Lumenal and transmembrane domains play a role in sorting type I membrane proteins on endocytic pathways

Comparative study of the steady-state subcellular distribution of lysosome-associated membrane glycoprotein-2 (LAMP-2) isoforms with GYXXΦ-type tyrosine-based motifs that interact differently with four adaptor protein (AP) complexes

Lysosome-associated membrane protein-1 and -2 (LAMP-1 and LAMP-2, respectively) are type I transmembrane proteins. LAMP-2 comprises three splice isoforms (LAMP-2A, -B and-C) with different cytoplasmic tails (CTs). These three CTs possess different tyrosine-based motifs (GYXXΦ, where Φ is a bulky hydrophobic amino acid) at their C-termini. Interactions between tyrosine-based motifs and μ-subunits of four tetrameric adaptor protein (AP) complexes are necessary for their vesicular transport to lysosomes. Little is known about how the interaction strengths of these tyrosine motifs with μ-subunits affect the localization of isoforms to lysosomes. The interactions were first investigated using a yeast two-hybrid system to address this question. LAMP-2A-CT interacted with all four μ-subunits (μ1, μ2, μ3A and μ4 of AP-1, AP-2, AP-3 and AP-4, respectively). The interaction with μ3A was more robust than that with other μ-subunits. LAMP-2B-CT interacted exclusively and moderately with μ3A. LAMP-2C-CT did not detectably interact with any of the four μ-subunits. Immunofluorescence microscopy showed that all isoforms were localized in late endosomes and lysosomes. LAMP-2C was present in the plasma membrane and early endosomes; however, LAMP-2A and -2B were barely detectable in these organelles. In cell fractionation, LAMP-2A was the most abundant in the dense lysosomes, whereas LAMP-2C was significantly present in the low-density fraction containing the plasma membrane and early endosomes, in addition to the dense lysosomes. LAMP-2B considerably existed in the low-density late endosomal fraction. These data strongly suggest that the LAMP-2 isoforms are distributed differently in endocytic organelles depending on their interaction strengths with AP-3.

S-palmitoylation determines TMEM55B-dependent positioning of lysosomes

The spatiotemporal cellular distribution of lysosomes depends on active transport mainly driven by microtubule motors such as kinesins and dynein. Different protein complexes attach these molecular motors to their vesicular cargo. TMEM55B (also known as PIP4P1), as an integral lysosomal membrane protein, is a component of such a complex that mediates the retrograde transport of lysosomes by establishing interactions with the cytosolic scaffold protein JIP4 (also known as SPAG9) and dynein-dynactin. Here, we show that TMEM55B and its paralog TMEM55A (PIP4P2) are S-palmitoylated proteins that are lipidated at multiple cysteine residues. Mutation of all cysteines in TMEM55B prevents S-palmitoylation and causes retention of the mutated protein in the Golgi. Consequently, non-palmitoylated TMEM55B is no longer able to modulate lysosomal positioning and the perinuclear clustering of lysosomes. Additional mutagenesis of the dileucine-based lysosomal sorting motif in non-palmitoylated TMEM55B leads to partial missorting to the plasma membrane instead of retention in the Golgi, implicating a direct effect of S-palmitoylation on the adaptor protein-dependent sorting of TMEM55B. Our data suggest a critical role for S-palmitoylation in the trafficking of TMEM55B and TMEM55B-dependent lysosomal positioning.

Structural basis of the intracellular sorting of the SNARE VAMP7 by the AP3 adaptor complex

VAMP7 is involved in the fusion of late endocytic compartments with other membranes. One possible mechanism of VAMP7 delivery to these late compartments is via the AP3 trafficking adaptor. We show that the linker of the δ-adaptin subunit of AP3 binds the VAMP7 longin domain and determines the structure of their complex. Mutation of residues on both partners abolishes the interaction in vitro and in vivo. The binding of VAMP7 to δ-adaptin requires the VAMP7 SNARE motif to be engaged in SNARE complex formation and hence AP3 must transport VAMP7 when VAMP7 is part of a cis-SNARE complex. The absence of δ-adaptin causes destabilization of the AP3 complex in mouse mocha fibroblasts and mislocalization of VAMP7. The mislocalization can be rescued by transfection with wild-type δ-adaptin but not by δ-adaptin containing mutations that abolish VAMP7 binding, despite in all cases intact AP3 being present and LAMP1 trafficking being rescued.

Rab9-dependent retrograde transport and endosomal sorting of the endopeptidase furin

The endopeptidase furin and the trans-Golgi network protein TGN38 are membrane proteins that recycle between the TGN and plasma membrane. TGN38 is transported by a retromer-dependent pathway from early endosomes to the TGN, whereas the intracellular transport of furin is poorly defined. Here we have identified the itinerary and transport requirements of furin. Using internalisation assays, we show that furin transits the early and late endosomes en route to the TGN. The GTPase Rab9 and the TGN golgin GCC185, components of the late endosome-to-TGN pathway, were required for efficient TGN retrieval of furin. By contrast, TGN38 trafficking was independent of Rab9 and GCC185. To identify the sorting signals for the early endosome-to-TGN pathway, the trafficking of furin-TGN38 chimeras was investigated. The diversion of furin from the Rab9-dependent late-endosome-to-TGN pathway to the retromer-dependent early-endosome-to-TGN pathway required both the transmembrane domain and cytoplasmic tail of TGN38. We present evidence to suggest that the length of the transmembrane domain is a contributing factor in endosomal sorting. Overall, these data show that furin uses the Rab9-dependent pathway from late endosomes and that retrograde transport directly from early endosomes is dependent on both the transmembrane domain and the cytoplasmic tail.

