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Amajor function of the endocytic system is the sorting of cargo to various organelles. Endocytic sorting of the yeast reductive iron transporter, which is composed of the Fet3 and Ftr1 proteins, is regulated by available iron. When iron is provided to iron-starved cells, Fet3p–Ftr1p is targeted to the lysosome-like vacuole and degraded. In contrast, when iron is not available, Fet3p–Ftr1p is maintained on the plasma membrane via an endocytic recycling pathway requiring the sorting nexin Grd19/Snx3p, the pentameric retromer complex, and the Ypt6p Golgi Rab GTPase module. A recycling signal in Ftr1p was identified and found to bind directly to Grd19/Snx3p. Retromer and Grd19/Snx3p partially colocalize to tubular endosomes, where they are physically associated. After export from the endosome, Fet3p–Ftr1p transits through the Golgi apparatus for resecretion. Thus, Grd19/Snx3p, functions as a cargo-specific adapter for the retromer complex, establishing a precedent for a mechanism by which sorting nexins expand the repertoire of retromer-dependent cargos.

Endocytic sorting of signaling receptors between recycling and degradative pathways is a key cellular process controlling the surface complement of receptors and, accordingly, the cell’s ability to respond to specific extracellular stimuli. The beta-2 adrenergic receptor (β2AR) is a prototypical seven-transmembrane signaling receptor that recycles rapidly and efficiently to the plasma membrane after ligand-induced endocytosis. β2AR recycling is dependent on the receptor’s C-terminal PDZ ligand and Rab41,2. This active sorting process is required for functional resensitization of β2AR-mediated signaling3,4. Here we show that sequence-directed sorting occurs at the level of entry into retromer tubules and that retromer tubules are associated with Rab4. Further, we show that sorting nexin 27 (SNX27) serves as an essential adapter protein linking β2ARs to the retromer tubule. SNX27 does not appear to directly interact with the retromer core complex, but does interact with the retromer associated Wiskott-Aldrich Syndrome Protein and SCAR Homolog (WASH) complex. The present results identify a role for retromer in endocytic trafficking of signaling receptors, in regulating a receptor-linked signaling pathway, and in mediating direct endosome-to-plasma membrane traffic.

Endosomes are the primary organelle where decisions are made as to whether endocytosed proteins will be sorted into degradative trafficking pathways or recycled back to the plasma membrane. This balance between cellular uptake and recycling regulates the plasma membrane composition and is therefore critical for many cellular processes such as nutrient uptake, neuronal transmission and cell migration.1 In addition to its well-known role in membrane trafficking, the endosome is increasingly being recognized as a critical cellular domain for regulated cell signaling. We recently showed that several proteins that regulate endosomal recycling, SNX17, SNX27 and SNX31 are structurally and functionally related.2 These proteins use an unusual FERM domain to bind specific endosomal cargo molecules, and most interestingly, we also found that these proteins use the same FERM domain to associate with the activated Ras small GTPase. Here we speculate on the potential dual role of the PX-FERM proteins in endosomal transport and as scaffolds that may be involved in endosomal Ras signaling processes.

G protein–coupled receptors rely on the PDZ domain of SNX27 for endosomal recycling.

Postsynaptic density 95/discs large/zonus occludens-1 (PDZ) domain–interacting motifs, in addition to their well-established roles in protein scaffolding at the cell surface, are proposed to act as cis-acting determinants directing the molecular sorting of transmembrane cargo from endosomes to the plasma membrane. This hypothesis requires the existence of a specific trans-acting PDZ protein that mediates the proposed sorting operation in the endosome membrane. Here, we show that sorting nexin 27 (SNX27) is required for efficient PDZ-directed recycling of the β2-adrenoreceptor (β2AR) from early endosomes. SNX27 mediates this sorting function when expressed at endogenous levels, and its recycling activity requires both PDZ domain–dependent recognition of the β2AR cytoplasmic tail and Phox homology (PX) domain–dependent association with the endosome membrane. These results identify a discrete role of SNX27 in PDZ-directed recycling of a physiologically important signaling receptor, and extend the concept of cargo-specific molecular sorting in the recycling pathway.

