delta 329 truncation) prevented intracellular localization of the CAM. These data show that AT(1A) CAMs are constitutively and permanently internalized and recycled. This mechanism is different from the down-regulation observed for CAMs of other G protein-coupled receptors and thus defines a new paradigm for the cellular regulation of CAMs.

The constitutive activation of G-protein-coupled receptors is a major new approach to investigating their physiopathology and pharmacology. A large number of spontaneous and site-directed mutations resulting in constitutive activity have been identified, but systematic mapping of the amino acids involved for a given receptor would be extremely useful for complete elucidation of the molecular mechanisms underlying its activation. We carried out such mapping for the angiotensin II type 1A (AT 1A ) receptor by screening a randomly mutated cDNA library after expressing the mutated clones in eukaryotic cells. To test the AT 1A mutants generated, we developed an original, specific, and highly sensitive assay based on the properties of CGP42112A. This classical AT 2 agonist is a weak partial agonist of the wild-type AT 1A receptor and becomes a full agonist for constitutively active AT 1A mutants, as shown experimentally and in allostery-based theoretical models. Activation of the mutated receptors by CGP42112A was monitored by using the bioluminescent protein æquorin, a very sensitive and specific sensor of intracellular calcium mobilization. The screening of 4,800 clones, providing an exhaustive coverage of all of the mutations generated, led to the identification of 16 mutations in sequences encoding the transmembrane domains that were responsible for high sensitivity to CGP42112A. The constitutive activity was confirmed by agonist-independent production of inositol phosphates, which showed that at least half of the clones had significantly increased basal activity. These data demonstrate that this new type of approach is very efficient for the systematic identification of constitutively active mutants of G-protein-coupled receptors.

BACKGROUND INFORMATION: Comprehensive libraries of plasmids for SARS-CoV-2 proteins with various tags (e.g., Strep, HA, Turbo) are now available. They enable the identification of numerous potential protein-protein interactions between the SARS-CoV-2 virus and host proteins. RESULTS: We present here a large library of SARS CoV-2 protein constructs fused with green and red fluorescent proteins and their initial characterisation in various human cell lines including lung epithelial cell models (A549, BEAS-2B), as well as in budding yeast. The localisation of a few SARS-CoV-2 proteins matches their proposed interactions with host proteins. These include the localisation of Nsp13 to the centrosome, Orf3a to late endosomes and Orf9b to mitochondria. CONCLUSIONS AND SIGNIFICANCE: This library should facilitate further cellular investigations, notably by imaging techniques.

The two ubiquitous isoforms of the RAB6 GTPase, RAB6A and RAB6A’, are associated with Golgi membranes and regulate anterograde and retrograde transport pathways from and to the Golgi. In vitro, RAB6A/A’ have been reported to be involved in several cellular functions, including, in addition to transport, cell division, migration, adhesion and polarity. However, their role remains poorly described in vivo, in particular in epithelial tissues. Here, we generated BlgCre; Rab6aF/F mouse presenting a specific deletion of Rab6a in the mammary luminal secretory lineage during gestation and lactation. Rab6a loss severely impaired the differentiation, maturation and maintenance of the secretory tissue, compromising lactation. It led to a decreased activation of STAT5, a key regulator of the lactogenic process primarily governed by prolactin. Additional data obtained with a human mammary epithelial cell line suggested that defective STAT5 activation might originate from a perturbed transport of the prolactin receptor, altering its membrane expression and signaling cascade. Despite the major functional defects observed upon Rab6a deletion, the polarized organization of the mammary epithelial bilayer was preserved. Altogether, our data reveal a crucial role for RAB6A/A’ in the lactogenic function of the mammary gland. They also suggest that trafficking pathways controlled by RAB6A/A’ depend on cell type specialization and tissue context., Les deux isoformes ubiquitaires de la GTPase RAB6, RAB6A et RAB6A’, sont localisées sur les membranes de l’appareil de Golgi et contrôlent des voies de transport antérograde et rétrograde vers et à partir de l’appareil de Golgi. In vitro, RAB6A/A’ ont été impliquées dans plusieurs fonctions cellulaires, incluant, outre les mécanismes du transport, la division, la migration, l'adhésion et la polarité cellulaires. Toutefois, leur rôle reste peu documenté in vivo, notamment dans les tissus épithéliaux. La glande mammaire est constituée d’un réseau de canaux et d’alvéoles qui se développe majoritairement après la naissance, sous contrôle hormonal. La bicouche épithéliale mammaire comprend des cellules myoépithéliales reposant sur une membrane basale et des cellules luminales organisées autour d’une lumière. Au cours de la lactation, les cellules luminales alvéolaires, hautement polarisées, synthétisent et secrètent les composants du lait. Pour étudier le rôle de RAB6A/A’ (RAB6A) dans le développement et la fonction de la glande mammaire, nous avons généré des souris BlgCre; Rab6aF/F présentant une délétion spécifique de RAB6A/A’ dans le lignage luminal sécrétoire au cours de la gestation et de la lactation. L’absence de Rab6a affecte sévèrement la différenciation, la maturation et le maintien du tissu sécrétoire, et compromet la lactation. Elle entraine une diminution de l’activation de STAT5, un régulateur essentiel au processus lactogénique majoritairement contrôlé par la prolactine. Les données obtenues sur une lignée épithéliale mammaire humaine suggèrent que le défaut d’activation de STAT5 a probablement pour origine initiale une perturbation de transport du récepteur à la prolactine, entrainant une réduction de son expression membranaire et de sa cascade de signalisation. Malgré les défauts fonctionnels majeurs observés en l’absence de Rab6a, l’organisation polarisée de la bicouche épithéliale mammaire est conservée. Dans leur ensemble, nos données révèlent un rôle crucial pour RAB6A dans la fonction lactogénique de la glande mammaire. De plus, elles suggèrent que la fonction de RAB6A dépend de la spécialisation cellulaire et du contexte tissulaire où elle s’exerce.

Antigen-presenting cells have the remarkable capacity to transfer exogenous antigens to the cytosol for processing by proteasomes and subsequent presentation on major histocompatibility complex class-I (MHC-I) molecules, a process termed cross-presentation. This is the target of biomedical approaches that aim to trigger a therapeutic immune response. The receptor-binding B-subunit of Shiga toxin (STxB) has been developed as an antigen delivery tool for such immunotherapy applications. In this study, we have analyzed pathways and trafficking factors that are involved in this process. A covalent conjugate between STxB and saporin was generated to quantitatively sample the membrane translocation step to the cytosol in differentiated monocyte-derived THP-1 cells. We have found that retrograde trafficking to the Golgi complex was not required for STxB–saporin translocation to the cytosol or for STxB-dependent antigen cross-presentation. Depletion of endosomal Rab7 inhibited, and lowering membrane cholesterol levels favored STxB–saporin translocation. Interestingly, experiments with reducible and non-reducible linker-arm–STxB conjugates led to the conclusion that after translocation, STxB remains associated with the cytosolic membrane leaflet. In summary, we report new facets of the endosomal escape process bearing relevance to antigen cross-presentation.