Two dileucine motifs mediate late endosomal/lysosomal targeting of transmembrane protein 192 (TMEM192) and a C-terminal cysteine residue is responsible for disulfide bond formation in TMEM192 homodimers

TMEM192 (transmembrane protein 192) is a novel constituent of late endosomal/lysosomal membranes with four potential transmembrane segments and an unknown function that was initially discovered by organellar proteomics. Subsequently, localization in late endosomes/lysosomes has been confirmed for overexpressed and endogenous TMEM192, and homodimers of TMEM192 linked by disulfide bonds have been reported. In the present study the molecular determinants of TMEM192 mediating its transport to late endosomes/lysosomes were analysed by using CD4 chimaeric constructs and mutagenesis of potential targeting motifs in TMEM192. Two directly adjacent N-terminally located dileucine motifs of the DXXLL-type were found to be critical for transport of TMEM192 to late endosomes/lysosomes. Whereas disruption of both dileucine motifs resulted in mistargeting of TMEM192 to the plasma membrane, each of the two motifs was sufficient to ensure correct targeting of TMEM192. In order to study disulfide bond formation, mutagenesis of cysteine residues was performed. Mutation of Cys266 abolished disulfide bridge formation between TMEM192 molecules, indicating that TMEM192 dimers are linked by a disulfide bridge between their C-terminal tails. According to the predicted topology, Cys266 would be localized in the reductive milieu of the cytosol where disulfide bridges are generally uncommon. Using immunogold labelling and proteinase protection assays, the localization of the N- and C-termini of TMEM192 on the cytosolic side of the late endosomal/lysosomal membrane was experimentally confirmed. These findings may imply close proximity of the C-termini in TMEM192 dimers and a possible involvement of this part of the protein in dimer assembly.

A dileucine signal situated in the C-terminal tail of the lysosomal membrane protein p40 is responsible for its targeting to lysosomes

Transport of newly synthesized lysosomal membrane proteins from the TGN (trans-Golgi network) to the lysosomes is due to the presence of specific signals in their cytoplasmic domains that are recognized by cytosolic adaptors. p40, a hypothetical transporter of 372 amino acids localized in the lysosomal membrane, contains four putative lysosomal sorting motifs in its sequence: three of the YXXphi-type (Y(6)QLF, Y(106)VAL, Y(333)NGL) and one of the [D/E]XXXL[L/I]-type (EQERL(360)L(361)). To test the role of these motifs in the biosynthetic transport of p40, we replaced the most critical residues of these consensus sequences, the tyrosine residue or the leucine-leucine pair, by alanine or alanine-valine respectively. We analysed the subcellular localization of the mutated p40 proteins in transfected HeLa cells by confocal microscopy and by biochemical approaches (subcellular fractionation on self-forming Percoll density gradients and cell surface biotinylation). The results of the present study show that p40 is mistargeted to the plasma membrane when its dileucine motif is disrupted. No role of the tyrosine motifs could be put forward. Taken together, our results provide evidence that the sorting of p40 from the TGN to the lysosomes is directed by the dileucine EQERL(360)L(361) motif situated in its C-terminal tail.

Modulation of host cell endocytosis by the type III cytotoxin, Pseudomonas ExoS

Pseudomonas aeruginosa ExoS is a bifunctional type III cytotoxin that possesses Rho GTPase-activating protein (RhoGAP) and ADP-ribosyltransferase (ADPr) activities. In the current study, the RhoGAP and ADPr activities of ExoS were tested for the ability to disrupt mammalian epithelial cell physiology. RhoGAP, but not ADPr, inhibited internalization/phagocytosis of bacteria, while ADPr, but not RhoGAP, inhibited vesicle trafficking, both general fluid-phase uptake and EGF-activated EGF receptor (EGFR) degradation. In ADPr-intoxicated cells, upon EGF activation, EGFR co-localized with clathrin-coated vesicles (CCV), which did not mature into Rab5-positive early endosomes. Constitutively, active Rab5 recruited EGFR from CCV to early endosomes. Consistent with the inhibition of Rab5 function by ADPr, several Rab proteins including Rab5 and 9, but not Rab4, were ADP ribosylated by ExoS. Thus, the two enzymatic activities of ExoS have different effects on epithelial cells with RhoGAP inhibiting bacterial internalization and ADPr interfering with CCV maturation. The ability ADPr to inhibit mammalian vesicle trafficking provides a new mechanism for bacterial toxin-mediated virulence.

Intracellular trafficking of the HCMV immunoevasin UL16 depends on elements present in both its cytoplasmic and transmembrane domains

Expression of the UL16 glycoprotein leads to down-regulation of NKG2D-ligands from the surface of the human cytomegalovirus (HCMV)-infected cell. The molecular elements responsible for UL16 trafficking and intracellular localization were investigated by preparing various chimeric proteins and mutants, using CD8 as a reporter molecule. A YQRL motif, present in UL16's cytoplasmic tail was functional for internalization, but the presence of the transmembrane domain modified the fate of the molecule after internalization. Various elements of the transmembrane domain that affected the trafficking of the protein were identified; however, their influence was modified in turn by the presence of the cytoplasmic tail of UL16. Strikingly, the extremely slow maturation rate of the native viral protein was only reproduced by the chimera that contained both transmembrane and cytoplasmic regions of UL16. These findings add data to a topic of increasing interest and importance: the role of the transmembrane domain of a protein in controlling its intracellular trafficking. In addition, they provide a new insight into the mechanism of action of the viral immunoevasin UL16.