Endocytosed proteins are either delivered to the lysosome to be degraded or are exported from the endosomal system and delivered to other organelles. Sorting of the Saccharomyces cerevisiae reductive iron transporter, composed of the Fet3 and Ftr1 proteins, in the endosomal system is regulated by available iron; in iron-starved cells, Fet3-Ftr1 is sorted by Snx3/Grd19 and retromer into a recycling pathway that delivers it back to the plasma membrane, but when starved cells are exposed to iron, Fet3-Ftr1 is targeted to the lysosome-like vacuole and is degraded. We report that iron-induced endocytosis of Fet3-Ftr1 is independent of Fet3-Ftr1 ubiquitylation, and after endocytosis, degradation of Fet3-Ftr1 is mediated by the multivesicular body (MVB) sorting pathway. In mutant cells lacking any component of the ESCRT protein-dependent MVB sorting machinery, the Rsp5 ubiquitin ligase, or in wild-type cells expressing Fet3-Ftr1 lacking cytosolic lysyl ubiquitin acceptor sites, Fet3-Ftr1 is constitutively sorted into the recycling pathway independent of iron status. In the presence and absence of iron, Fet3-Ftr1 transits an endosomal compartment where a subunit of the MVB sorting receptor (Vps27), Snx3/Grd19, and retromer proteins colocalize. We propose that this endosome is where Rsp5 ubiquitylates Fet3-Ftr1 and where the recycling and degradative pathways diverge.

Summary

Early endosome-to-trans-Golgi network (TGN) transport is organized by the retromer complex. Consisting of cargo-selective and membrane-bound subcomplexes, retromer coordinates sorting with membrane deformation and carrier formation. Here, we describe four mammalian retromers whose membrane-bound subcomplexes contain specific combinations of the sorting nexins (SNX), SNX1, SNX2, SNX5, and SNX6. We establish that retromer requires a dynamic spatial organization of the endosomal network, which is regulated through association of SNX5/SNX6 with the p150glued component of dynactin, an activator of the minus-end directed microtubule motor dynein; an association further defined through genetic studies in C. elegans. Finally, we also establish that the spatial organization of the retromer pathway is mediated through the association of SNX1 with the proposed TGN-localized tether Rab6-interacting protein-1. These interactions describe fundamental steps in retromer-mediated transport and establish that the spatial organization of the retromer network is a critical element required for efficient retromer-mediated sorting.

The functional consequences of signaling receptor endocytosis are determined by the endosomal sorting of receptors between degradation and recycling pathways. How receptors recycle efficiently, in a sequence-dependent manner that is distinct from bulk membrane recycling, is not known. Here, in live cells, we visualize the sorting of a prototypical sequence-dependent recycling receptor, the beta-2 adrenergic receptor, from bulk recycling proteins and the degrading delta-opioid receptor. Our results reveal a remarkable diversity in recycling routes at the level of individual endosomes, and indicate that sequence-dependent recycling is an active process mediated by distinct endosomal sub-domains distinct from those mediating bulk recycling. We identify a specialized subset of tubular microdomains on endosomes, stabilized by a highly localized but dynamic actin machinery, that mediate this sorting, and provide evidence that these actin-stabilized domains provide the physical basis for a two-step kinetic and affinity-based model for protein sorting into the sequence-dependent recycling pathway.

