Syntaxin 6 functions in trans-Golgi network vesicle trafficking

A role of LIM kinase 1/cofilin pathway in regulating endocytic trafficking of EGF receptor in human breast cancer cells

We have previously shown that overexpression of LIM kinase1 (LIMK1) resulted in a marked retardation of the internalization of the receptor-mediated endocytic tracer, Texas red-labeled epidermal growth factor (EGF) in low-invasive human breast cancer cell MCF-7. We thereby postulate that LIMK1 signaling plays an important role in the regulation of ligand-induced endocytosis of EGF receptor (EGFR) in tumor cells by reorganizing and influencing actin-filament dynamics. In the present study, we further assessed the effect of wild-type LIMK1, a kinase-deficient dominant negative mutant of LIMK1 (DN-LIMK1) and an active, unphosphorylatable cofilin mutant (S3A cofilin) on internalization of EGF-EGFR in MDA-MB-231, a highly invasive human breast cancer cell line. We demonstrate here that a marked delay in the receptor-mediated internalization of Texas red-labeled EGF was observed in the wild-type LIMK1 transfectants, and that most of the internalized EGF staining were accumulated within transferrin receptor-positive early endosomes even after 30 min internalization. In contrast, the expression of dominant-negative LIMK1 mutant rescued the efficient endocytosis of Texas red-EGF, and large amounts of Texas red-EGF staining already reached LIMPII-positive late endosomes/lysosomal vacuoles after 15 min internalization. We further analyzed the effect of S3A cofilin mutant on EGFR trafficking, and found an efficient delivery of Texas red-EGF into late endosomes/lysosomes at 15-30 min after internalization. Taken together, our novel findings presented in this paper implicate that LIMK1 signaling indeed plays a pivotal role in the regulation of EGFR trafficking through the endocytic pathway in invasive tumor cells.

Difference in distribution of membrane proteins between low- and high-density secretory granules in parotid acinar cells

Secretory granules (SGs) are considered to be generated as immature granules and to mature by condensation of their contents. In this study, SGs of parotid gland were separated into low-, medium-, and high-density granule fractions by Percoll-density gradient centrifugation, since it was proposed that the density corresponds to the degree of maturation. The observation with electron microscopy showed that granules in the three fractions were very similar. The average diameter of high-density granules was a little but significantly larger than that of low-density granules. Although the three fractions contained amylase, suggesting that they are all SGs, distribution of membrane proteins was markedly different. Syntaxin6 and VAMP4 were localized in the low-density granule fraction, while VAMP2 was concentrated in the high-density granule fraction. Immunoprecipitation with anti-syntaxin6 antibody caused coprecipitation of VAMP2 from the medium-density granule fraction without solubilization, but not from Triton X-100-solubilized fraction, while VAMP4 was coprecipitated from both fractions. Therefore, VAMP2 is present on the same granules, but is separated from syntaxin6 and VAMP4, which are expected to be removed from immature granules. These results suggest that the medium-density granules are intermediates from low- to high-density granules, and that the membrane components of SGs dynamically change by budding and fusion during maturation.

Synaptotagmin IV is necessary for the maturation of secretory granules in PC12 cells

In neuroendocrine PC12 cells, immature secretory granules (ISGs) mature through homotypic fusion and membrane remodeling. We present evidence that the ISG-localized synaptotagmin IV (Syt IV) is involved in ISG maturation. Using an in vitro homotypic fusion assay, we show that the cytoplasmic domain (CD) of Syt IV, but not of Syt I, VII, or IX, inhibits ISG homotypic fusion. Moreover, Syt IV CD binds specifically to ISGs and not to mature secretory granules (MSGs), and Syt IV binds to syntaxin 6, a SNARE protein that is involved in ISG maturation. ISG homotypic fusion was inhibited in vivo by small interfering RNA–mediated depletion of Syt IV. Furthermore, the Syt IV CD, as well as Syt IV depletion, reduces secretogranin II (SgII) processing by prohormone convertase 2 (PC2). PC2 is found mostly in the proform, suggesting that activation of PC2 is also inhibited. Granule formation, and the sorting of SgII and PC2 from the trans-Golgi network into ISGs and MSGs, however, is not affected. We conclude that Syt IV is an essential component for secretory granule maturation.

