Vps26A and Vps26B subunits define distinct retromer complexes

The trimeric Vps29-Vps35-Vps26 sub-complex of retromer mediates retrograde transport of transmembrane proteins from endosomes to the trans-Golgi network. Our group has recently identified a Vps26 paralogue, Vps26B, which is able to suppress the expression of Vps26A when exogenously expressed in mammalian cells and defines a distinct retromer complex (Vps26B-retromer) in vivo and in vitro. In this study, we use HEK293 cells stably expressing either Vps26A-myc or Vps26B-myc to address the role of retromer cargo transport and subcellular localization of the two core retromer complexes as defined by the two mammalian-specific Vps26 paralogues. Vps26B-retromer, like Vps26A-retromer, associates with TBC1D5 and GOLPH3. In contrast, no interaction between Vps26B-retromer and cation-independent mannose 6-phosphate receptor (CI-M6PR) was detected, leading to a degradation of this receptor and an increase in cathepsin D secretion. Colocalization of Vps26 paralogues with different endosomally located Rab proteins shows prolonged association of Vps26B-retromer with maturing endosomes relative to Vps26A-retromer. Interestingly, the cycling of CI-M6PR is restored upon deletion of the variable Vps26B C-terminal region indicating that this region is directly responsible for the differential function of the two paralogues. In summary, we show that the two distinct retromer complexes defined by different Vps26 paralogues are not functionally equivalent and that the Vps26B C-terminal region can control cargo selection of the Vps26B-retromer.     

Structure of Vps26B and mapping of its interaction with the retromer protein complex

Retromer is a heteromeric protein complex with important roles in endosomal membrane trafficking, most notably in the retrograde transport of lysosomal hydrolase receptors from endosomes to the Golgi. The core of retromer is composed of three subunits vacuolar protein sorting (Vps)35, Vps26 and Vps29, and in mammals, there are two paralogues of the medium subunit Vps26A and Vps26B. We find that both Vps26A and Vps26B bind to Vps35/Vps29 with nanomolar affinity and compete for a single-binding site to define distinct retromer complexes in vitro and in vivo. We have determined the crystal structure of mouse Vps26B and compare this structure with that of Vps26A. Vps26 proteins have a striking similarity to the arrestin family of proteins that regulate the signalling and endocytosis of G-protein-coupled receptors, although we observe that surface residues involved in arrestin function are not conserved in Vps26. Using structure-based mutagenesis, we show that both Vps26A and Vps26B are incorporated into retromer complexes through binding of Vps35 to a highly conserved surface patch within the C-terminal subdomain and that this interaction is required for endosomal recruitment of the proteins.     

Dominant-negative behavior of mammalian Vps35 in yeast requires a conserved PRLYL motif involved in retromer assembly

The retromer protein complex assists in recycling selected integral membrane proteins from endosomes to the trans Golgi network. One protein subcomplex (Vps35p, Vps26p and Vps29p) combines with a second (Vps17p and Vps5p) to form a coat involved in sorting and budding of endosomal vesicles. Yeast Vps35p (yVps35) exhibits similarity to human Vps35 (hVps35), especially in a completely conserved PRLYL motif contained within an amino-terminal domain. Companion studies indicate that an R(98)W mutation in yVps35 causes defective retromer assembly in Saccharomyces cerevisiae. Herein, we find that the expression of hVps35 in yeast confers dominant-negative vacuolar proenzyme secretion and defective secretory proprotein processing. The mutant phenotype appears to be driven by hVps35 competing with endogenous yVps35, becoming incorporated into defective retromer complexes and causing proteasomal degradation of endogenous Vps26 and Vps29. Increased expression of yVps35 displaces some hVps35 to a 100 000 x g supernatant and suppresses the dominant-negative phenotype. Remarkably, mutation of the conserved R(107)W of hVps35 displaces some of the protein to the 100 000 x g supernatant, slows protein turnover and restores stability of Vps26p and Vps29p and completely abrogates dominant-negative trafficking behavior. We show that hVps35 coprecipitates Vps26, whereas the R(107)W mutant does not. In pancreatic beta cells, the R(107)W mutant shifts hVps35 from peripheral endosomes to a juxtanuclear compartment, affecting both mannose phosphate receptors and insulin. These data underscore importance of the Vps35 PRLYL motif in retromer subcomplex interactions and function.     