Two di-leucine motifs regulate trafficking of mucolipin-1 to lysosomes

Mutations in the mucolipin-1 gene have been linked to mucolipidosis type IV, a lysosomal storage disorder characterized by severe neurological and ophthalmologic abnormalities. Mucolipin-1 is a membrane protein containing six putative transmembrane domains with both its N- and C-termini localized facing the cytosol. To gain information on the sorting motifs that mediate the trafficking of this protein to lysosomes, we have generated chimeras in which the N- and C- terminal tail portions of mucolipin-1 were fused to a reporter gene. In this article, we report the identification of two separate di-leucine-type motifs that co-operate to regulate the transport of mucolipin-1 to lysosomes. One di-leucine motif is positioned at the N-terminal cytosolic tail and mediates direct transport to lysosomes, whereas the other di-leucine motif is found at the C-terminal tail and functions as an adaptor protein 2-dependent internalization motif. We have also found that the C-terminal tail of mucolipin-1 is palmitoylated and that this modification might regulate the efficiency of endocytosis. Finally, the mutagenesis of both di-leucine motifs abrogated lysosomal accumulation and resulted in cell-surface redistribution of mucolipin-1. Taken together, these results reveal novel information regarding the motifs that regulate mucolipin-1 trafficking and suggest a role for palmitoylation in protein sorting.

The NH(2)-terminal transmembrane and lumenal domains of LGP85 are needed for the formation of enlarged endosomes/lysosomes

LGP85 is a lysosomal membrane protein possessing a type III topology and is also known as a member of the CD36 superfamily of proteins, such as CD36 and the scavenger-receptor BI (SR-BI). We have recently demonstrated that overexpression of LGP85 in various mammalian cell lines causes the enlargement of endosomal/lysosomal compartments (ELCs). Using chimeras and deletion mutants, we show here that the lumenal region of LGP85 is necessary, but not sufficient, for the development of ELCs. Effective formation of enlarged ELC was largely dependent on the presence of a preceding NH(2)-terminal transmembrane segment. Analyses of deletion mutants within the lumenal domain further revealed a requirement of the NH(2)-terminal transmembrane proximal lumenal region, with high sequence similarity with SR-BI for the enlargement of ELC. These results suggest that an interaction of the NH(2)-terminal transmembrane proximal lumenal domain of LGP85 with the inner leaflet of endosomal/lysosomal membranes through the connection with the transmembrane domain is an essential determinant for the regulation of endosomal/lysosomal membrane traffic. Interestingly, although the NH(2)-terminal transmembrane domain itself was not sufficient for the enlargement of ELCs, it appeared to be required for direct targeting of LGP85 from the trans-Golgi network to late endosomes/lysosomes. Taken together, these results indicate the involvement of distinct domain of LGP85 in the targeting to, and biogenesis and maintenance of, ELC.

Role of the endocytic machinery in the sorting of lysosome-associated membrane proteins

The limiting membrane of the lysosome contains a group of transmembrane glycoproteins named lysosome-associated membrane proteins (Lamps). These proteins are targeted to lysosomes by virtue of tyrosine-based sorting signals in their cytosolic tails. Four adaptor protein (AP) complexes, AP-1, AP-2, AP-3, and AP-4, interact with such signals and are therefore candidates for mediating sorting of the Lamps to lysosomes. However, the role of these complexes and of the coat protein, clathrin, in sorting of the Lamps in vivo has either not been addressed or remains controversial. We have used RNA interference to show that AP-2 and clathrin-and to a lesser extent the other AP complexes-are required for efficient delivery of the Lamps to lysosomes. Because AP-2 is exclusively associated with plasma membrane clathrin coats, our observations imply that a significant population of Lamps traffic via the plasma membrane en route to lysosomes.

Differential Use of Two AP-3-mediated Pathways by Lysosomal Membrane Proteins

The adaptor protein complex AP-3 is involved in the sorting of lysosomal membrane proteins to late endosomes/lysosomes. It is unclear whether AP-3-containing vesicles form at the trans-Golgi network (TGN) or early endosomes. We have compared the trafficking routes of endolyn/CD164 and 'typical' lysosomal membrane glycoproteins (lgp120/lamp-1 and CD63/lamp-3) containing cytosolic YXXPhi-targeting motifs preceded by asparagine and glycine, respectively. Endolyn, which has a NYHTL-motif, is concentrated in lysosomes, but also occurs in endosomes and at the cell surface. We observed predominant interaction of the NYHTL-motif with the mu-subunits of AP-3 in the yeast two-hybrid system. Endolyn was mislocalized to the cell surface in AP-3-deficient pearl cells, confirming a major role of AP-3 in endolyn traffic. However, lysosomal delivery of endolyn (or a NYHTL-reporter), but not GYXXPhi-containing proteins, was practically abolished when AP-2-mediated endocytosis or traffic from early to late endosomes was inhibited in NRK and 3T3 cells. This indicates that endolyn is mostly transported along the indirect lysosomal pathway (via the cell surface), rather than directly from the TGN to late endosomes/lysosomes. Our results suggest that AP-3 mediates lysosomal sorting of some membrane proteins in early endosomes in addition to sorting of proteins with intrinsically strong AP-3-interacting lysosomal targeting motifs at the TGN.