Complexes of syndapin II with dynamin II promote vesicle formation at the trans-Golgi network

The role of dynamin and so-called accessory proteins in endocytosis is well established. However, molecular details of the function(s) of dynamin II at the Golgi are largely unclear. We demonstrate that the ubiquitously expressed syndapin II isoform interacts with the proline-rich domain (PRD) of dynamin II through its Src-homology 3 (SH3) domain. Co-immunoprecipitation of endogenous syndapin II and dynamin II, and successful reconstitutions of such complexes at membranes in COS-7 cells, show the in vivo relevance of the interaction. Syndapin II can associate with Golgi membranes and this association increases upon Golgi exit block. Brefeldin A treatment clearly shows that the observed perinuclear localization of syndapin II co-localizing with syntaxin 6 reflects the Golgi complex and that it requires functional integrity of the Golgi. Syndapins are crucial for Golgi vesicle formation because anti-syndapin antibodies, used either in in vitro reconstitutions or in living cells, inhibited this process. Both types of assays additionally revealed the essential role of syndapin II SH3 interactions with the dynamin II PRD in vesicle formation. An excess of the syndapin SH3 domain strongly inhibited budding from Golgi membranes in vitro. Likewise, overexpression of the syndapin SH3 domain or of a dynamin II variant incapable of associating with syndapin II (dynamin IIΔPRD) impaired trafficking of vesicular stomatitis virus glycoprotein (VSVG)-GFP in vivo. By contrast, full-length syndapin II-l had no negative effect, and instead promoted VSVG-GFP export from the Golgi. Importantly, a cytosolic fraction containing endogenous syndapin-dynamin complexes was sufficient to promote vesicle formation from Golgi membranes in a syndapin-dependent manner. Thus, syndapin-dynamin complexes are crucial and sufficient to promote vesicle formation from the trans-Golgi network.

Sustained neurotensin exposure promotes cell surface recruitment of NTS2 receptors

In this study, we investigated whether persistent agonist stimulation of NTS2 receptors gives rise to down-regulation, in light of reports that their activation induced long-lasting effects. To address this issue, we incubated COS-7 cells expressing the rat NTS2 with neurotensin (NT) for up to 24 h and measured resultant cell surface [125I]-NT binding. We found that NTS2-expressing cells retained the same surface receptor density despite efficient internalization mechanisms. This preservation was neither due to NTS2 neosynthesis nor recycling since it was not blocked by cycloheximide or monensin. However, it appeared to involve translocation of spare receptors from internal stores, as NT induced NTS2 migration from trans-Golgi network to endosome-like structures. This stimulation-induced regulation of cell surface NTS2 receptors was even more striking in rat spinal cord neurons. Taken together, these results suggest that sustained NTS2 activation promotes recruitment of intracellular receptors to the cell surface, thereby preventing functional desensitization.

Ubiquitously expressed secretory carrier membrane proteins (SCAMPs) 1-4 mark different pathways and exhibit limited constitutive trafficking to and from the cell surface

Secretory carrier membrane proteins (SCAMPs) 1-4 are ubiquitously expressed and are major components of the eukaryotic cell surface recycling system. We investigated whether different SCAMPs function along distinct pathways and whether they behave like itinerant cargoes or less mobile trafficking machinery. In NRK cells, we show by immunofluorescence microscopy that different SCAMPs are concentrated mostly adjacent to one another in the trans-Golgi network and endosomal recycling compartment. By immunoelectron microscopy, they were shown to be close neighbors on individual transferrin-containing endosomal elements and on the plasma membrane. Within the internal endosomal network, SCAMPs are located distal to rab5-containing endosomes, and the individual isoforms appear to mark pathways that diverge from the constitutive recycling route and that may be distinguished by different adaptors, especially AP-1 and AP-3. Based on comparisons of SCAMP localization with endocytosed transferrin as well as live imaging of GFP-SCAMP1, we show that SCAMPs are concentrated within the motile population of early and recycling endosomes; however, they are not detected in newly formed transferrin-containing endocytic vesicles or in vesicles recycling transferrin to the surface. Also, they are not detected in constitutive secretory carriers marked by VSV-G. Their minimal recycling to the surface is reflected by their inability to relocate to the plasma membrane upon inhibition of endocytosis. Thus SCAMPs exhibit limited exchange between the cell surface and internal recycling systems, but within each of these sites, they form a mosaic with individual isoforms marking distinct pathways and potentially functioning as trafficking machinery at sites of vesicle formation and fusion. A corollary of these findings is that early endosomes exist as a distinct SCAMP-containing compartment and are not formed de novo by fusion of endocytic vesicles.