Recent advances in retromer biology

The endosomal network is an organized array of intracellular, membranous compartments that function as sorting sites for endosomal and biosynthetic cargo. The fate of endocytic cargo is reliant upon interactions with a number of molecularly distinct sorting complexes, which tightly control the relationship between sorting of their respective cargo and the physical process of membrane re-scuplturing required for the formation of transport carries. One such complex, retromer, mediates retrograde transport from endosomes to the trans-Golgi network (TGN). Disregulation of retromer has been implicated in a host of disease states including late-onset Alzheimer's. Rather than give a broad overview of retromer biology, here we aim to outline the recent advances in understanding this complex, focussing on the involvement of both clathrin and the cytoskeleton in retromer function.     

The structure and function of the retromer protein complex

The transport of transmembrane proteins and associated ligands through the endosomal system is governed by a number of different protein assemblies. One such assembly is retromer, a peripheral membrane protein complex that has important roles in endosomal sorting of a variety of cargo molecules. Retromer was first shown to control the endosome-to-Golgi retrieval of lysosomal hydrolase receptors, and over the past few years, it has been found to play a similar role in the transport of many other proteins in all eukaryotes from simple amoeba to plants and mammals. Recent structural studies of the core retromer complex have revealed both unexpected similarities and intriguing differences between retromer and other regulators of membrane trafficking and are beginning to open the door to a mechanistic understanding of retromer-mediated endosomal transport.     

Sorting nexins--unifying trends and new perspectives

The sorting nexins (SNXs) are a family of PX domain-containing proteins found in yeast and mammalian cells that have been proposed to regulate intracellular trafficking. Mammalian SNXs have been suggested to function variously in pro-degradative sorting, internalization, endosomal recycling, or simply in endosomal sorting. In yeast, the defining function for these proteins is a regulation of cargo retrieval. Here we examine recent data on the SNX family of proteins and attempt to draw out unifying themes between the work performed in yeast and mammalian systems.     

Rabankyrin-5 interacts with EHD1 and Vps26 to regulate endocytic trafficking and retromer function

Rabankyrin-5 (Rank-5) has been implicated as an effector of the small GTPase Rab5 and plays an important role in macropinocytosis. We have now identified Rank-5 as an interaction partner for the recycling regulatory protein, Eps15 homology domain 1 (EHD1). We have demonstrated this interaction by glutathione S-transferase-pulldown, yeast two-hybrid assay, isothermal calorimetry and co-immunoprecipitation, and found that the binding occurs between the EH domain of EHD1 and the NPFED motif of Rank-5. Similar to EHD1, we found that Rank-5 colocalizes and interacts with components of the retromer complex such as vacuolar protein sorting 26 (Vps26), suggesting a role for Rank-5 in retromer-based transport. Indeed, depletion of Rank-5 causes mislocalization of Vps26 and affects both the retrieval of mannose 6-phosphate receptor transport to the Golgi from endosomes and biosynthetic transport. Moreover, Rank-5 is required for normal retromer distribution, as overexpression of a wild-type Rank-5-small interfering RNA-resistant construct rescues retromer mislocalization. Finally, we show that depletion of either Rank-5 or EHD1 impairs secretion of vesicular stomatitis virus glycoprotein. Overall, our data identify a new interaction between Rank-5 and EHD1, and novel endocytic regulatory roles that include retromer-based transport and secretion.     

