Lysosome biogenesis requires Rab9 function and receptor recycling from endosomes to the trans-Golgi network

Shiga toxins: intracellular trafficking to the ER leading to activation of host cell stress responses

Despite efforts to improve hygenic conditions and regulate food and drinking water safety, the enteric pathogens, Shiga toxin-producing Escherichia coli (STEC) and Shigella dysenteriae serotype 1 remain major public health concerns due to widespread outbreaks and the severity of extra-intestinal diseases they cause, including acute renal failure and central nervous system complications. Shiga toxins are the key virulence factors expressed by these pathogens mediating extra-intestinal disease. Delivery of the toxins to the endoplasmic reticulum (ER) results in host cell protein synthesis inhibition, activation of the ribotoxic stress response, the ER stress response, and in some cases, the induction of apoptosis. Intrinsic and/or extrinsic apoptosis inducing pathways are involved in executing cell death following intoxication. In this review we provide an overview of the current understanding Shiga toxin intracellular trafficking, host cellular responses to the toxin and ER stress-induced apoptosis with an emphasis on recent findings.

Rab7b controls trafficking from endosomes to the TGN

Rab7b is a recently identified member of the Rab GTPase protein family and has high similarity to Rab7. It has been reported that Rab7b is lysosome associated, that it is involved in monocytic differentiation and that it promotes lysosomal degradation of TLR4 and TLR9. Here we investigated further the localization and function of this GTPase. We found that wild-type Rab7b is lysosome associated whereas an activated, GTP-bound form of Rab7b localizes to the Golgi apparatus. In contrast to Rab7, Rab7b is not involved in EGF and EGFR degradation. Depletion of Rab7b or expression of Rab7b T22N, a Rab7b dominant-negative mutant, impairs cathepsin-D maturation and causes increased secretion of hexosaminidase. Moreover, expression of Rab7b T22N or depletion of Rab7b alters TGN46 distribution, cation-independent mannose-6-phosphate receptor (CI-MPR) trafficking, and causes an increase in the levels of the late endosomal markers CI-MPR and cathepsin D. Vesicular stomatitis virus G protein (VSV-G) trafficking, by contrast, is normal in Rab7b-depleted or Rab7b-T22N-expressing cells. In addition, depletion of Rab7b prevents cholera toxin B-subunit from reaching the Golgi. Altogether, these data indicate that Rab7b is required for normal lysosome function, and, in particular, that it is an essential factor for retrograde transport from endosomes to the trans-Golgi network (TGN).

The clathrin heavy chain isoform CHC22 functions in a novel endosomal sorting step

Clathrin heavy chain 22 (CHC22) is an isoform of the well-characterized CHC17 clathrin heavy chain, a coat component of vesicles that mediate endocytosis and organelle biogenesis. CHC22 has a distinct role from CHC17 in trafficking glucose transporter 4 (GLUT4) in skeletal muscle and fat, though its transfection into HEK293 cells suggests functional redundancy. Here, we show that CHC22 is eightfold less abundant than CHC17 in muscle, other cell types have variably lower amounts of CHC22, and endogenous CHC22 and CHC17 function independently in nonmuscle and muscle cells. CHC22 was required for retrograde trafficking of certain cargo molecules from endosomes to the trans-Golgi network (TGN), defining a novel endosomal-sorting step distinguishable from that mediated by CHC17 and retromer. In muscle cells, depletion of syntaxin 10 as well as CHC22 affected GLUT4 targeting, establishing retrograde endosome-TGN transport as critical for GLUT4 trafficking. Like CHC22, syntaxin 10 is not expressed in mice but is present in humans and other vertebrates, implicating two species-restricted endosomal traffic proteins in GLUT4 transport.

[Sorting, recycling and WNT signaling: Wntless and retromer functions]

Cell-cell signaling is essential for the development of multi-cellular organisms. Indeed, membrane traffic is required for the correct sorting and function of receptors and ligands. In the past decades, many genetic screens performed in C. elegans and Drosophila have been crucial to identify the role of intracellular traffic in signaling. In this review, we discuss recent work that led to the identification of Wntless, a sorting receptor for WNT, and of the retromer, a protein complex required for the recycling of Wntless from endosomes to the trans-Golgi network.

Patched regulates Smoothened trafficking using lipoprotein-derived lipids

Hedgehog (Hh) is a lipoprotein-borne ligand that regulates both patterning and proliferation in a wide variety of vertebrate and invertebrate tissues. When Hh is absent, its receptor Patched (Ptc) represses Smoothened (Smo) signaling by an unknown catalytic mechanism that correlates with reduced Smo levels on the basolateral membrane. Ptc contains a sterol-sensing domain and is similar to the Niemann-Pick type C-1 protein, suggesting that Ptc might regulate lipid trafficking to repress Smo. However, no endogenous lipid regulators of Smo have yet been identified, nor has it ever been shown that Ptc actually controls lipid trafficking. Here, we show that Drosophila Ptc recruits internalized lipoproteins to Ptc-positive endosomes and that its sterol-sensing domain regulates trafficking of both lipids and Smo from this compartment. Ptc utilizes lipids derived from lipoproteins to destabilize Smo on the basolateral membrane. We propose that Ptc normally regulates Smo degradation by changing the lipid composition of endosomes through which Smo passes, and that the presence of Hh on lipoproteins inhibits utilization of their lipids by Ptc.

On the fate of early endosomes

Proteins are endocytosed by various pathways into the cell. All these pathways converge at the level of the early endosome. The fate of the early endosome and how proteins are sorted into recycling and late endosomes/multi-vesicular body is a matter of debate and intense research. Obviously, the transition from early to late endosome poses an interesting logistic problem and would merit attention on an intellectual level. Numerous diseases are also caused by defects in turning off/over signaling molecules or mis-sorting of proteins at the level of the early endosome. This brief review aims to discuss different molecular mechanisms whereby early-to-late endosome transition could be achieved.

Multiple routes of protein transport from endosomes to the trans Golgi network

Proteins use multiple routes for transport from endosomes to the Golgi complex. Shiga and cholera toxins and TGN38/46 are routed from early and recycling endosomes, while mannose 6-phosphate receptors are routed from late endosomes. The identification of distinct molecular requirements for each of these pathways makes it clear that mammalian cells have evolved more complex targeting mechanisms and routes than previously anticipated.