The retromer is a cytosolic/peripheral membrane protein complex that mediates the retrieval of the cation-independent mannose 6-phosphate receptor from endosomes to the trans-Golgi network (TGN) in mammalian cells. Previous studies showed that the mammalian retromer comprises three proteins, named Vps26, Vps29, and Vps35, plus the sorting nexin, SNX1. There is conflicting evidence, however, as to whether a homologous sorting nexin, SNX2, is truly a component of the retromer. In addition, the nature of the subunit interactions and assembly of the mammalian retromer complex are poorly understood. We have addressed these issues by performing biochemical and functional analyses of endogenous retromers in the human cell line HeLa. We found that the mammalian retromer complex consists of two autonomously assembling subcomplexes, namely, a Vps26-Vps29-Vps35 obligate heterotrimer and a SNX1/2 alternative heterodimer or homodimer. The association of Vps26-Vps29-Vps35 with endosomes requires the presence of either SNX1 or SNX2, whereas SNX1/2 can be recruited to endosomes independently of Vps26-Vps29-Vps35. We also found that the presence of either SNX1 or SNX2 is essential for the retrieval of the cation-independent mannose 6-phosphate receptor to the TGN. These observations indicate that the mammalian retromer complex assembles by sequential association of SNX1/2 and Vps26-Vps29-Vps35 subcomplexes on endosomal membranes and that SNX1 and SNX2 play interchangeable but essential roles in retromer structure and function.

We present evidence that ubiquitination controls sorting of the ABC-transporter Ste6 in the early endocytic pathway. The intracellular distribution of Ste6 variants with reduced ubiquitination was examined. In contrast to wild-type Ste6, which was mainly localized to internal structures, these variants accumulated at the cell surface in a polar manner. When endocytic recycling was blocked by Ypt6 inactivation, the ubiquitination deficient variants were trapped inside the cell. This indicates that the polar distribution is maintained dynamically through endocytic recycling and localized exocytosis (“kinetic polarization”). Ste6 does not appear to recycle through late endosomes, because recycling was not blocked in class E vps (vacuolar protein sorting) mutants (Δvps4, Δvps27), which are affected in late endosome function and in the retromer mutant Δvps35. Instead, recycling was partially affected in the sorting nexin mutant Δsnx4, which serves as an indication that Ste6 recycles through early endosomes. Enhanced recycling of wild-type Ste6 was observed in class D vps mutants (Δpep12, Δvps8, and Δvps21). The identification of putative recycling signals in Ste6 suggests that recycling is a signal-mediated process. Endocytic recycling and localized exocytosis could be important for Ste6 polarization during the mating process.

Plasma membrane proteins that are internalized independently of clathrin, such as major histocompatibility complex class I (MHCI), are internalized in vesicles that fuse with the early endosomes containing clathrin-derived cargo. From there, MHCI is either transported to the late endosome for degradation or is recycled back to the plasma membrane via tubular structures that lack clathrin-dependent recycling cargo, e.g., transferrin. Here, we show that the small GTPase Rab22a is associated with these tubular recycling intermediates containing MHCI. Expression of a dominant negative mutant of Rab22a or small interfering RNA-mediated depletion of Rab22a inhibited both formation of the recycling tubules and MHCI recycling. By contrast, cells expressing the constitutively active mutant of Rab22a exhibited prominent recycling tubules and accumulated vesicles at the periphery, but MHCI recycling was still blocked. These results suggest that Rab22a activation is required for tubule formation and Rab22a inactivation for final fusion of recycling membranes with the surface. The trafficking of transferrin was only modestly affected by these treatments. Dominant negative mutant of Rab11a also inhibited recycling of MHCI but not the formation of recycling tubules, suggesting that Rab22a and Rab11a might coordinate different steps of MHCI recycling.