Syntaxin 9 is Enriched in Skin Hair Follicle Epithelium and Interacts With the Epidermal Growth Factor Receptor

We describe a novel syntaxin family member, syntaxin 9 (Syn 9), which does not possess a typical C-terminal hydrophobic tail anchor. Syn 9 has, however, a Q-SNARE domain and an overall homology to syntaxins (with the highest overall homology with mammalian syntaxin 11). Syn 9 is enriched in some epithelial cells, particularly that of the stomach lining and the skin. At the skin, it is found in the epidermal layers as well as structures associated with hair follicles. A biochemical interaction screen revealed that Syn 9 interacts specifically with the epidermal growth factor (EGF) receptor. Overexpression of Syn 9 perturbed EGF receptor endocytosis but does not appear to affect the internalization of the transferrin receptor. Syn 9 may therefore have a role in EGF receptor transport and signaling in certain epithelial cell types.

Mannose 6-phosphate receptors, Niemann-Pick C2 protein, and lysosomal cholesterol accumulation

Niemann-Pick disease type C (NPC), caused by mutations in the NPC1 gene or the NPC2 gene, is characterized by the accumulation of unesterified cholesterol and other lipids in endo/lysosomal compartments. NPC2 is a small, soluble, lysosomal protein that is targeted to this compartment via a mannose 6-phosphate-inhibitable pathway. To obtain insight into the roles of mannose 6-phosphate receptors (MPRs) in NPC2 targeting, we here examine the trafficking and function of NPC2 in fibroblast lines deficient in one or both of the two MPRs, MPR46 and MPR300. We demonstrate that either MPR alone is sufficient to transport NPC2 to the endo/lysosomal compartment, although MPR300 seems to be more efficient than MPR46. In the absence of both MPRs, NPC2 is secreted into the culture medium, and only a small amount of intracellular NPC2 can be detected, mainly in the endoplasmic reticulum. This leads to massive accumulation of unesterified cholesterol in the endo/lysosomal compartment of the MPR46/300-deficient fibroblasts, a phenotype similar to that of the NPC patient fibroblasts. In addition, we observed an upregulation of NPC1 protein and mRNA in the MPR-double-deficient cells. Taken together, our results suggest that the lysosomal targeting of NPC2 is strictly dependent on MPRs in fibroblasts.

Characterization of the Role of the Rab GTPase-activating Protein AS160 in Insulin-regulated GLUT4 Trafficking

Insulin stimulates the translocation of the glucose transporter GLUT4 from intracellular vesicles to the plasma membrane. In the present study we have conducted a comprehensive proteomic analysis of affinity-purified GLUT4 vesicles from 3T3-L1 adipocytes to discover potential regulators of GLUT4 trafficking. In addition to previously identified components of GLUT4 storage vesicles including the insulin-regulated aminopeptidase insulin-regulated aminopeptidase and the vesicle soluble N-ethylmaleimide factor attachment protein (v-SNARE) VAMP2, we have identified three new Rab proteins, Rab10, Rab11, and Rab14, on GLUT4 vesicles. We have also found that the putative Rab GTPase-activating protein AS160 (Akt substrate of 160 kDa) is associated with GLUT4 vesicles in the basal state and dissociates in response to insulin. This association is likely to be mediated by the cytosolic tail of insulin-regulated aminopeptidase, which interacted both in vitro and in vivo with AS160. Consistent with an inhibitory role of AS160 in the basal state, reduced expression of AS160 in adipocytes using short hairpin RNA increased plasma membrane levels of GLUT4 in an insulin-independent manner. These findings support an important role for AS160 in the insulin regulated trafficking of GLUT4.

NH2-terminal signals in ATP7B Cu-ATPase mediate its Cu-dependent anterograde traffic in polarized hepatic cells