Expression of Btcl2, a novel member of Btcl gene family, during development of the central nervous system

Cell-cell interactions are primarily mediated by secreted and transmembrane proteins which play essential roles in the neuronal circuit formation. However, molecular mechanisms underlying neuronal circuit formation, which is mediated by the cell-cell interactions, remain largely elusive. We isolated and characterized a novel gene, Btcl2 (brain-specific transmembrane protein containing CUB [complement subcomponent C1r/C1s, sea urchin protein Uegf, and BMP-1] and LDLa [low-density lipoprotein receptor domain class A] domains 2), using the signal sequence trap (SST) method. The extracellular domain of BTCL2 contains two CUB domains and an LDLa domain. BTCL2 and BTCL1 have similar domain structures, sharing 51% overall identity. The CUB1, CUB2, and LDLa domains of these two proteins share 63%, 72%, and 84% identity, respectively. The CUB domains of BTCL1 and BTCL2 share significant identity with those of neuropilins. Btcl2 mRNA was detected as a single 6-kb transcript in Northern blot analysis. In situ hybridization (ISH) analysis revealed that both Btcl1 and Btcl2 mRNAs were observed restrictively in brain throughout embryonic and postnatal stages. Btcl1 and Btcl2 mRNAs were expressed uniquely in the pontine nucleus and subplate, which are required for establishing the neuronal circuit formation. These results will aid in resolving the mechanisms underlying neuronal circuit formations (e.g., pontocerebellar and thalamocortical axon guidance) and permit more precise studies aimed at understanding the role of BTCL1 and BTCL2 in the central nervous system.

ADP-ribosylation factor 1-independent protein sorting and export from the trans-Golgi network

Polarized epithelial cells efficiently sort newly synthesized apical and basolateral proteins into distinct transport carriers that emerge from the trans-Golgi network (TGN), and this sorting is recapitulated in nonpolarized cells. While the targeting signals of basolaterally destined proteins are generally cytoplasmically disposed, apical sorting signals are not typically accessible to the cytosol, and the transport machinery required for segregation and export of apical cargo remains largely unknown. Here we investigated the molecular requirements for TGN export of the apical marker influenza hemagglutinin (HA) in HeLa cells using an in vitro reconstitution assay. HA was released from the TGN in intact membrane-bound compartments, and export was dependent on addition of an ATP-regenerating system and exogenous cytosol. HA release was inhibited by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) as well as under conditions known to negatively regulate apical transport in vivo, including expression of the acid-activated proton channel influenza M2. Interestingly, release of HA was unaffected by depletion of ADP-ribosylation factor 1, a small GTPase that has been implicated in the recruitment of all known adaptors and coat proteins to the Golgi complex. Furthermore, regulation of HA release by GTPgammaS or M2 expression was unaffected by cytosolic depletion of ADP-ribosylation factor 1, suggesting that HA sorting remains functionally intact in the absence of the small GTPase. These data suggest that TGN sorting and export of influenza HA does not require classical adaptors involved in the formation of other classes of exocytic carriers and thus appears to proceed via a novel mechanism.

Lysosome associated membrane protein 1 (Lamp1) traffics directly from the TGN to early endosomes

The precise trafficking routes followed by newly synthesized lysosomal membrane proteins after exit from the Golgi are unclear. To study these events we created a novel chimera (YAL) having a lumenal domain comprising two tyrosine sulfation motifs fused to avidin, and the transmembrane and cytoplasmic domains of lysosome associated membrane protein 1 (Lamp1). The newly synthesized protein rapidly transited from the trans- Golgi Network (TGN) to lysosomes (t(1/2) approximately 30 min after a lag of 15-20 min). However, labeled chimera was captured by biotinylated probes endocytosed for only 5 min, indicating that the initial site of entry into the endocytic pathway was early endosomes. Capture required export of YAL from the TGN, and endocytosis of the biotinylated reagent, and was essentially quantitative within 2 h of chase, suggesting that all molecules were following an identical route. There was no evidence of YAL trafficking via the cell surface. Fusion of TGN-derived vesicles with 5 min endosomes could be recapitulated in vitro, but neither late endosomes nor lysosomes could serve as acceptor compartments. This suggests that contrary to previous conclusions, most if not all newly synthesized Lamp1 traffics from the TGN to early endosomes prior to delivery to late endosomes and lysosomes.

Aggregation and lack of secretion of most newly synthesized proinsulin in non-beta-cell lines

Myoblasts transfected with HB10D insulin secrete more hormone than those transfected with wild-type insulin, as published previously, indicating that production of wild-type insulin is not efficient in these cells. The ability of non-beta-cells to produce insulin was examined in several cell lines. In clones of neuroendocrine GH(4)C(1) cells stably transfected with proinsulin, two thirds of (35)S-proinsulin was degraded within 3 h of synthesis, whereas (35)S-prolactin was stable. In transiently transfected neuroendocrine AtT20 cells, half of (35)S-proinsulin was degraded within 3 h after synthesis, whereas (35)S-GH was stable. In transiently transfected fibroblast COS cells, (35)S-proinsulin was stable for longer, but less than 10% was secreted 8 h after synthesis. Proinsulin formed a concentrated patch detected by immunofluorescence in transfected cells that did not colocalize with calreticulin or BiP, markers for the endoplasmic reticulum, but did colocalize with membrin, a marker for the cis-medial Golgi complex. Proinsulin formed a Lubrol-insoluble aggregate within 30 min after synthesis in non-beta-cells but not in INS-1E cells, a beta-cell line that normally produces insulin. More than 45% of (35)S-HB10D proinsulin was secreted from COS cells 3 h after synthesis, and this mutant formed less Lubrol-insoluble aggregate in the cells than did wild-type hormone. These results indicate that proinsulin production from these non-beta-cells is not efficient and that proinsulin aggregates in their secretory pathways. Factors in the environment of the secretory pathway of beta-cells may prevent aggregation of proinsulin to allow efficient production.