The internalization of essential nutrients, lipids and receptors is a crucial process for all eukaryotic cells. Accordingly, endocytosis is highly conserved across cell types and species. Once internalized, small cargo-containing vesicles fuse with early endosomes (also known as sorting endosomes), where they undergo segregation to distinct membrane regions and are sorted and transported on through the endocytic pathway. Although the mechanisms that regulate this sorting are still poorly understood, some receptors are directed to late endosomes and lysosomes for degradation, whereas other receptors are recycled back to the plasma membrane; either directly or through recycling endosomes. The Rab family of small GTP-binding proteins plays crucial roles in regulating these trafficking pathways. Rabs cycle from inactive GDP-bound cytoplasmic proteins to active GTP-bound membrane-associated proteins, as a consequence of the activity of multiple specific GTPase-activating proteins (GAPs) and GTP exchange factors (GEFs). Once bound to GTP, Rabs interact with a multitude of effector proteins that carry out Rab-specific functions. Recent studies have shown that some of these effectors are also interaction partners for the C-terminal Eps15 homology (EHD) proteins, which are also intimately involved in endocytic regulation. A particularly interesting example of common Rab-EHD interaction partners is the MICAL-like protein, MICAL-L1. MICAL-L1 and its homolog, MICAL-L2, belong to the larger MICAL family of proteins, and both have been directly implicated in regulating endocytic recycling of cell surface receptors and junctional proteins, as well as controlling cytoskeletal rearrangement and neurite outgrowth. In this review, we summarize the functional roles of MICAL and Rab proteins, and focus on the significance of their interactions and the implications for endocytic transport.

Recent evidence suggests that apical and basolateral endocytic pathways in epithelia converge in an apically located, pericentriolar endosomal compartment termed the apical recycling endosome. In this compartment, apically and basolaterally internalized membrane constituents are thought to be sorted for recycling back to their site of origin or for transcytosis to the opposite plasma membrane domain. We report here that in the epithelial cell line Madin–Darby Canine Kidney (MDCK), antibodies to Rab11a label an apical pericentriolar endosomal compartment that is dependent on intact microtubules for its integrity. Furthermore, this compartment is accessible to a membrane-bound marker (dimeric immunoglobulin A [IgA]) internalized from either the apical or basolateral pole, functionally defining it as the apical recycling endosome. We have also examined the role of a closely related epithelial-specific Rab, Rab25, in the regulation of membrane recycling and transcytosis in MDCK cells. When cDNA encoding Rab25 was transfected into MDCK cells, the protein colocalized with Rab11a in subapical vesicles. Rab25 transfection also altered the distribution of Rab11a, causing the coalescence of immunoreactivity into multiple denser vesicular structures not associated with the centrosome. Nevertheless, nocodazole still dispersed these vesicles, and dimeric IgA internalized from either the apical or basolateral membrane was detected in endosomes labeled with antibodies to both Rab11a and Rab25. Overexpression of Rab25 decreased the rate of IgA transcytosis and of apical, but not basolateral, recycling of internalized ligand. Conversely, expression of the dominant-negative Rab25T26N did not alter either apical recycling or transcytosis. These results indicate that both Rab11a and Rab25 associate with the apical recycling system of epithelial cells and suggest that Rab25 may selectively regulate the apical recycling and/or transcytotic pathways.

A systematic analysis of the dimerization, membrane remodelling and higher order assembly properties of all 12 human SNX-BAR sorting nexins reveals how different SNX-BAR combinations allow the formation of distinct tubular subdomains from the same endosomal vacuole during cargo sorting.

Sorting nexins (SNXs) are regulators of endosomal sorting. For the SNX-BAR subgroup, a Bin/Amphiphysin/Rvs (BAR) domain is vital for formation/stabilization of tubular subdomains that mediate cargo recycling. Here, by analysing the in vitro membrane remodelling properties of all 12 human SNX-BARs, we report that some, but not all, can elicit the formation of tubules with diameters that resemble sorting tubules observed in cells. We reveal that SNX-BARs display a restricted pattern of BAR domain-mediated dimerization, and by resolving a 2.8 Å structure of a SNX1-BAR domain homodimer, establish that dimerization is achieved in part through neutralization of charged residues in the hydrophobic BAR-dimerization interface. Membrane remodelling also requires functional amphipathic helices, predicted to be present in all SNX-BARs, and the formation of high order SNX-BAR oligomers through selective ‘tip–loop' interactions. Overall, the restricted and selective nature of these interactions provide a molecular explanation for how distinct SNX-BAR-decorated tubules are nucleated from the same endosomal vacuole, as observed in living cells. Our data provide insight into the molecular mechanism that generates and organizes the tubular endosomal network.