Endosomal receptor trafficking: Retromer and beyond

The tubular endolysosomal network is a quality control system that ensures the proper delivery of internalized receptors to specific subcellular destinations in order to maintain cellular homeostasis. Although retromer was originally described in yeast as a regulator of endosome‐to‐Golgi receptor recycling, mammalian retromer has emerged as a central player in endosome‐to‐plasma membrane recycling of a variety of receptors. Over the past decade, information regarding the mechanism by which retromer facilitates receptor trafficking has emerged, as has the identification of numerous retromer‐associated molecules including the WASH complex, sorting nexins (SNXs) and TBC1d5. Moreover, the recent demonstration that several SNXs can directly interact with retromer cargo to facilitate endosome‐to‐Golgi retrieval has provided new insight into how these receptors are trafficked in cells. The mechanism by which SNX17 cargoes are recycled out of the endosomal system was demonstrated to involve a retromer‐like complex termed the retriever, which is recruited to WASH positive endosomes through an interaction with the COMMD/CCDC22/CCDC93 (CCC) complex. Lastly, the mechanisms by which bacterial and viral pathogens highjack this complex sorting machinery in order to escape the endolysosomal system or remain hidden within the cells are beginning to emerge. In this review, we will highlight recent studies that have begun to unravel the intricacies by which the retromer and associated molecules contribute to receptor trafficking and how deregulation at this sorting domain can contribute to disease or facilitate pathogen infection.      

Functions of sorting nexin 17 domains and recognition motif for P-selectin trafficking

SNX17 is a member of the sorting nexin family (SNX), a group of hydrophilic proteins whose common characteristic property is a phox homology (PX) domain. The PX domain directs SNXs to phosphatidylinositides containing membranes of the endosomal compartment, where the SNXs are involved in the sorting of transmembrane proteins. SNX17 is known to interact with P-selectin and the LDL receptor family. Here, we report that the PX domain of SNX17 specifically binds to phosphatidylinositol 3-phosphate-containing membranes. The functional part of SNX17 that binds P-selectin or Patched (PTCH) consists of a truncated FERM domain and a unique C terminus together (FC-unit). In a yeast two-hybrid analysis a putative recognition motif for the FC-unit was revealed within P-selectin as FxNaa(F/Y). When HepG2 cells overexpress P-selectin together with SNX17, SNX17 changes its distribution from early endosomes to lysobisphosphatidic acid-containing late endosomes. Furthermore, overexpressed SNX17 restrains P-selectin in the outer membrane of the late endosomal compartment, thus preventing the normal lysosomal accumulation of P-selectin. These results suggest that the PX domain is necessary for the intracellular localisation, while the FC-unit is required for cargo recognition. We hypothesise that the expression level of SNX17 may regulate the lysosomal degradation, at least for P-selectin, by suppressing its entry into the inner vesicles of the multi-vesicular bodies (MVBs).     

Vps26p, a component of retromer, directs the interactions of Vps35p in endosome-to-Golgi retrieval

Endosome-to-Golgi retrieval of the carboxypeptidase Y receptor Vps10p is mediated by a recently discovered membrane coat complex termed retromer. Retromer comprises five conserved proteins: Vps35p, Vps29p, Vps5p, Vps17p, and Vps26p. Vps35p recognizes cargo molecules such as Vps10p and interacts strongly with Vps29p. Vps5p forms a subcomplex with Vps17p and has been proposed to play a structural role by self-assembling into large multimeric structures. The function of Vps26p is currently unknown. We have investigated the role that Vps26p plays in retromer-mediated endosome-to-Golgi transport by analyzing dominant negative alleles of Vps26p. These mutants have identified a crucial region of Vps26p that plays an important role in its function. Functional domains of Vps26p have been investigated by the creation of yeast-mouse hybrid molecules in which domains of Vps26p have been replaced by the similar domain in the protein encoded by the mouse VPS26 gene, Hbeta58. These domain swap experiments have shown that Vps26p promotes the interactions between the cargo-selective component Vps35p and the structural components Vps5p/Vps17p.     

Phospholipase D and retromer promote recycling of TRPL ion channel via the endoplasmic reticulum

Plasma membrane protein trafficking is of fundamental importance for cell function and cell integrity of neurons and includes regulated protein recycling. In this work, we report a novel role of the endoplasmic reticulum (ER) for protein recycling as discovered in trafficking studies of the ion channel TRPL in photoreceptor cells of Drosophila. TRPL is located within the rhabdomeric membrane from where it is endocytosed upon light stimulation and stored in the cell body. Conventional immunohistochemistry as well as stimulated emission depletion super-resolution microscopy revealed TRPL storage at the ER after illumination, suggesting an unusual recycling route of TRPL. Our results also imply that both phospholipase D (PLD) and retromer complex are required for correct recycling of TRPL to the rhabdomeric membrane. Loss of PLD activity in PLD^3.1 mutants results in enhanced degradation of TRPL. In the retromer mutant vps35^MH20, TRPL is trapped in a Rab5-positive compartment. Evidenced by epistatic analysis in the double mutant PLD^3.1 vps35^MH20, PLD activity precedes retromer function. We propose a model in which PLD and retromer function play key roles in the transport of TRPL to an ER enriched compartment.     