Analysis of post-lysosomal compartments

Lysosomes are acidic intracellular compartments and are regarded as degradative and the end point, of the endocytic pathway. Here we provide evidence for the generation of acid hydrolase poor and non-acidic post-lysosomal compartments in NRK cells that have accumulated non-digestible macromolecules, Texas red-dextran (TR-Dex), within lysosomes. When TR-Dex was fed to the cells for 6h, most of the internalized TR-Dex colocalized with a lysosomal enzyme, cathepsin D. With an increase in the chase period, however, the internalized TR-Dex gradually accumulated in cathepsin D-negative vesicles. These vesicles were positive for a lysosomal membrane protein, LGP85, and their formation was inhibited by treatment of the cells with U18666A, which impairs membrane transport out of late endosomal/lysosomal compartments, thereby suggesting that the vesicles are derived from lysosomes. Interestingly, these compartments are non-acidic as judged for the DAMP staining. The results, therefore, suggest that the excess accumulation of non-digestible macromolecules within lysosomes induces the formation of acid hydrolase poor and non-acidic post-lysosomal compartments. The fact that treatment of the cells with lysosomotropic amines or a microtubule-depolymerization agent resulted in extensive colocalization of TR-Dex with cathepsin D further indicates that the formation of the post-lysosomal compartments depends on the lysosomal acidification and microtubule organization. Furthermore, these results suggest bi-directional membrane transport between lysosomes and the post-lysosomal compartments, which implies that the latter are not resting compartments.

Two motifs target Batten disease protein CLN3 to lysosomes in transfected nonneuronal and neuronal cells

Batten disease is a neurodegenerative disorder resulting from mutations in CLN3, a polytopic membrane protein, whose predominant intracellular destination in nonneuronal cells is the lysosome. The topology of CLN3 protein, its lysosomal targeting mechanism, and the development of Batten disease are poorly understood. We provide experimental evidence that both the N and C termini and one large loop domain of CLN3 face the cytoplasm. We have identified two lysosomal targeting motifs that mediate the sorting of CLN3 in transfected nonneuronal and neuronal cells: an unconventional motif in the long C-terminal cytosolic tail consisting of a methionine and a glycine separated by nine amino acids [M(X)9G], and a more conventional dileucine motif, located in the large cytosolic loop domain and preceded by an acidic patch. Each motif on its own was sufficient to mediate lysosomal targeting, but optimal efficiency required both. Interestingly, in primary neurons, CLN3 was prominently seen both in lysosomes in the cell body and in endosomes, containing early endosomal antigen-1 along neuronal processes. Because there are few lysosomes in axons and peripheral parts of dendrites, the presence of CLN3 in endosomes of neurons may be functionally important. Endosomal association of the protein was independent of the two lysosomal targeting motifs.

Distribution and trafficking of MPR300 is normal in cells with cholesterol accumulated in late endocytic compartments: evidence for early endosome-to-TGN trafficking of MPR300

It has been reported that an accumulation of cholesterol within late endosomes/lysosomes in Niemann-Pick type C (NPC) fibroblasts and U18666A-treated cells causes impairment of retrograde trafficking of the cation-independent mannose 6-phosphate/IGF-II receptor (MPR300) from late endosomes to the trans-Golgi network (TGN). In apparent conflict with these results, here we show that as in normal fibroblasts, MPR300 localizes exclusively to the TGN in NPC fibroblasts as well as in normal fibroblasts treated with U18666A. This localization can explain why several lysosomal properties and functions, such as intracellular lysosomal enzyme activity and localization, the biosynthesis of cathepsin D, and protein degradation, are all normal in NPC fibroblasts. These results, therefore, suggest that the accumulation of cholesterol in late endosomes/lysosomes does not affect the retrieval of MPR300 from endosomes to the TGN. Furthermore, treatment of normal and NPC fibroblasts with chloroquine, which inhibits membrane traffic from early endosomes to the TGN, resulted in a redistribution of MPR300 to EEA1 and internalized transferrin-positive, but LAMP-2-negative, early-recycling endosomes. We propose that in normal and NPC fibroblasts, MPR300 is exclusively targeted from the TGN to early endosomes, from where it rapidly recycles back to the TGN without being delivered to late endosomes. This notion provides important insights into the definition of late endosomes, as well as the biogenesis of lysosomes.

The p55 tumour necrosis factor receptor TNFR1 contains a trans-Golgi network localization signal in the C-terminal region of its cytoplasmic tail

It has been reported in some human cells that, in addition to a plasma membrane localization, members of the tumour necrosis factor receptor superfamily may be localized to the Golgi complex. We have shown by immunofluorescence and immunoelectron microscopy that the p55 tumour necrosis factor receptor, TNFR1, is principally localized to the trans-Golgi network in the human breast carcinoma cell line, MCF7. Chimaeras consisting of the extracellular and transmembrane domains of CD8 together with the cytoplasmic tail of TNFR1 were targeted to the trans-Golgi network in stably transfected rat fibroblastic cells. Deletions in the cytoplasmic tails of these chimaeras demonstrated the requirement for the C-terminal sequence of 23 amino acids for this targeting. The 23 amino acid sequence is mostly outside the death domain and contains both an acid patch and a dileucine motif. Interaction of this sequence with membrane traffic adaptor proteins may play an important role in controlling the responses of cells to tumour necrosis factor, since binding of signalling adaptor proteins has only been demonstrated for plasma membrane, and not Golgi-localized, TNFR1.