The retromer complex mediates retrograde transport of transmembrane cargo from endosomes to the trans-Golgi network (TGN). Mammalian retromer is composed of a sorting nexin (SNX) dimer that binds to phosphatidylinositol 3-phosphate–enriched endosomal membranes and a vacuolar protein sorting (Vps) 26/29/35 trimer that participates in cargo recognition. The mammalian SNX dimer is necessary but not sufficient for recruitment of the Vps26/29/35 trimer to membranes. In this study, we demonstrate that the guanosine triphosphatase Rab7 contributes to this recruitment. The Vps26/29/35 trimer specifically binds to Rab7–guanosine triphosphate (GTP) and localizes to Rab7-containing endosomal domains. Interference with Rab7 function causes dissociation of the Vps26/29/35 trimer but not the SNX dimer from membranes. This blocks retrieval of mannose 6-phosphate receptors to the TGN and impairs cathepsin D sorting. Rab5-GTP does not bind to the Vps26/29/35 trimer, but perturbation of Rab5 function causes dissociation of both the SNX and Vps26/29/35 components from membranes through inhibition of a pathway involving phosphatidylinositol 3-kinase. These findings demonstrate that Rab5 and Rab7 act in concert to regulate retromer recruitment to endosomes.

Summary

The retromer is a heteropentameric complex that associates with the cytosolic face of endosomes and mediates retrograde transport of transmembrane cargo from endosomes to the trans-Golgi network. The mammalian retromer complex comprises a sorting nexin dimer composed of a still undefined combination of SNX1, SNX2, SNX5 and SNX6, and a cargo-recognition trimer composed of Vps26, Vps29 and Vps35. The SNX subunits contain PX and BAR domains that allow binding to PI(3)P enriched, highly curved membranes of endosomal vesicles and tubules, while Vps26, Vps29 and Vps35 have arrestin, phosphoesterase and α-solenoid folds, respectively. Recent studies have implicated retromer in a broad range of physiological, developmental and pathological processes, underscoring the critical nature of retrograde transport mediated by this complex.

Sorting endosomes and the endocytic recycling compartment are critical intracellular stores for the rapid recycling of internalized membrane receptors to the cell surface in multiple cell types. However, the molecular mechanisms distinguishing fast receptor recycling from sorting endosomes and slow receptor recycling from the endocytic recycling compartment remain poorly understood. We previously reported that Rab15 differentially regulates transferrin receptor trafficking through sorting endosomes and the endocytic recycling compartment, suggesting a role for distinct Rab15-effector interactions at these endocytic compartments. In this study, we identified the novel protein Rab15 effector protein (REP15) as a binding partner for Rab15-GTP. REP15 is compartment specific, colocalizing with Rab15 and Rab11 on the endocytic recycling compartment but not with Rab15, Rab4, or early endosome antigen 1 on sorting endosomes. REP15 interacts directly with Rab15-GTP but not with Rab5 or Rab11. Consistent with its localization, REP15 overexpression and small interfering RNA-mediated depletion inhibited transferrin receptor recycling from the endocytic recycling compartment, without affecting receptor entry into or recycling from sorting endosomes. Our data identify REP15 as a compartment-specific protein for receptor recycling from the endocytic recycling compartment, highlighting that the rapid and slow modes of transferrin receptor recycling are mechanistically distinct pathways.

Retromer complex mediates the sorting of cargo from the endosome to the Golgi apparatus. At the endosome, recognition of Ypt7 (Rab7) by the Vps35 retromer subunit is essential for the cargo export step of the retromer functional cycle. Retromer also controls Ypt7-regulated fusion dynamics of the late endovacuolar system.