Discovery of Atg5/Atg7-independent alternative macroautophagy

Macroautophagy is a process that leads to the bulk degradation of subcellular constituents by producing autophagosomes/autolysosomes. It is believed that Atg5 (ref. 4) and Atg7 (ref. 5) are essential genes for mammalian macroautophagy. Here we show, however, that mouse cells lacking Atg5 or Atg7 can still form autophagosomes/autolysosomes and perform autophagy-mediated protein degradation when subjected to certain stressors. Although lipidation of the microtubule-associated protein light chain 3 (LC3, also known as Map1lc3a) to form LC3-II is generally considered to be a good indicator of macroautophagy, it did not occur during the Atg5/Atg7-independent alternative process of macroautophagy. We also found that this alternative process of macroautophagy was regulated by several autophagic proteins, including Unc-51-like kinase 1 (Ulk1) and beclin 1. Unlike conventional macroautophagy, autophagosomes seemed to be generated in a Rab9-dependent manner by the fusion of isolation membranes with vesicles derived from the trans-Golgi and late endosomes. In vivo, Atg5-independent alternative macroautophagy was detected in several embryonic tissues. It also had a function in clearing mitochondria during erythroid maturation. These results indicate that mammalian macroautophagy can occur through at least two different pathways: an Atg5/Atg7-dependent conventional pathway and an Atg5/Atg7-independent alternative pathway.

Membrane recruitment of the cargo-selective retromer subcomplex is catalysed by the small GTPase Rab7 and inhibited by the Rab-GAP TBC1D5

Retromer is a membrane-associated heteropentameric coat complex that functions in the endosome-to-Golgi retrieval of the cation-independent mannose-6-phosphate receptor, the Wntless protein and other membrane proteins of physiological significance. Retromer comprises two functional subcomplexes: the cargo-selective subcomplex is a trimer of the VPS35, VPS29, VPS26 proteins, whereas the sorting nexin proteins, Snx1 and Snx2 function to tubulate the endosomal membrane. Unlike the sorting nexins, which contain PtdIns3P-binding PX domains, the cargo-selective VPS35/29/26 complex has no lipid-binding domains and its recruitment to the endosomal membrane remains mechanistically uncharacterised. In this study we show that the VPS35/29/26 complex interacts with the small GTPase Rab7 and requires Rab7 for its recruitment to the endosome. We show that the Rab7K157N mutant that causes the peripheral neuropathy, Charcot-Marie-Tooth disease, does not interact with the VPS35/29/26 complex, resulting in a weakened association with the membrane. We have also identified a novel retromer-interacting protein, TBC1D5, which is a member of the Rab GAP family of proteins that negatively regulates VPS35/29/26 recruitment and causes Rab7 to dissociate from the membrane. We therefore propose that recruitment of the cargo-selective VPS35/29/26 complex is catalysed by Rab7 and inhibited by the Rab-GAP protein, TBC1D5.

Bric-a-brac at the Golgi

Until now, Rho proteins were known as GTPases involved in cell polarity and morphogenesis. In a recent issue of Cell, Espinosa and coworkers show that RhoBTB3, a member of this family, is an ATPase involved in endosome-to-Golgi transport.

RhoBTB3: a Rho GTPase-family ATPase required for endosome to Golgi transport

Rho GTPases are key regulators of the actin-based cytoskeleton; Rab GTPases are key regulators of membrane traffic. We report here that the atypical Rho GTPase family member, RhoBTB3, binds directly to Rab9 GTPase and functions with Rab9 in protein transport from endosomes to the trans Golgi network. Gene replacement experiments show that RhoBTB3 function in cultured cells requires both RhoBTB3's N-terminal, Rho-related domain and C-terminal sequences that are important for Rab9 interaction. Biochemical analysis reveals that RhoBTB3 binds and hydrolyzes ATP rather than GTP. Rab9 binding opens the autoinhibited RhoBTB3 protein to permit maximal ATP hydrolysis. Because RhoBTB3 interacts with TIP47 on membranes, we propose that it may function to release this cargo selection protein from vesicles to permit their efficient docking and fusion at the Golgi.

Deficiency of niemann-pick type C-1 protein impairs release of human immunodeficiency virus type 1 and results in Gag accumulation in late endosomal/lysosomal compartments

Human immunodeficiency virus type 1 (HIV-1) relies on cholesterol-laden lipid raft membrane microdomains for entry into and egress out of susceptible cells. In the present study, we examine the need for intracellular cholesterol trafficking pathways with respect to HIV-1 biogenesis using Niemann-Pick type C-1 (NPC1)-deficient (NPCD) cells, wherein these pathways are severely compromised, causing massive accumulation of cholesterol in late endosomal/lysosomal (LE/L) compartments. We have found that induction of an NPC disease-like phenotype through treatment of various cell types with the commonly used hydrophobic amine drug U18666A resulted in profound suppression of HIV-1 release. Further, NPCD Epstein-Barr virus-transformed B lymphocytes and fibroblasts from patients with NPC disease infected with a CD4-independent strain of HIV-1 or transfected with an HIV-1 proviral clone, respectively, replicated HIV-1 poorly compared to normal cells. Infection of the NPCD fibroblasts with a vesicular stomatitis virus G-pseudotyped strain of HIV-1 produced similar results, suggesting a postentry block to HIV-1 replication in these cells. Examination of these cells using confocal microscopy showed an accumulation and stabilization of Gag in LE/L compartments. Additionally, normal HIV-1 production could be restored in NPCD cells upon expression of a functional NPC1 protein, and overexpression of NPC1 increased HIV-1 release. Taken together, our findings demonstrate that intact intracellular cholesterol trafficking pathways mediated by NPC1 are needed for efficient HIV-1 production.

Phylogeny and evolution of Rab7 and Rab9 proteins

Background

An important role in the evolution of intracellular trafficking machinery in eukaryotes played small GTPases belonging to the Rab family known as pivotal regulators of vesicle docking, fusion and transport. The Rab family is very diversified and divided into several specialized subfamilies. We focused on the VII functional group comprising Rab7 and Rab9, two related subfamilies, and analysed 210 sequences of these proteins. Rab7 regulates traffic from early to late endosomes and from late endosome to vacuole/lysosome, whereas Rab9 participates in transport from late endosomes to the trans-Golgi network.

Results

Although Rab7 and Rab9 proteins are quite small and show heterogeneous rates of substitution in different lineages, we found a phylogenetic signal and inferred evolutionary relationships between them. Rab7 proteins evolved before radiation of main eukaryotic supergroups while Rab9 GTPases diverged from Rab7 before split of choanoflagellates and metazoans. Additional duplication of Rab9 and Rab7 proteins resulting in several isoforms occurred in the early evolution of vertebrates and next in teleost fishes and tetrapods. Three Rab7 lineages emerged before divergence of monocots and eudicots and subsequent duplications of Rab7 genes occurred in particular angiosperm clades. Interestingly, several Rab7 copies were identified in some representatives of excavates, ciliates and amoebozoans. The presence of many Rab copies is correlated with significant differences in their expression level. The diversification of analysed Rab subfamilies is also manifested by non-conserved sequences and structural features, many of which are involved in the interaction with regulators and effectors. Individual sites discriminating different subgroups of Rab7 and Rab9 GTPases have been identified.