Lysosome-associated protein transmembrane 4 alpha (LAPTM4 alpha) requires two tandemly arranged tyrosine-based signals for sorting to lysosomes

Lysosome-associated protein transmembrane 4 alpha (LAPTM4 alpha) and homologues comprise a family of conserved proteins, which are found in mammals, insects and nematodes. LAPTM4 alpha functions to regulate the intracellular compartmentalization of amphipathic solutes and possibly the sensitivity of cells toward anthracyclines, antibiotics, ionophores, nucleobases and organic cations. This is similar to the multidrug-resistance phenotype exhibited by cells synthesizing high levels of P-glycoprotein. Accordingly, it is possible that LAPTM4 alpha may be a suitable target for development of novel chemotherapeutic agents. LAPTM4 alpha contains four putative membrane-spanning domains and a 55 amino acid C-terminal region that faces the cytoplasm. Localization of LAPTM4 alpha to endosomes and lysosomes appears to be tightly controlled as transient high-level expression of LAPTM4 alpha in cultured cells resulted in no detectable protein on the cell surface. Mutagenic analysis of the C-terminus of LAPTM4 alpha indicated that two tandomly arranged tyrosine-containing motifs in the cytoplasmic domain are required for efficient localization of LAPTM4 alpha to vesicles containing the lysosomal marker lysosomal glycoprotein 120. Although a number of membrane proteins that localize to endosomes/lysosomes contain more than one independently functioning sorting signal, to our knowledge, LAPTM4 alpha is the first example of a membrane protein that requires two tandemly arranged tyrosine-based sorting signals for efficient localization in these compartments.

CD95 (Fas) expression is regulated by sequestration in the Golgi complex in B-cell lymphoma

The CD95 (Fas) molecule transmits apoptotic signals important in B-cell development and the genesis of B-cell lymphoma. We have investigated the surface and intracellular expression of CD95 in Burkitt's lymphoma (BL) cells, an important non-Hodgkin's lymphoma of B-cell origin. Group I BL cells did not express CD95 at the cell surface, but contained high levels of this receptor in the cytoplasm. In contrast, group III BL cells expressed CD95 intracellularly and at the cell surface. In group I and group III BL cells, cytoplasmic CD95 was localized to the Golgi complex, as assessed by confocal immunofluorescence microscopy and subcellular fractionation followed by immunoblotting. Trafficking through the Golgi complex is regulated by elements within the target protein and cellular sorting mechanisms. CD95 contains candidate signals for interaction with trafficking machinery. Group I BL cells can be induced to upregulate surface expression of CD95 following CD40 ligation and certain group I BL cell lines drift invitro to a group III phenotype, with consequent surface expression of CD95. Taken together, these observations show that CD95 can either be retained in the Golgi complex or exported to the cell surface, and suggest that membrane trafficking has an important and previously unrecognized role in regulating CD95 expression in B lymphocytes.

Role of adaptor complex AP-3 in targeting wild-type and mutated CD63 to lysosomes

CD63 is a lysosomal membrane protein that belongs to the tetraspanin family. Its carboxyterminal cytoplasmic tail sequence contains the lysosomal targeting motif GYEVM. Strong, tyrosine-dependent interaction of the wild-type carboxyterminal tail of CD63 with the AP-3 adaptor subunit mu 3 was observed using a yeast two-hybrid system. The strength of interaction of mutated tail sequences with mu 3 correlated with the degree of lysosomal localization of similarly mutated human CD63 molecules in stably transfected normal rat kidney cells. Mutated CD63 containing the cytosolic tail sequence GYEVI, which interacted strongly with mu 3 but not at all with mu 2 in the yeast two-hybrid system, localized to lysosomes in transfected normal rat kidney and NIH-3T3 cells. In contrast, it localized to the cell surface in transfected cells of pearl and mocha mice, which have genetic defects in genes encoding subunits of AP-3, but to lysosomes in functionally rescued mocha cells expressing the delta subunit of AP-3. Thus, AP-3 is absolutely required for the delivery of this mutated CD63 to lysosomes. Using this AP-3-dependent mutant of CD63, we have shown that AP-3 functions in membrane traffic from the trans-Golgi network to lysosomes via an intracellular route that appears to bypass early endosomes.

Targeting to rhoptry organelles of Toxoplasma gondii involves evolutionarily conserved mechanisms

Intracellular parasites of the phylum Apicomplexa contain specialized rhoptry secretory organelles that have a crucial function in host-cell invasion and establishment of the parasitophorous vacuole. Here we show that localization of the Toxoplasma gondii rhoptry protein ROP2 is dependent on a YEQL sequence in the cytoplasmic tail that binds to micro-chain subunits of T. gondii and mammalian adaptors, and conforms to the YXXstraight phi mammalian sorting motif. Chimaeric reporters, containing the transmembrane domains and cytoplasmic tails of the low-density lipoprotein receptor and of Lamp-1, are sorted to the Golgi or the trans-Golgi network (TGN), and partially to apical microneme organelles of the parasite, respectively. Targeting of these reporters is mediated by YXXstraight phi- and NPXY-type signals. This is the first demonstration of tyrosine-dependent sorting in protozoan parasites, indicating that T. gondii proteins may be targeted to, and involved in biogenesis of, morphologically unique organelles through the use of evolutionarily conserved signals and machinery.

Endolyn is a mucin-like type I membrane protein targeted to lysosomes by its cytoplasmic tail

Endolyn (endolyn-78) is a membrane protein found in lysosomal and endosomal compartments of mammalian cells. Unlike 'classical' lysosomal membrane proteins, such as lysosome-associated membrane protein (lamp)-1, it is also present in a subapical compartment in polarized WIF-B hepatocytes. The structural features that determine sorting of endolyn are unknown. We have identified a rat endolyn cDNA by expression screening. The cDNA encodes a ubiquitously expressed type I membrane protein with a short cytoplasmic tail of 13 amino acids and many putative sites for N- and O-linked glycosylation in the predicted luminal domain. Endolyn is closely related to two human mucin-like proteins, multi-glycosylated core protein (MGC)-24 and CD164 (MGC-24v), expressed in gastric carcinoma cells and bone marrow stromal and haematopoietic precursor cells respectively. The predicted transmembrane and cytoplasmic tail domains of endolyn, as well as parts of its luminal domain, also show some similarities with lamp-1 and lamp-2. Like these and other known lysosomal membrane proteins, endolyn contains a YXXO motif at the C-terminus of its cytoplasmic tail (where O is a bulky hydrophobic amino acid), but with no preceding glycine. Nonetheless, the last ten amino acids of this tail, when transplanted on to human CD8, caused efficient targeting of the chimaeric protein to endosomes and lysosomes in transfected normal rat kidney cells.