Regulation of P2Y1 Receptor Traffic by Sorting Nexin 1 is Retromer Independent

The activity and traffic of G‐protein coupled receptors (GPCRs) is tightly controlled. Recent work from our laboratory has shown that P2Y₁ and P2Y₁₂ responsiveness is rapidly and reversibly modulated in human platelets and that the underlying mechanism requires receptor trafficking as an essential part of this process. However, little is known about the molecular mechanisms underlying P2Y receptor traffic. Sorting nexin 1 (SNX1) has been shown to regulate the endosomal sorting of cell surface receptors either to lysosomes where they are downregulated or back to the cell surface. These functions may in part be due to interactions of SNX1 with the mammalian retromer complex. In this study, we investigated the role of SNX1 in P2Y receptor trafficking. We show that P2Y₁ receptors recycle via a slow recycling pathway that is regulated by SNX1, whereas P2Y₁₂ receptors return to the cell surface via a rapid route that is SNX1 independent. SNX1 inhibition caused a dramatic increase in the rate of P2Y₁ receptor recycling, whereas inhibition of Vps26 and Vps35 known to be present in retromer had no effect, indicating that SNX1 regulation of P2Y₁ receptor recycling is retromer independent. In addition, inhibition of SNX4, 6 and 17 proteins did not affect P2Y₁ receptor recycling. SNX1 has also been implicated in GPCR degradation; however, we provide evidence that P2Y receptor degradation is SNX1 independent. These data describe a novel function of SNX1 in the regulation of P2Y₁ receptor recycling and suggest that SNX1 plays multiple roles in endocytic trafficking of GPCRs.     

Overexpression of a novel sorting nexin, SNX15, affects endosome morphology and protein trafficking

Sorting nexin (SNX) 15 is a novel member of the SNX family of proteins. Although the functions of most SNXs have not yet been determined, several family members (e.g., SNX1, SNX2, SNX3, and SNX8) are orthologs of yeast proteins involved in protein trafficking. Overexpression of myc-tagged SNX15 in COS-7 cells altered the morphology of several endosomal compartments. In transient transfection experiments, myc-SNX15 was first seen in small punctate spots and small ring structures. Later, myc-SNX15 was found in larger rings. Finally, myc-SNX15 was observed in large, amorphous membrane-limited structures. These structures contained proteins from lysosomes, late endosomes, early endosomes, and the trans-Golgi network. However, the morphology of the endoplasmic reticulum and Golgi was not affected by overexpression of myc-SNX15. In myc-SNX15-overexpressing cells, the endocytosis of transferrin was severely inhibited and endocytosis of tac-trans-Golgi network (TGN) 38 and tac-furin was slowed. In addition, the recycling of internalized tac-TGN38 and tac-furin was also inhibited. Both the morphological and biochemical data indicate that SNX15 plays a crucial role in trafficking through the endocytic pathway. This is the first demonstration that a mammalian SNX protein is involved in protein trafficking.     

Implication of mouse Vps26b-Vps29-Vps35 retromer complex in sortilin trafficking

The retromer complex, which mediates retrograde transport from endosomes to the trans-Golgi network, is a heteropentameric complex that contains a multifunctional cargo recognition heterotrimer consisted of the vacuolar protein sorting (Vps) subunits Vps26, Vps29, and Vps35. In mammals, there are two different isoforms of Vps26, Vps26a and Vps26b, that localize to the endosome, and to the plasma membrane, respectively. To elucidate the biological significance of the Vps26b isoform, we generated Vps26b knockout mice and studied their molecular, histological, and behavioral phenotypes. We found that the loss of Vps26b results in no significant defects in the behavior, body size, and health of the mice. Vps26b-deficient mice showed a severe reduction of Vps35 protein at cellular level and lacked the Vps26b-Vps29-Vps35 retromer complex, despite the normal presence of the Vps26a-Vps29-Vps35 retromer complex. Relatively, the amount of sortilin was increased approximately 20% in the Vps26b-deficient mice, whereas the sorLA was normal. These results suggest that mouse Vps26b-Vps29-Vps35 retromer complex is implicated in the transport of sortilin from endosomes to the trans-Golgi network (TGN).     