The integral membrane of lysosomes: its proteins and their roles in disease

The protein composition of the integral lysosomal membrane and the membrane-associated compartment have been defined in part by proteomics approaches. While the role of its constituent hydrolases in a large array of human disorders has been well-documented, the manner in which membrane proteins are integrated into the organelle, the multiprotein complexes that form at the organelle's cytosolic surface and their roles in the biogenesis and functional control of the organelle are now emerging. Defining cytosolic targeting complexes that affect the function of the lysosomal/endosomal compartment may help to identify the lysosome's role in a variety of human pathologies.

Sorting of lysosomal proteins

Lysosomes are composed of soluble and transmembrane proteins that are targeted to lysosomes in a signal-dependent manner. The majority of soluble acid hydrolases are modified with mannose 6-phosphate (M6P) residues, allowing their recognition by M6P receptors in the Golgi complex and ensuing transport to the endosomal/lysosomal system. Other soluble enzymes and non-enzymatic proteins are transported to lysosomes in an M6P-independent manner mediated by alternative receptors such as the lysosomal integral membrane protein LIMP-2 or sortilin. Sorting of cargo receptors and lysosomal transmembrane proteins requires sorting signals present in their cytosolic domains. These signals include dileucine-based motifs, DXXLL or [DE]XXXL[LI], and tyrosine-based motifs, YXXØ, which interact with components of clathrin coats such as GGAs or adaptor protein complexes. In addition, phosphorylation and lipid modifications regulate signal recognition and trafficking of lysosomal membrane proteins. The complex interaction of both luminal and cytosolic signals with recognition proteins guarantees the specific and directed transport of proteins to lysosomes.

Identification of rab20 as a potential regulator of connexin 43 trafficking

Connexin oligomerization and trafficking are regulated processes. To identify proteins that control connexin 43 (Cx43), a screen was designed using HeLa cells expressing a Cx43 construct with di-lysine endoplasmic reticulum (ER)-retention/retrieval motif, Cx43-HKKSL. At moderate levels of expression, Cx43-HKKSL is retained in the ER as monomers; however, Cx43-HKKSL stably overexpressed by HeLa cells localizes to the perinuclear region and oligomerizes. HeLa/Cx43-HKKSL overexpressors were transiently transfected with pooled clones from a human kidney cDNA library and used immunofluorescence microscopy to identify cDNAs that enabled overexpressed Cx43-HKKSL to convert from a perinuclear to ER localization pattern. Using this approach, a small molecular weight GTPase, rab20, was identified as a candidate protein with the ability to regulate Cx43 trafficking. Enhanced green fluorescent protein (EGFP)-tagged rab20 showed a predominantly perinuclear and ER localization pattern and caused wild-type Cx43 to be retained inside the cell. By contrast, mutant EGFP-rab20T19N, which lacks the ability to bind GTP, had no effect on Cx43. These results suggest Cx43 is transported through an intracellular compartment regulated by rab20 along the secretory pathway.

Using fluorescent sphingolipid analogs to study intracellular lipid trafficking

Sphingolipids (SLs), including glycosphingolipids, are found on the plasma membrane where they play important roles in a wide variety of cell functions, including cell-cell communication, cell growth and differentiation, host-pathogen interactions, and cell-signaling events. This unit illustrates the use of fluorescent SL analogs to identify the mechanisms underlying SL endocytosis and subsequent intracellular trafficking. Techniques used to study SL domain formation at the plasma membrane, endocytic mechanisms and intracellular transport steps are highlighted. The use of biochemical treatments and dominant-negative protein expression to block specific steps in lipid trafficking are also discussed.

Leishmania requires Rab7-mediated degradation of endocytosed hemoglobin for their growth

Leishmania is unable to synthesize heme and must acquire it from exogenous source, the mechanism of which is not known. We have shown that Leishmania endocytoses hemoglobin (Hb) and subsequently degrade it probably to generate heme. To understand how internalized Hb is degraded, we have cloned and expressed Rab7 homolog from Leishmania donovani. Interestingly, Rab7 in Leishmania is found to be localized both on early and late endocytic compartment and regulates both uptake and degradation of endocytosed Hb demonstrating that Rab7 in Leishmania play a very unique role connecting both early and late events of Hb endocytosis. Our data also indicate that overexpression of Rab7:WT in Leishmania induces transport of Hb to lysosomes and rapidly degrade internalized Hb. Whereas Hb transport to lysosomes and its degradation is significantly inhibited in cells overexpressing Rab7:T21N, a GDP locked mutant of Rab7. Moreover, cells overexpressing Rab7:T21N grow at a slower rate (<50%) compared with control Leishmania. Addition of exogenous hemin recovers the growth of Rab7:T21N mutant cells almost to the control level, suggesting that intracellular heme generated by Rab7-mediated Hb degradation is required for optimal growth of the parasites. Thus, our results identify a potential target which might be exploited to suppress the growth of Leishmania.

A syntaxin 10-SNARE complex distinguishes two distinct transport routes from endosomes to the trans-Golgi in human cells

Mannose 6-phosphate receptors (MPRs) are transported from endosomes to the Golgi after delivering lysosomal enzymes to the endocytic pathway. This process requires Rab9 guanosine triphosphatase (GTPase) and the putative tether GCC185. We show in human cells that a soluble NSF attachment protein receptor (SNARE) complex comprised of syntaxin 10 (STX10), STX16, Vti1a, and VAMP3 is required for this MPR transport but not for the STX6-dependent transport of TGN46 or cholera toxin from early endosomes to the Golgi. Depletion of STX10 leads to MPR missorting and hypersecretion of hexosaminidase. Mouse and rat cells lack STX10 and, thus, must use a different target membrane SNARE for this process. GCC185 binds directly to STX16 and is competed by Rab6. These data support a model in which the GCC185 tether helps Rab9-bearing transport vesicles deliver their cargo to the trans-Golgi and suggest that Rab GTPases can regulate SNARE–tether interactions. Importantly, our data provide a clear molecular distinction between the transport of MPRs and TGN46 to the trans-Golgi.