The retromer complex, composed of sorting nexin subunits and a Vps26/Vps29/Vps35 trimer, mediates sorting of retrograde cargo from the endosome to the trans-Golgi network. The retromer trimer subcomplex is an effector of Rab7 (Ypt7 in yeast). Whereas endosome targeting of human retromer has been shown to require Rab7-GTP, targeting of yeast retromer to the endosome is independent of Ypt7-GTP and requires the Vps5 and Vps17 retromer sorting nexin subunits. An evolutionarily conserved amino acid segment within Vps35 is required for Ypt7/Rab7 recognition in vivo by both yeast and human retromer, establishing that Rab recognition is a conserved feature of this subunit. Recognition of Ypt7 by retromer is required for its function in retrograde sorting, and in yeast cells lacking the guanine nucleotide exchange factor for Ypt7, retrograde cargo accumulates in endosomes that are decorated with retromer, revealing an additional role for Rab recognition at the cargo export stage of the retromer functional cycle. In addition, yeast retromer trimer antagonizes Ypt7-regulated organelle tethering and fusion of endosomes/vacuoles via recognition of Ypt7. Thus retromer has dual roles in retrograde cargo export and in controlling the fusion dynamics of the late endovacuolar system.

The plasma membrane dopamine transporter (DAT) takes extracellular dopamine back up into dopaminergic neurons. Although the number of DATs at the cell surface is regulated by endocytosis and recycling, the molecular mechanisms that control this endocytic trafficking of DAT are not defined. To map the sequence motifs that are involved in constitutive DAT endocytosis, mutagenesis of human DAT tagged with yellow fluorescent protein (YFP) and an extracellular HA epitope was performed. Removal of the entire amino-terminus of DAT resulted in accumulation of the resulting DAT mutant (YFP-HA-ΔN-DAT) in early and recycling endosomes in HeLa and PAE cells, and in primary rat mesencephalic-striatal neuronal co-cultures. This endosomal accumulation was due to rapid constitutive internalization of YFP-HA-ΔN-DAT by the clathrin-dependent pathway. Small deletions and multi-alanine substitutions in the amino-terminus revealed two molecular determinants within the membrane proximal residues 60–65 that are important for preventing rapid internalization of DAT. First, mutations of Arg60 or Trp63, leading to disruption of the “outward facing” DAT conformation, correlated with an increased pool of mobile DATs in the plasma membrane and accelerated constitutive internalization of the DAT mutants. Second, mutation of Lys65 also correlated with elevated endocytosis. While none of these mutations alone recapitulated the marked endocytic phenotype of YFP-HA-ΔN-DAT, simultaneous elimination of both the outward conformation of DAT and Lys65 resulted in DAT mutants that were rapidly internalized. Thus, our studies reveal a new link between DAT endocytosis and conformation-dependent uptake activity that represents a novel mode for regulating DAT function.

Retrieval of β integrins from the lysosomal degradation pathway mediated by sorting nexin-17 is important for integrin recycling and regulation of cell migration.

The FERM-like domain–containing sorting nexins of the SNX17/SNX27/SNX31 family have been proposed to mediate retrieval of transmembrane proteins from the lysosomal pathway. In this paper, we describe a stable isotope labeling with amino acids in culture–based quantitative proteomic approach that allows an unbiased, global identification of transmembrane cargoes that are rescued from lysosomal degradation by SNX17. This screen revealed that several integrins required SNX17 for their stability, as depletion of SNX17 led to a loss of β1 and β5 integrins and associated a subunits from HeLa cells as a result of increased lysosomal degradation. SNX17 bound to the membrane distal NPXY motif in β integrin cytoplasmic tails, thereby preventing lysosomal degradation of β integrins and their associated a subunits. Furthermore, SNX17-dependent retrieval of integrins did not depend on the retromer complex. Consistent with an effect on integrin recycling, depletion of SNX17 also caused alterations in cell migration. Our data provide mechanistic insight into the retrieval of internalized integrins from the lysosomal degradation pathway, a prerequisite for subsequent recycling of these matrix receptors.

The WASH complex controls actin dynamics on endosomes, and its functional mechanism is poorly defined. The WASH complex subunit Fam21 bears many copies of a novel motif that directly interacts with the retromer cargo-selective complex. Endosomal localization of FAM21 requires both the retromer and multivalency of the repeat elements.