The Rous sarcoma virus Env glycoprotein contains a highly conserved motif homologous to tyrosine-based endocytosis signals and displays an unusual internalization phenotype

The cytoplasmic domains of retroviral transmembrane (TM) glycoproteins contain conserved sequence motifs that resemble tyrosine-based (YXXO-type) endocytosis signals. We have previously described a mutant Rous sarcoma virus (RSV) Env protein, Env-mu26, with an L165R mutation in the membrane-spanning domain (MSD) of TM, that exhibited dramatically decreased steady-state surface expression (G. L. Davis and E. Hunter, J. Cell Biol. 105:1191-1203, 1987; P. B. Johnston, J. Y. Dong, and E. Hunter, Virology 206:353-361, 1995). We now demonstrate that the tyrosine of the Y(190)RKM motif in the RSV TM cytoplasmic domain is crucial for the mu26 phenotype and is part of an efficient internalization signal in the context of a mutant MSD. In contrast, despite the presence of the Y(190)RKM motif, wild-type RSV Env is constitutively internalized at a slow rate (1.1%/min) more characteristic of bulk uptake during membrane turnover than of active clustering into endocytic vesicles. The mu26 mutation and two MSD mutations that abrogate palmitoylation of TM resulted in enhanced Env endocytosis indicative of active concentration into coated pits. Surprisingly, an Env-Y190A mutant was apparently excluded from coated pits since its uptake rate of 0.3%/min was significantly below that expected for the bulk rate. We suggest that in RSV Env an inherently functional endocytosis motif is silenced by a counteracting determinant in the MSD that acts to prevent clustering of Env into endocytic vesicles. Mutations in either the cytoplasmic tail or the MSD that inactivate one of the two counteracting signals would thus render the remaining determinant dominant.

MAL, an integral element of the apical sorting machinery, is an itinerant protein that cycles between the trans-Golgi network and the plasma membrane

The MAL proteolipid is a nonglycosylated integral membrane protein found in glycolipid-enriched membrane microdomains. In polarized epithelial Madin-Darby canine kidney cells, MAL is necessary for normal apical transport and accurate sorting of the influenza virus hemagglutinin. MAL is thus part of the integral machinery for glycolipid-enriched membrane-mediated apical transport. At steady state, MAL is predominantly located in perinuclear vesicles that probably arise from the trans-Golgi network (TGN). To act on membrane traffic and to prevent their accumulation in the target compartment, integral membrane elements of the protein-sorting machinery should be itinerant proteins that cycle between the donor and target compartments. To establish whether MAL is an itinerant protein, we engineered the last extracellular loop of MAL by insertion of sequences containing the FLAG epitope or with sequences containing residues that became O-glycosylated within the cells or that displayed biotinylatable groups. The ectopic expression of these modified MAL proteins allowed us to investigate the surface expression of MAL and its movement through different compartments after internalization with the use of a combination of assays, including surface biotinylation, surface binding of anti-FLAG antibodies, neuraminidase sensitivity, and drug treatments. Immunofluorescence and flow cytometric analyses indicated that, in addition to its Golgi localization, MAL was also expressed on the cell surface, from which it was rapidly internalized. This retrieval implies transport through the endosomal pathway and requires endosomal acidification, because it can be inhibited by drugs such as chloroquine, monensin, and NH(4)Cl. Resialylation experiments of surface MAL treated with neuraminidase indicated that approximately 30% of the internalized MAL molecules were delivered to the TGN, probably to start a new cycle of cargo transport. Together, these observations suggest that, as predicted for integral membrane members of the late protein transport machinery, MAL is an itinerant protein cycling between the TGN and the plasma membrane.

The CD3ε Subunit of the TCR Contains Endocytosis Signals

Ligand binding to TCR induces its internalization and cell surface down-modulation. These phenomena contribute to the extinction of activation signals. Due to the multicomponent nature of the TCR-CD3 complex, its internalization may be mediated by one or several of its subunits. Although it has been reported that CD3γ and CD3δ contain endocytosis motifs involved in the internalization of the TCR-CD3 complex, other subunits could also be involved in this process. For instance, CD3ε and CDζ display amino acid sequences reminiscent of internalization motifs. To investigate whether CD3ε bears endocytosis signals, we have analyzed the internalization capacity of a panel of deletion and point mutants of CD3ε that were expressed on the cell surface independently of other TCR-CD3 subunits. Here we report that CD3ε displays endocytosis determinants. These data indicate that CD3ε could contribute to the internalization and cell surface down-regulation of TCR-CD3 complexes. Moreover, the existence of endocytosis signals in this polypeptide could serve to retrieve unassembled CD3ε subunits or partial CD3 complexes from the plasma membrane, thus restricting the expression on the cell surface to fully functional TCR-CD3 complexes.