Constitutive overexpression of a stress-inducible small GTP-binding protein PgRab7 from Pennisetum glaucum enhances abiotic stress tolerance in transgenic tobacco

The Rab GTPases are important components of endocytic network in plant cells. Endocytosis participates in the cell's reaction to extracellular stimuli by desensitizing, down-regulating or recycling receptors and membrane proteins. Rab7 is a small GTP-binding protein involved in intracellular vesicle trafficking from late endosome to the vacuole. We have isolated Rab7 cDNA from Pennisetum glaucum, a relatively drought-stress tolerant food grain crop grown commonly in India, during cDNA-subtractive hybridization of dehydration-stress treated plants. The PgRab7 ORF, encoding 207 aminoacids, was over-expressed in E. coli. The recombinant PgRab7 protein showed GTP-binding and GTPase activity. Transcript expression of PgRab7 gene was differentially up-regulated by different environmental stimuli such as cold, dehydration and NaCl and also by a plant hormone IAA. Overexpression of PgRab7 gene enhanced tolerance to NaCl and mannitol in transgenic tobacco. Transgenic plants also had increased alkaline phosphatase (ALP) activity. These results show that PgRab7 is a potential candidate gene for developing both salinity and dehydration tolerance in planta.

Intracellular localization of type III-delivered Pseudomonas ExoS with endosome vesicles

ExoS (453 amino acids) is a bi-functional type III cytotoxin produced by Pseudomonas aeruginosa. Residues 96-219 include the Rho GTPase-activating protein (RhoGAP) domain, and residues 234-453 include the 14-3-3-dependent ADP-ribosyltransferase domain. Earlier studies also identified an N-terminal domain (termed the membrane localization domain) that comprises residues 51-77 and includes a novel leucine-rich motif that targets ExoS to the perinuclear region of cultured cells. There is limited information on how ExoS or other type III cytotoxins enter and target intracellular host proteins. Type III-delivered ExoS localized to both plasma membrane and perinuclear region, whereas ExoS(DeltaMLD) was localized to the cytosol. Plasma membrane localization of ExoS was transient and had a half-life of approximately 20 min. Type III-delivered ExoS co-immunoprecipitated 14-3-3 proteins and Rab9, Rab6, and Rab5. Immunofluorescence experiments showed that ExoS colocalized with Rab9, Rab6, and Rab5. Fluorescent energy transfer was detected between ExoS and 14-3-3 proteins but not between ExoS and Rabs proteins. Together, these results indicate that type III-delivered ExoS localizes on the host endosomes and utilizes multiple pathways to traffic from the plasma membrane to the perinuclear region of intoxicated host cells.

A loss-of-function screen reveals SNX5 and SNX6 as potential components of the mammalian retromer

The mammalian retromer is a multimeric protein complex involved in mediating endosome-to-trans-Golgi-network retrograde transport of the cation-independent mannose-6-phosphate receptor. The retromer is composed of two subcomplexes, one containing SNX1 and forming a membrane-bound coat, the other comprising VPS26, VPS29 and VPS35 and being cargo-selective. In yeast, an additional sorting nexin--Vps17p--is a component of the membrane bound coat. It remains unclear whether the mammalian retromer requires a functional equivalent of Vps17p. Here, we have used an RNAi loss-of-function screen to examine whether any of the other 30 mammalian sorting nexins are required for retromer-mediated endosome-to-trans-Golgi-network retrieval of the cation-independent mannose-6-phosphate receptor. Using this screen, we identified two proteins, SNX5 and SNX6, that, when suppressed, induced a phenotype similar to that observed upon suppression of known retromer components. Whereas SNX5 and SNX6 colocalised with SNX1 on early endosomes, in immunoprecipitation experiments only SNX6 appeared to exist in a complex with SNX1. Interestingly, suppression of SNX5 and/or SNX6 resulted in a significant loss of SNX1, an effect that seemed to result from post-translational regulation of the SNX1 level. Such data suggest that SNX1 and SNX6 exist in a stable, endosomally associated complex that is required for retromer-mediated retrieval of the cation-independent mannose-6-phosphate receptor. SNX5 and SNX6 may therefore constitute functional equivalents of Vps17p in mammals.

Rab10 is involved in basolateral transport in polarized Madin-Darby canine kidney cells

The sorting of newly synthesized membrane proteins to the cell surface is an important mechanism of cell polarity. To identify more of the molecular machinery involved, we investigated the function of the small GTPase Rab10 in polarized epithelial Madin-Darby canine kidney cells. We find that GFP-tagged Rab10 localizes primarily to the Golgi during early cell polarization. Expression of an activated Rab10 mutant inhibits biosynthetic transport from the Golgi and missorts basolateral cargo to the apical membrane. Depletion of Rab10 by RNA interference has only mild effects on biosynthetic transport and epithelial polarization, but simultaneous inhibition of Rab10 and Rab8a more strongly impairs basolateral sorting. These results indicate that Rab10 functions in trafficking from the Golgi at early stages of epithelial polarization, is involved in biosynthetic transport to the basolateral membrane and may co-operate with Rab8.

Retrograde transport from endosomes to the trans-Golgi network

A subset of intracellular transmembrane proteins such as acid-hydrolase receptors, processing peptidases and SNAREs, as well as extracellular protein toxins such as Shiga toxin and ricin, undergoes 'retrograde' transport from endosomes to the trans-Golgi network. Here, we discuss recent studies that have begun to unravel the molecular machinery that is involved in this process. We also propose a central role for a 'tubular endosomal network' in sorting to recycling pathways that lead not only to the trans-Golgi network but also to different plasma-membrane domains and to specialized storage vesicles.

A functional role for the GCC185 golgin in mannose 6-phosphate receptor recycling

Mannose 6-phosphate receptors (MPRs) deliver newly synthesized lysosomal enzymes to endosomes and then recycle to the Golgi. MPR recycling requires Rab9 GTPase; Rab9 recruits the cytosolic adaptor TIP47 and enhances its ability to bind to MPR cytoplasmic domains during transport vesicle formation. Rab9-bearing vesicles then fuse with the trans-Golgi network (TGN) in living cells, but nothing is known about how these vesicles identify and dock with their target. We show here that GCC185, a member of the Golgin family of putative tethering proteins, is a Rab9 effector that is required for MPR recycling from endosomes to the TGN in living cells, and in vitro. GCC185 does not rely on Rab9 for its TGN localization; depletion of GCC185 slightly alters the Golgi ribbon but does not interfere with Golgi function. Loss of GCC185 triggers enhanced degradation of mannose 6-phosphate receptors and enhanced secretion of hexosaminidase. These data assign a specific pathway to an interesting, TGN-localized protein and suggest that GCC185 may participate in the docking of late endosome-derived, Rab9-bearing transport vesicles at the TGN.