Wiskott–Aldrich syndrome protein (WASPs) control actin dynamics in cellular processes, including cell motility, receptor-mediated endocytosis, bacterial invasion, and vesicular trafficking. We demonstrated that WASH, a recently identified WASP family protein, colocalizes on endosomal subdomains with the cargo-selective complex (CSC) of the retromer, where it regulates retrograde sorting from endosomes in an actin-dependent manner. However, the mechanism of WASH recruitment to these retromer-enriched endosomal subdomains is unclear. Here we show that a component of the WASH regulatory complex (SHRC), FAM21, which contains 21 copies of a novel L-F-[D/E]3-10-L-F motif, directly interacts with the retromer CSC protein VPS35. Endosomal localization of FAM21 is VPS35 dependent and relies on multivalency of FAM21 repeat elements. Using a combination of pull-down assays and isothermal calorimetry, we demonstrate that individual repeats can bind CSC, and binding affinity varies among different FAM21 repeats. A high-affinity repeat can be converted into a low-affinity one by mutation of a hydrophobic residue within the motif. These in vitro data mirror the localization of FAM21 to retromer-coated vesicles in cells. We propose that multivalency enables FAM21 to sense the density of retromer on membranes, allowing coordination of SHRC recruitment, and consequent actin polymerization, with retromer sorting domain organization/maturation.

Although endosomal compartments have been suggested to play a role in unconventional protein secretion, there is scarce experimental evidence for such involvement. Here we report that recycling endosomes are essential for externalization of cytoplasmic secretory protein tissue transglutaminase (tTG). The de novo synthesized cytoplasmic tTG does not follow the classical ER/Golgi-dependent secretion pathway, but is targeted to perinuclear recycling endosomes, and is delivered inside these vesicles prior to externalization. On its route to the cell surface tTG interacts with internalized β1 integrins inside the recycling endosomes and is secreted as a complex with recycled β1 integrins. Inactivation of recycling endosomes, blocking endosome fusion with the plasma membrane, or downregulation of Rab11 GTPase that controls outbound trafficking of perinuclear recycling endosomes, all abrogate tTG secretion. The initial recruitment of cytoplasmic tTG to recycling endosomes and subsequent externalization depend on its binding to phosphoinositides on endosomal membranes. These findings begin to unravel the unconventional mechanism of tTG secretion which utilizes the long loop of endosomal recycling pathway and indicate involvement of endosomal trafficking in non-classical protein secretion.

Low-density lipoprotein receptor–related protein 1 (LRP1) is an endocytic recycling receptor with two cytoplasmic tyrosine-based basolateral sorting signals. Here we show that during biosynthetic trafficking LRP1 uses AP1B adaptor complex to move from a post-TGN recycling endosome (RE) to the basolateral membrane. Then it recycles basolaterally from the basolateral sorting endosome (BSE) involving recognition by sorting nexin 17 (SNX17). In the biosynthetic pathway, Y29 but not N26 from a proximal NPXY directs LRP1 basolateral sorting from the TGN. A N26A mutant revealed that this NPXY motif recognized by SNX17 is required for the receptor's exit from BSE. An endocytic Y63ATL66 motif also functions in basolateral recycling, in concert with an additional endocytic motif (LL86,87), by preventing LRP1 entry into the transcytotic apical pathway. All this sorting information operates similarly in hippocampal neurons to mediate LRP1 somatodendritic distribution regardless of the absence of AP1B in neurons. LRP1 basolateral distribution results then from spatially and temporally segregation steps mediated by recognition of distinct tyrosine-based motifs. We also demonstrate a novel function of SNX17 in basolateral/somatodendritic recycling from a different compartment than AP1B endosomes.