Identification of a di-leucine motif within the C terminus domain of the Menkes disease protein that mediates endocytosis from the plasma membrane

The protein encoded by the Menkes disease gene (MNK) is localised to the Golgi apparatus and cycles between the trans-Golgi network and the plasma membrane in cultured cells on addition and removal of copper to the growth medium. This suggests that MNK protein contains active signals that are involved in the retention of the protein to the trans-Golgi network and retrieval of the protein from the plasma membrane. Previous studies have identified a signal involved in Golgi retention within transmembrane domain 3 of MNK. To identify a motif sufficient for retrieval of MNK from the plasma membrane, we analysed the cytoplasmic domain, downstream of transmembrane domain 7 and 8. Chimeric constructs containing this cytoplasmic domain fused to the reporter molecule CD8 localised the retrieval signal(s) to 62 amino acids at the C terminus. Further studies were performed on putative internalisation motifs, using site-directed mutagenesis, protein expression, chemical treatment and immunofluorescence. We observed that a di-leucine motif (L1487L1488) was essential for rapid internalisation of chimeric CD8 proteins and the full-length Menkes cDNA from the plasma membrane. We suggest that this motif mediates the retrieval of MNK from the plasma membrane into the endocytic pathway, via the recycling endosomes, but is not sufficient on its own to return the protein to the Golgi apparatus. These studies provide a basis with which to identify other motifs important in the sorting and delivery of MNK from the plasma membrane to the Golgi apparatus.

The tyrosinase tail mediates sorting to the lysosomal compartment in MDCK cells via a di-leucine and a tyrosine-based signal

Tyrosinase is a type I membrane protein found in melanosomes, which are lysosomal-like organelles and specific for pigment cells. A mutation of mouse tyrosinase, platinum (cp), leads to truncation of tyrosinase's cytosolic tail, and results in misrouting to the cell periphery. In this study, we expressed chimeras of wild-type and mutant cytosolic tails of mouse tyrosinase fused to rat lysosome-associated membrane protein-1 luminal and transmembrane domain to study sorting of tyrosinase in Madin-Darby canine kidney cells. The study shows that the mouse tyrosinase cytosolic tail is necessary and sufficient to mediate sorting of a heterologous type I membrane protein to compartments of the lysosomal lineage. Whereas deletions of 7 or 10 C-terminal amino acids of the tail still result in sorting to lysosomes, a deletion mutant corresponding to platinum (cp) tail fails to sort correctly and corroborates the in situ findings in cp homozygous mutant mice. Correct sorting of tyrosinase-lysosome-associated membrane protein-1 chimeras is mediated by the interplay of a di-leucine signal and a tyrosine motif of the Y-X-X-O type.

Specificity of interaction between adaptor-complex medium chains and the tyrosine-based sorting motifs of TGN38 and lgp120

Multiple sorting steps within eukaryotic cells are mediated by tyrosine-based sorting motifs. Motifs conforming to the consensus -YXXO- (where O indicates a bulky hydrophobic residue) have been shown to specify high-efficiency internalization from the plasma membrane, targeting from the plasma membrane to the trans-Golgi network and targeting to lysosomal compartments as well as being involved in basolateral sorting in polarized cells. These motifs are recognized by the medium-chain subunits of heterotetrameric adaptor complexes. Whereas these motifs have been shown to be sufficient to mediate interaction with the mu-chains, we and others have shown that their context is important in determining the affinity of interaction. In this study we have investigated the interaction between the tyrosine motifs of the type-1 integral membrane proteins TGN38 and lgp120 with medium-chain subunits using the yeast two-hybrid system. Whereas the wild-type version of the cytosolic domain of TGN38 interacts with highest affinity with mu2, we show that the cytosolic domain of lgp120 interacts almost exclusively with mu3A. The specificity of binding of tyrosine-based sorting motifs to mu-chains is shown to be highly sensitive to the context in which the motif lies. For example, the -YQTI- motif of lgp120 is effectively non-functional with regard to mu-chain binding when placed in the context of the TGN38 cytosolic domain. Deletion of four amino acids (NLKL) at the extreme C-terminus of TGN38, leaving the YXXO motif as the C-terminus, greatly enhances the affinity of interaction with mu2. Furthermore, addition of these same residues to the extreme C-terminus of lgp120 effectively abolishes the interaction of the cytosolic domain of lgp120 with mu-chains. We also show that the newly identified mu-adaptin-related protein 2 (mu4) only interacts weakly with tyrosine-based sorting motifs.

Efficient trafficking of TGN38 from the endosome to the trans-Golgi network requires a free hydroxyl group at position 331 in the cytosolic domain

TGN38 is one of the few known resident integral membrane proteins of the trans-Golgi network (TGN). Since it cycles constitutively between the TGN and the plasma membrane, TGN38 is ideally suited as a model protein for the identification of post-Golgi trafficking motifs. Several studies, employing chimeric constructs to detect such motifs within the cytosolic domain of TGN38, have identified the sequence 333YQRL336 as an autonomous signal capable of localizing reporter proteins to the TGN. In addition, one group has found that an upstream serine residue, S331, may also play a role in TGN38 localization. However, the nature and degree of participation of S331 in the localization of TGN38 remain uncertain, and the effect has been studied in chimeric constructs only. Here we investigate the role of S331 in the context of full-length TGN38. Mutations that abolish the hydroxyl moiety at position 331 (A, D, and E) lead to missorting of endocytosed TGN38 to the lysosome. Conversely, mutation of S331 to T has little effect on the endocytic trafficking of TGN38. Together, these findings indicate that the S331 hydroxyl group has a direct or indirect effect on the ability of the cytosolic tail of TGN38 to interact with trafficking and/or sorting machinery at the level of the early endosome. In addition, mutation of S331 to either A or D results in increased levels of TGN38 at the cell surface. The results confirm that S331 plays a critical role in the intracellular trafficking of TGN38 and further reveal that TGN38 undergoes a signal-mediated trafficking step at the level of the endosome.