Rabs and their effectors: achieving specificity in membrane traffic

Rab proteins constitute the largest branch of the Ras GTPase superfamily. Rabs use the guanine nucleotide-dependent switch mechanism common to the superfamily to regulate each of the four major steps in membrane traffic: vesicle budding, vesicle delivery, vesicle tethering, and fusion of the vesicle membrane with that of the target compartment. These different tasks are carried out by a diverse collection of effector molecules that bind to specific Rabs in their GTP-bound state. Recent advances have not only greatly extended the number of known Rab effectors, but have also begun to define the mechanisms underlying their distinct functions. By binding to the guanine nucleotide exchange proteins that activate the Rabs certain effectors act to establish positive feedback loops that help to define and maintain tightly localized domains of activated Rab proteins, which then serve to recruit other effector molecules. Additionally, Rab cascades and Rab conversions appear to confer directionality to membrane traffic and couple each stage of traffic with the next along the pathway.

Cholesterol accumulation sequesters Rab9 and disrupts late endosome function in NPC1-deficient cells

Niemann-Pick type C disease is an autosomal recessive disorder that leads to massive accumulation of cholesterol and glycosphingolipids in late endosomes and lysosomes. To understand how cholesterol accumulation influences late endosome function, we investigated the effect of elevated cholesterol on Rab9-dependent export of mannose 6-phosphate receptors from this compartment. Endogenous Rab9 levels were elevated 1.8-fold in Niemann-Pick type C cells relative to wild type cells, and its half-life increased 1.6-fold, suggesting that Rab9 accumulation is caused by impaired protein turnover. Reduced Rab9 degradation was accompanied by stabilization on endosome membranes, as shown by a reduction in the capacity of Rab9 for guanine nucleotide dissociation inhibitor-mediated extraction from Niemann-Pick type C membranes. Cholesterol appeared to stabilize Rab9 directly, as liposomes loaded with prenylated Rab9 showed decreased extractability with increasing cholesterol content. Rab9 is likely sequestered in an inactive form on Niemann-Pick type C membranes, as cation-dependent mannose 6-phosphate receptors were missorted to the lysosome for degradation, a process that was reversed by overexpression of GFP-tagged Rab9. In addition to using primary fibroblasts isolated from Niemann-Pick type C patients, RNA interference was utilized to recapitulate the disease phenotype in cultured cells, greatly facilitating the analysis of cholesterol accumulation and late endosome function. We conclude that cholesterol contributes directly to the sequestration of Rab9 on Niemann-Pick type C cell membranes, which in turn, disrupts mannose 6-phosphate receptor trafficking.

Plant Prevacuolar/Endosomal Compartments

Multivesicular endosomes or prevacuolar compartments (PVCs) are membrane-bound organelles that play an important role in mediating protein traffic in the secretory and endocytic pathways of eukaryotic cells. PVCs function as an intermediate compartment for sorting proteins from the Golgi apparatus to vacuoles, sending missorted proteins back to the Golgi from the PVC, and receiving proteins from plasma membrane in the endocytic pathway. PVCs have been identified as multivesicular bodies in mammalian cells and yeast and more recently in plant cells. Whereas much is known about PVC-mediated protein trafficking and PVC biogenesis in mammalian cells and yeast, relatively little is known about the molecular mechanism of plant PVCs. In this review, we summarize and discuss our understanding of the plant PVC and compare it with its counterparts in yeast and mammalian cells.

A model for Rab GTPase localization

The human genome encodes almost 70 Rab GTPases. These proteins are C-terminally geranylgeranylated and are localized to the surfaces of distinct membrane-bound compartments in eukaryotic cells. This mini review presents a working model for how Rabs achieve and maintain their steady-state localizations. Data from a number of laboratories suggest that Rabs participate in the generation of macromolecular assemblies that generate functional microdomains within a given membrane compartment. Our data suggest that these complex interactions are important for the cellular localization of Rab proteins at steady state.

Melanotrope cells as a model to understand the (patho)physiological regulation of hormone secretion

Regulation of hormone secretion is a complex process that comprises the sequential participation of numerous subcellular mechanisms. Hormone secretion is dictated by extracellular stimuli that are transduced intracellularly into activation/deactivation of different mechanisms, such as hormone expression, processing and exocytosis, which will ultimately determine the precise availability of hormone to be secreted. Malfunction in any of these steps may result in deficient or excessive hormone release and the subsequent appearance of endocrine disorders. Given the complexity of this system, it is difficult to find appropriate cellular models wherein to investigate the multiple components of the secretory process in a physiologically relevant, experimentally manipulable setting. In this review, we present recent evidence on the use of the intermediate lobe (IL) of the pituitary as a powerful tool to understand different aspects of the regulated secretory pathway. IL is composed of a single endocrine cell type, alpha-melanocyte stimulating hormone (alpha-MSH)-producing melanotropes, a fact that greatly facilitates its study. Furthermore, melanotropes can be separated using classic cell separation techniques into two cell subtypes showing opposite morphophysiological phenotypes of hypo- and hypersecretory cells. Comparison of their gene expression fingerprints has unveiled the existence of certain genes preferentially expressed in each melanotrope subtype. Because of their direct participation in the secretory pathway, we postulate that characterization of these gene products in an endocrine cell type may represent novel and useful markers for reliably determining the general secretory status in an endocrine gland, as well as a valuable new tool to further investigate this complex process.

Lysosomal membrane proteomics and biogenesis of lysosomes

This review focuses on events involved in the biogenesis of the lysosome. This organelle contains a diverse array of soluble, luminal proteins capable of digesting all the macromolecules in the cell. Altered function of lysosomes or its constituent enzymes has been implicated in a host of human pathologies, including storage diseases, cancer, and infectious and neurodegenerative diseases. Luminal enzymes are well-characterized, and aspects of how they are incorporated into lysosomes are known. However, little is known about the composition of the membrane surrounding the organelle or how the membrane is assembled. Our starting point to study lysosome biogenesis is to define the composition of the membrane by the use of proven methods for purification of lysosomes to near homogeneity and then to characterize membrane-associated and integral lysosomal membrane proteins. This has been achieved using advanced proteomics (electrophoretic or chromatographic separations of proteins followed by time-of-flight mass spectrometric identification of peptide sequences). To date, we have identified 55 proteins in the membrane-associated fraction and 215 proteins in the integral membrane. By applying these methods to mouse models of lysosome dysgenesis (such as BEIGE, Pale Ear, PEARL) that are related to human diseases such as Chediak-Higashi and Hermansky-Pudlak syndromes, it may be possible to define the membrane protein composition of lysosomes in each of these mutants and to determine how they differ from normal. Identifying proteins affected in the respective mutants may provide hints about how they are targeted to the lysosomal membrane and how failure to target them leads to disease; these features are pivotal to understanding lysosome biogenesis and have the potential to implicate lysosomes in a broad range of human pathologies.