The mammalian retromer is an evolutionally conserved protein complex composed of a vacuolar protein sorting trimer (Vps 26/29/35) that participates in cargo recognition and a sorting nexin (SNX) dimer that binds to endosomal membranes. The retromer plays an important role in efficient retrograde transport for endosome-to-Golgi retrieval of the cation-independent mannose-6-phosphate receptor (CI-MPR), a receptor for lysosomal hydrolases, and other endosomal proteins. This ultimately contributes to the control of cell growth, cell adhesion, and cell migration. The herpesvirus saimiri (HVS) tyrosine kinase-interacting protein (Tip), required for the immortalization of primary T lymphocytes, targets cellular signaling molecules, including Lck tyrosine kinases and the p80 endosomal trafficking protein. Despite the pronounced effects of HVS Tip on T cell signal transduction, the details of its activity on T cell immortalization remain elusive. Here, we report that the amino-terminal conserved, glutamate-rich sequence of Tip specifically interacts with the retromer subunit Vps35 and that this interaction not only causes the redistribution of Vps35 from the early endosome to the lysosome but also drastically inhibits retromer activity, as measured by decreased levels of CI-MPR and lower activities of cellular lysosomal hydrolases. Physiologically, the inhibition of intracellular retromer activity by Tip is ultimately linked to the downregulation of CD4 surface expression and to the efficient in vitro immortalization of primary human T cells to interleukin-2 (IL-2)-independent permanent growth. Therefore, HVS Tip uniquely targets the retromer complex to impair the intracellular trafficking functions of infected cells, ultimately contributing to efficient T cell transformation.

Cell surface proteins are internalized into the cell through endocytosis and either degraded within lysosomes or recycled back to the plasma membrane. While perturbations in endosomal internalization are known to modulate renal function, it is not known whether similar alterations in recycling affect renal function. Rififylin is a known regulator of endocytic recycling with E3 ubiquitin protein ligase activity. In this study, using two genetically similar strains, the Dahl Salt-sensitive rat and an S.LEW congenic strain, which had allelic variants within a < 330 kb segment containing rififylin, we tested the hypothesis that alterations in endosomal recycling affect renal function. The congenic strain had 1.59-fold higher renal expression of rififylin. Transcriptome analysis indicated that components of both endocytosis and recycling were upregulated in the congenic strain. Transcription of Atp1a1 and cell surface content of the protein product of Atp1a1, the alpha subunit of Na+K+ATPase were increased in the proximal tubules from the congenic strain. Because rififylin does not directly regulate endocytosis and it is also a differentially expressed gene within the congenic segment, we reasoned that the observed alterations in the transcriptome of the congenic strain constitute a feedback response to the primary functional alteration of recycling caused by rififylin. To test this, recycling of transferrin was studied in isolated proximal tubules. Recycling was significantly delayed within isolated proximal tubules of the congenic strain, which also had a higher level of polyubiquitinated proteins and proteinuria compared with S. These data provide evidence to suggest that delayed endosomal recycling caused by excess of rififylin indirectly affects endocytosis, enhances intracellular protein polyubiquitination and contributes to proteinuria.

Many endocytosed proteins in yeast travel to the vacuole, but some are recycled to the plasma membrane. We have investigated the recycling of chimeras containing green fluorescent protein (GFP) and the exocytic SNARE Snc1p. GFP-Snc1p moves from the cell surface to internal structures when Golgi function or exocytosis is blocked, suggesting continuous recycling via the Golgi. Internalization is mediated by a conserved cytoplasmic signal, whereas diversion from the vacuolar pathway requires sequences within and adjacent to the transmembrane domain. Delivery from the Golgi to the surface is also influenced by the transmembrane domain, but the requirements are much less specific. Recycling requires the syntaxins Tlg1p and Tlg2p but not Pep12p or proteins such as Vps4p and Vps5p that have been implicated in late endosome–Golgi traffic. Subtle changes to the recycling signal cause GFP-Snc1p to accumulate preferentially in punctate internal structures, although it continues to recycle to the surface. The internal GFP-Snc1p colocalizes with Tlg1p, and immunofluorescence and immunoelectron microscopy reveal structures that contain Tlg1p, Tlg2p, and Kex2p but lack Pep12p and Sec7p. We propose that these represent early endosomes in which sorting of Snc1p and late Golgi proteins occurs, and that transport can occur directly from them to the Golgi apparatus.