A Novel,Retromer‐Independent Role for Sorting Nexins 1 and 2 in RhoG‐Dependent Membrane Remodeling

The sorting nexins SNX1 and SNX2 are members of the retromer complex involved in protein sorting within the endocytic pathway. While retromer‐dependent functions of SNX1 and SNX2 have been well documented, potential retromer‐independent roles remain unclear. Here, we show that SNX1 and SNX2 interact with the Rac1 and RhoG guanine nucleotide exchange factor Kalirin‐7. Simultaneous overexpression of SNX1 or SNX2 and Kalirin‐7 in epithelial cells causes partial redistribution of both SNX isoforms to the plasma membrane, and results in RhoG‐dependent lamellipodia formation that requires functional Phox homology (PX) and Bin/Amphiphysin/Rvs (BAR) domains of SNX, but is Rac1‐ and retromer‐independent. Conversely, depletion of endogenous SNX1 or SNX2 inhibits Kalirin‐7‐mediated lamellipodia formation. Finally, we demonstrate that SNX1 and SNX2 interact directly with inactive RhoG, suggesting a novel role for these SNX proteins in recruiting an inactive Rho GTPase to its exchange factor.     

Effects of deficiencies of STAMs and Hrs,mammalian class E Vps proteins,on receptor downregulation

The STAM family proteins, STAM1 and STAM2/EAST/Hbp, are phosphotyrosine proteins that contain SH3 domains and ubiquitin-interacting motifs. Their yeast homologue, Hse1, and its binding protein, Vps27, are involved in the vacuolar membrane transport machinery. Here we show that STAM1 and STAM2 are localized to the endosomal membrane. Some of these complexes contain Eps15, an endocytic protein, which accumulates in clumps upon expression of a dominant-negative form of Vps4-A, an AAA-type ATPase, that is required for normal endosome function. These results support the idea that the STAMs are mammalian vacuolar protein sorting (Vps) proteins. We also demonstrate that ligand-mediated epidermal growth factor receptor (EGFR) degradation is partially but not completely impaired in both Hrs(-/-) and STAM1(-/-)STAM2(-/-) mouse embryonic fibroblasts. Furthermore, endosome swelling is seen in both Hrs(-/-) and STAM1(-/-)STAM2(-/-) cells. These results suggest that the STAMs and Hrs play important roles in the mammalian endosomal/vacuolar protein sorting pathway.     

The Microtubule-Associated Protein CLASP Sustains Cell Proliferation through a Brassinosteroid Signaling Negative Feedback Loop

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Drosophila ATP6AP2/VhaPRR functions both as a novel planar cell polarity core protein and a regulator of endosomal trafficking

Planar cell polarity (PCP) controls the orientation of cells within tissues and the polarized outgrowth of cellular appendages. So far, six PCP core proteins including the transmembrane proteins Frizzled (Fz), Strabismus (Stbm) and Flamingo (Fmi) have been identified. These proteins form asymmetric PCP domains at apical junctions of epithelial cells. Here, we demonstrate that VhaPRR, an accessory subunit of the proton pump V‐ATPase, directly interacts with the protocadherin Fmi through its extracellular domain. It also shows a striking co‐localization with PCP proteins during all pupal wing stages in Drosophila. This localization depends on intact PCP domains. Reversely, VhaPRR is required for stable PCP domains, identifying it as a novel PCP core protein. VhaPRR performs an additional role in vesicular acidification as well as endolysosomal sorting and degradation. Membrane proteins, such as E‐Cadherin and the Notch receptor, accumulate at the surface and in intracellular vesicles of cells mutant for VhaPRR. This trafficking defect is shared by other V‐ATPase subunits. By contrast, the V‐ATPase does not seem to have a direct role in PCP regulation. Together, our results suggest two roles for VhaPRR, one for PCP and another in endosomal trafficking. This dual function establishes VhaPRR as a key factor in epithelial morphogenesis.