Genetic evidence for a mammalian retromer complex containing sorting nexins 1 and 2

We have previously shown that the putative mammalian retromer components sorting nexins 1 and 2 (Snx1 and Snx2) result in embryonic lethality when simultaneously targeted for deletion in mice, whereas others have shown that Hbeta58 (also known as mVps26), another retromer component, results in similar lethality when targeted for deletion. In the current study, we address the genetic interaction of these mammalian retromer components in mice. Our findings reveal a functional interaction between Hbeta58, SNX1, and SNX2 and strongly suggest that SNX2 plays a more critical role than SNX1 in retromer activity during embryonic development. This genetic evidence supports the existence of mammalian retromer complexes containing SNX1 and SNX2 and identifies SNX2 as an important mediator of retromer biology. Moreover, we find that mammalian retromer complexes containing SNX1 and SNX2 have an essential role in embryonic development that is independent of cation-independent mannose 6-phosphate receptor trafficking.

Mucolipidosis II is caused by mutations in GNPTA encoding the alpha/beta GlcNAc-1-phosphotransferase

Mucolipidosis II (ML II) is a fatal lysosomal storage disorder resulting from defects in the multimeric GlcNAc-1-phosphotransferase responsible for the initial step in the generation of the mannose 6-phosphate (M6P) recognition marker. M6P residues on oligosaccharides of newly synthesized lysosomal enzymes are essential for efficient receptor-mediated transport to lysosomes. We used the recombinant GlcNAc-1-phosphotransferase gamma subunit as an affinity matrix to purify an unknown protein identified as the product of GNPTA (encoding GNPTA, previously known as MGC4170). The cDNA encodes a protein of 1,256 amino acids with two putative transmembrane domains and a complex preserved modular structure comprising at least six domains. The N-terminal domain of GNPTA, interrupted by a long insertion, shows similarities to bacterial capsule biosynthesis proteins. We identified seven mutations in GNPTA that lead to premature translational termination in six individuals with ML II. Retroviral transduction of fibroblasts from an individual with ML II resulted in the expression and localization of GNPTA in the Golgi apparatus, accompanied by the correction of hypersecretion of lysosomal enzymes. Our results provide evidence that GNPTA encodes a subunit of GlcNAc-1-phosphotransferase defective in individuals with ML II.

Protein transduction of Rab9 in Niemann-Pick C cells reduces cholesterol storage

Niemann-Pick disease type C (NPC) is a genetic disorder in which patient cells exhibit lysosomal accumulation of cholesterol and sphingolipids (SLs) caused by defects in either NPC1 or NPC2 proteins. We previously demonstrated that NPC1 human skin fibroblasts overexpressing endosomal Rab proteins (Rab7 or Rab9) showed a correction in the storage disease phenotype. In the current study, we used protein transduction to further investigate Rab9-mediated reduction of stored lipids in NPC cells. Recombinant human Rab9 fused with the herpes simplex virus VP22 protein fragment was overexpressed, purified, and added to culture medium to induce protein transduction. When VP22-Rab9 was transduced into NPC1 fibroblasts, nearly all cells showed significant reduction in cellular free cholesterol levels, with no cytotoxicity up to 5 microM. A fraction of the VP22-Rab9 that was transduced into the cells was shown to bind to rab GDP dissociation inhibitor, suggesting that this pool of VP22-Rab9 had become prenylated. The reduction in cellular free cholesterol was associated with correction of abnormal intracellular trafficking of BODIPY-lactosylceramide and an increase of sterols in the culture media. The clearance of lysosomal free cholesterol was also associated with a decrease in LDL-receptor levels. In addition, we demonstrated reduction of intracellular cholesterol by VP22-Rab9 transduction in NPC2 fibroblasts and in cultured mouse NPC1 neurons. These observations provide important new information about the correction of membrane traffic in NPC cells by Rab9 overexpression and may lead to new therapeutic approaches for treatment of this disease.

Large-scale profiling of Rab GTPase trafficking networks: the membrome

Rab GTPases and SNARE fusion proteins direct cargo trafficking through the exocytic and endocytic pathways of eukaryotic cells. We have used steady state mRNA expression profiling and computational hierarchical clustering methods to generate a global overview of the distribution of Rabs, SNAREs, and coat machinery components, as well as their respective adaptors, effectors, and regulators in 79 human and 61 mouse nonredundant tissues. We now show that this systems biology approach can be used to define building blocks for membrane trafficking based on Rab-centric protein activity hubs. These Rab-regulated hubs provide a framework for an integrated coding system, the membrome network, which regulates the dynamics of the specialized membrane architecture of differentiated cells. The distribution of Rab-regulated hubs illustrates a number of facets that guides the overall organization of subcellular compartments of cells and tissues through the activity of dynamic protein interaction networks. An interactive website for exploring datasets comprising components of the Rab-regulated hubs that define the membrome of different cell and organ systems in both human and mouse is available at http://www.membrome.org/.

Purification and analysis of TIP47 function in Rab9-dependent mannose 6-phosphate receptor trafficking

TIP47 (tail interacting protein of 47 kDa) is a cytosolic protein that is essential for the transport of mannose 6-phosphate receptors (MPRs) from endosomes to the trans-Golgi. This protein is recruited from the cytosol onto the surface of late endosomes by Rab9 GTPase, which enables TIP47 to bind to MPR cytoplasmic domains with enhanced affinity. A mutation in a deep hydrophobic cleft of TIP47 (F(236)C) confers enhanced affinity binding to MPR cytoplasmic domains and stabilizes MPRs in living cells. We describe the purification of native and recombinant TIP47 proteins and assays that we use to monitor the function of this protein in MPR transport in living cells.

Rab9 GTPase regulates late endosome size and requires effector interaction for its stability

Rab9 GTPase resides in a late endosome microdomain together with mannose 6-phosphate receptors (MPRs) and the tail-interacting protein of 47 kDa (TIP47). To explore the importance of Rab9 for microdomain establishment, we depleted the protein from cultured cells. Rab9 depletion decreased late endosome size and reduced the numbers of multilamellar and dense-tubule-containing late endosomes/lysosomes, but not multivesicular endosomes. The remaining late endosomes and lysosomes were more tightly clustered near the nucleus, implicating Rab9 in endosome localization. Cells displayed increased surface MPRs and lysosome-associated membrane protein 1. In addition, cells showed increased MPR synthesis in conjunction with MPR missorting to the lysosome. Surprisingly, Rab9 stability on late endosomes required interaction with TIP47. Rabs are thought of as independent, prenylated entities that reside either on membranes or in cytosol, bound to GDP dissociation inhibitor. These data show that Rab9 stability is strongly influenced by a specific effector interaction. Moreover, Rab9 and the proteins with which it interacts seem critical for the maintenance of specific late endocytic compartments and endosome/lysosome localization.

Cargo-selective endosomal sorting for retrieval to the Golgi requires retromer

fEndosome-to-Golgi retrieval of the mannose 6-phosphate receptor (MPR) is required for lysosome biogenesis. Currently, this pathway is poorly understood. Analyses in yeast identified a complex of proteins called “retromer” that is essential for endosome-to-Golgi retrieval of the carboxypeptidase Y receptor Vps10p. Retromer comprises five distinct proteins: Vps35p, 29p, 26p, 17p, and 5p, which are conserved in mammals. Here, we show that retromer is required for the efficient retrieval of the cation-independent MPR (CI-MPR). Cells lacking mammalian VPS26 fail to retrieve the CI-MPR, resulting in either rapid degradation of or mislocalization to the plasma membrane. We have localized mVPS26 to multivesicular body endosomes by electron microscopy, and through the use of CD8 reporter protein constructs have examined the effect of loss of mVPS26 upon the trafficking of membrane proteins that cycle between the endosome and the Golgi. The data presented here support the hypothesis that retromer performs a selective function in endosome-to-Golgi transport, mediating retrieval of the CI-MPR, but not furin.

Intracellular machinery for matrix degradation in bone-resorbing osteoclasts

In osteoclasts, TRACP co-localized with cathepsin K in transcytotic vesicles and was activated by cathepsin K in vitro, suggesting that TRACP may degrade organic matrix components in transcytotic vesicles in an event regulated by cathepsin K.

INTRODUCTION

TRACP is an enzyme with unknown biological function. In addition to its phosphatase activity, TRACP is capable of generating reactive oxygen species (ROS). Bone-resorbing osteoclasts contain large amounts of TRACP, and transgenic animal models suggest that TRACP has a role in bone resorption. Osteoclasts resorb bone by secreting acid and lysosomal enzymes such as cathepsin K into an extracellular resorption lacuna between the cell membrane and bone surface. Matrix degradation products are then endocytosed, transcytosed, and secreted through a functional secretory domain in the basolateral membrane facing bone marrow.

MATERIALS AND METHODS

We have studied intracellular localization of TRACP in osteoclasts with antibodies against various known endosomal and lysosomal proteins using confocal microscopy. We also studied co-localization of TRACP with cathepsin K and endocytosed bone matrix components and the effect of cathepsin K digestion on the ROS generating activity of TRACP in vitro.

RESULTS

Double-staining experiments of TRACP with endosomal and lysosomal markers showed that, although some endosomal staining was detected, TRACP was not present in lysosomes. However, TRACP was present in transcytotic vesicles, where it co-localized with cathepsin K. Cathepsin K digestion of TRACP in vitro increased the phosphatase activity by 5.6-fold and the ROS generating activity by 2.0-fold.

CONCLUSIONS

These results suggest that cathepsin K may activate the ROS-generating activity of TRACP in transcytotic vesicles of resorbing osteoclasts, the ROS being targeted to finalize degradation of organic bone matrix components during their transcytosis.

Cytotoxic ribosome-inactivating lectins from plants

A class of heterodimeric plant proteins consisting of a carbohydrate-binding B-chain and an enzymatic A-chain which act on ribosomes to inhibit protein synthesis are amongst the most toxic substances known. The best known example of such a toxic lectin is ricin, produced by the seeds of the castor oil plant, Ricinnus communis. For ricin to reach its substrate in the cytosol, it must be endocytosed, transported through the endomembrane system to reach the compartment from which it is translocated into the cytosol, and there avoid degradation making it possible for a few molecules to inactivate a large proportion of the ribosomes and hence kill the cell. Cell entry by ricin involves the following steps: (i) binding to cell-surface glycolipids and glycoproteins bearing beta-1,4-linked galactose residues through the lectin activity of the B-chain (RTB); (ii) uptake by endocytosis and entry into early endosomes; (iii) transfer by vesicular transport to the trans-Golgi network; (iv) retrograde vesicular transport through the Golgi complex and into the endoplasmic reticulum (ER); (v) reduction of the disulfide bond connecting the A- and B-chains; (vi) a partial unfolding of the A-chain (RTA) to enable it to translocate across the ER membrane via the Sec61p translocon using the pathway normally followed by misfolded ER proteins for targeting to the ER-associated degradation (ERAD) machinery; (vi) refolding in the cytosol into a protease-resistant, enzymatically active structure; (vii) interaction with the sarcin-ricin domain (SRD) of the large ribosome subunit RNA followed by cleavage of a single N-glycosidic bond in the RNA to generate a depurinated, inactive ribosome. In addition to the highly specific action on ribosomes, ricin and related ribosome-inactivating proteins (RIPs) have a less specific action in vitro on DNA and RNA substrates releasing multiple adenine, and in some instances, guanine residues. This polynucleotide:adenosine glycosidase activity has been implicated in the general antiviral, and specifically, the anti HIV-1 activity of several single-chain RIPs which are homologous to the A-chains of the heterodimeric lectins. However, in the absence of clear cause and effect evidence in vivo, such claims should be regarded with caution.

Numerous transcriptional alterations in liver persist after short-term enzyme-replacement therapy in a murine model of mucopolysaccharidosis type VII

The lysosomal storage disease MPS VII (mucopolysaccharidosis type VII) is caused by a deficiency in beta-glucuronidase activity, and results in the accumulation of partially degraded glycosaminoglycans in many cell types. Although MPS VII is a simple monogenetic disorder, the clinical presentation is complex and incompletely understood. ERT (enzyme replacement therapy) is relatively effective at improving the clinical course of the disease; however, some pathologies persist. In order to clarify the molecular events contributing to the disease phenotype and how ERT might impact upon them, we analysed liver tissue from untreated and treated MPS VII mice at both 2 and 5 months of age using biochemical assays and microarray analysis. Overall, as the disease progresses, more genes have altered expression and, at either age, numerous transcriptional changes in multiple pathways appear to be refractory to therapy. With respect to the primary site of disease, both transcriptional and post-transcriptional mechanisms are involved in the regulation of lysosomal enzymes and other lysosome-associated proteins. Many of the changes observed in both lysosome-associated mRNAs and proteins are normalized by enzyme replacement. In addition, gene expression changes in seemingly unrelated pathways may account for the complex metabolic phenotype of the MPS VII mouse. In particular, beta-glucuronidase deficiency appears to induce physiological malnutrition in MPS VII mice. Malnutrition may account for the pronounced adipose storage deficiency observed in this animal. Studying the molecular response to lysosomal storage, especially those changes recalcitrant to therapy, has revealed additional targets that may improve the efficacy of existing therapies.

High resolution crystal structure of human Rab9 GTPase: a novel antiviral drug target

Rab GTPases and their effectors facilitate vesicular transport by tethering donor vesicles to their respective target membranes. Rab9 mediates late endosome to trans-Golgi transport and has recently been found to be a key cellular component for human immunodeficiency virus-1, Ebola, Marburg, and measles virus replication, suggesting that it may be a novel target in the development of broad spectrum antiviral drugs. As part of our structure-based drug design program, we have determined the crystal structure of a C-terminally truncated human Rab9 (residues 1-177) to 1.25-A resolution. The overall structure shows a characteristic nucleotide binding fold consisting of a six-stranded beta-sheet surrounded by five alpha-helices with a tightly bound GDP molecule in the active site. Structure-based sequence alignment of Rab9 with other Rab proteins reveals that its active site consists of residues highly conserved in the Rab GTPase family, implying a common catalytic mechanism. However, Rab9 contains seven regions that are significantly different in conformation from other Rab proteins. Some of those regions coincide with putative effector-binding sites and switch I and switch II regions identified by structure/sequence alignments. The Rab9 structure at near atomic resolution provides an excellent model for structure-based antiviral drug design.

Destination 'lysosome': a target organelle for tumour cell killing?

Lysosomes and lysosome-related organelles constitute a system of acid compartments that interconnect the inside of the cell with the extracellular environment via endocytosis, phagocytosis and exocytosis. In recent decades it has been recognized that lysosomes are not just wastebaskets for disposal of unused cellular constituents, but that they are involved in several cellular processes such as post-translational maturation of proteins, degradation of receptors and extracellular release of active enzymes. By complementing the autophagic process, lysosomes actively contribute to the maintenance of cellular homeostasis. Proteolysis by lysosomal cathepsins has been shown to mediate the death signal of cytotoxic drugs and cytokines, as well as the activation of pro-survival factors. Secreted lysosomal cathepsins have been shown to degrade protein components of the extracellular matrix, thus contributing actively to its re-modelling in physiological and pathological processes. The malfunction of lysosomes can, therefore, impact on cell behaviour and fate. Here we review the role of lysosomal hydrolases in several aspects of the malignant phenotype including loss of cell growth control, altered regulation of cell death, acquisition of chemoresistance and of metastatic potential. Based on these observations, the lysosome is proposed as a potential target organelle for the chemotherapy of tumours. We will also present some recent data concerning the technologies for delivering chemotherapeutic drugs to the endosomal-lysosomal compartment and the strategies to improve their efficacy.

Dominant negative Rab3D mutants reduce GTP-bound endogenous Rab3D in pancreatic acini

Two dominant negative mutants of Rab3D, N135I and T36N were recently reported to inhibit the early phase of regulated amylase secretion from mouse pancreatic acini (Chen, X., Edwards, J. A., Logsdon, C. D., Ernst, S. A., and Williams, J. A. (2002) J. Biol. Chem. 277, 18002-18009). Immunocytochemical studies showed that while the wild-type Rab3D localized to zymogen granules, the two dominant negative mutants did not localize to granules and were primarily in the basolateral regions of the cell. The present study, therefore, evaluated the potential mechanisms by which the dominant negative mutants might act. An affinity precipitation assay based on the property of the Rab3 effector Rim1 to interact only with GTP-bound Rab3D was developed. 78.9 +/- 4.5% of total endogenous Rab3D was found in the GTP-bound form. Overexpression of HA-tagged Rab3D, and its Q81L, N135I, and T36N mutants had no effect on the total amount of endogenous Rab3D. However, the dominant negative mutants, T36N and N135I, reduced GTP-bound endogenous Rab3D by 70.0 +/- 3.5% and 72.7 +/- 1.2%, respectively, while the wild-type Rab3D and Q81L mutant had no effect. Triton X-114 phase separation and cell fractionation studies showed that dominant negative Rab3D mutants did not alter isoprenylation or membrane association of endogenous Rab3D. The dominant negative Rab3D did not affect the amount of endogenous Rab3D on purified zymogen granules as assessed by either Western blotting or immunocytochemistry, but reduced the GTP-bound form by 78.6 +/- 3.3%. The two dominant negative Rab3D mutants, therefore, interfere with endogenous Rab3D function by blocking the GDP/GTP exchange but not zymogen granule targeting of endogenous Rab3D.

Targeting of the human immunodeficiency virus type 1 envelope to the trans-Golgi network through binding to TIP47 is required for env incorporation into virions and infectivity

Here, we report that human immunodeficiency virus type 1 (HIV-1) Env glycoprotein is located mainly in the trans-Golgi network (TGN) due to determinants present in the cytoplasmic domain of the transmembrane gp41 glycoprotein (TMgp41). Internalization assays demonstrated that Env present at the cell surface returns to the TGN. We found that the cytoplasmic domain of TMgp41 binds to TIP47, a protein required for the transport of mannose-6-phosphate receptors from endosomes to the TGN. Overexpression of a mutant of TIP47 affected the transport of Env from endosomes to the TGN. Retrograde transport of Env to the TGN requires a Y(802)W(803) diaromatic motif present in the TMgp41 cytoplasmic domain. Mutation of this motif abolished both targeting to the TGN as well as interaction with TIP47. These data support the view that binding of TIP47 to HIV-1 Env facilitates its delivery to the TGN. Lastly, we show that virus mutated in the Y(802)W(803) motif is poorly infectious and presents a defect in Env incorporation, supporting a model in which retrograde transport of Env is implicated in the optimization of fully infectious HIV-1 production.

Membrane traffic exploited by protein toxins

A large number of protein toxins having enzymatically active A- and B-moieties that bind to cell surface receptors must be endocytosed before the A-moiety is translocated into the cytosol where it exerts its cytotoxic action. The accumulated information about the most well-studied toxins has provided a detailed picture of how they exploit the membrane trafficking systems of cells, and studies of toxin trafficking have revealed the existence of new pathways. The complexity of different endocytic mechanisms, as well as the multiple routes between endosomes and the Golgi apparatus and retrogradely to the endoplasmic reticulum (ER), are being unravelled by investigations of how toxins gain access to their targets. With increasing information about the internalization and intracellular trafficking of these opportunistic toxins, new avenues have been opened for their application in areas of medicine such as drug delivery and therapy.