Cu is an essential cofactor of cellular proteins but is toxic in its free state. The hepatic Cu-ATPase ATP7B has two functions in Cu homeostasis: it loads Cu+ onto newly synthesized apoceruloplasmin in the secretory pathway, thereby activating the plasma protein; and it participates in the excretion of excess Cu+ into the bile. To carry out these two functions, the membrane protein responds to changes in intracellular Cu levels by cycling between the Golgi and apical region. We used polarized hepatic WIF-B cells and high-resolution confocal microscopy to map the itinerary of endogenous and exogenous ATP7B under different Cu conditions. In Cu-depleted cells, ATP7B resided in a post-trans-Golgi network compartment that also contained syntaxin 6, whereas in Cu-loaded cells, the protein relocated to unique vesicles very near to the apical plasma membrane as well as the membrane itself. To determine the role of ATP7B's cytoplasmic NH2 terminus in regulating its intracellular movements, we generated seven mutations/deletions in this large [approximately 650 amino acid (AA)] domain and analyzed the Cu-dependent behavior of the mutant ATP7B proteins in WIF-B cells. Truncation of the ATP7B NH2 terminus up to the fifth copper-binding domain (CBD5) yielded an active ATPase that was insensitive to cellular Cu levels and constitutively trafficked to the opposite (basolateral) plasma membrane domain. Fusion of the NH2-terminal 63 AA of ATP7B to the truncated protein restored both its Cu responsiveness and correct intracellular targeting. These results indicate that important targeting information is contained in this relatively short sequence, which is absent from the related CuATPase, ATP7A.

Syntaxin 5 interacts specifically with presenilin holoproteins and affects processing of betaAPP in neuronal cells

The specific roles of syntaxin 5 (Syx 5) in the interaction with presenilin (PS) and the accumulation of beta-amyloid precursor protein (betaAPP), as well as the secretion of beta-amyloid peptide (Abeta peptide) were examined in NG108-15 cells. Syx 5, which localizes from the endoplasmic reticulum (ER) to the Golgi, bound to PS holoproteins, while the other Syxs studied did not. Among familial Alzheimer's disease (FAD)-linked PS mutants, PS1deltaE9, which lacks the endoproteolytic cleavage site, showed markedly decreased binding to Syx 5. The interaction domains in Syx 5 were mapped to the transmembrane region and to the cytoplasmic region containing the alpha-helical domains, which are distinct from the H3 (SNARE motif). Among all of the Syxs examined, only overexpression of Syx 5 resulted in the accumulation of betaAPP in the ER to cis-Golgi compartment, an attenuation of the amount of the C-terminal fragment (APP-CTF) of betaAPP, and a reduction in the secretion of Abeta peptides. Furthermore, co-expression of Syx 5 with C99 resulted in an increase in APP-CTF and suppressed Abeta secretion. Taken together, these results indicate that Syx 5 may play a specific role in the modulation of processing and/or trafficking of FAD-related proteins in neuronal cells by interaction with PS holoproteins in the early secretory compartment of neuronal cells.

Murine coronavirus requires lipid rafts for virus entry and cell-cell fusion but not for virus release

Thorp and Gallagher first reported that depletion of cholesterol inhibited virus entry and cell-cell fusion of mouse hepatitis virus (MHV), suggesting the importance of lipid rafts in MHV replication (E. B. Thorp and T. M. Gallagher, J. Virol. 78:2682-2692, 2004). However, the MHV receptor is not present in lipid rafts, and anchoring of the MHV receptor to lipid rafts did not enhance MHV infection; thus, the mechanism of lipid rafts involvement is not clear. In this study, we defined the mechanism and extent of lipid raft involvement in MHV replication. We showed that cholesterol depletion by methyl beta-cyclodextrin or filipin did not affect virus binding but reduced virus entry. Furthermore, MHV spike protein bound to nonraftraft membrane at 4 degrees C but shifted to lipid rafts at 37 degrees C, indicating a redistribution of membrane following virus binding. Thus, the lipid raft involvement in MHV entry occurs at a step following virus binding. We also found that the viral spike protein in the plasma membrane of the infected cells was associated with lipid rafts, whereas that in the Golgi membrane, where MHV matures, was not. Moreover, the buoyant density of the virion was not changed when MHV was produced from the cholesterol-depleted cells, suggesting that MHV does not incorporate lipid rafts into the virion. These results indicate that MHV release does not involve lipid rafts. However, MHV spike protein has an inherent ability to associate with lipid rafts. Correspondingly, cell-cell fusion induced by MHV was retarded by cholesterol depletion, consistent with the association of the spike protein with lipid rafts in the plasma membrane. These findings suggest that MHV entry requires specific interactions between the spike protein and lipid rafts, probably during the virus internalization step.

Vesicle-associated membrane protein 4, a positional candidate gene on 1q24-q25, is not associated with type 2 diabetes in the Old Order Amish

OBJECTIVE

The vesicle-associated membrane protein-4 (VAMP4) gene is an excellent type 2 diabetes (T2DM) positional candidate gene. It is located on chromosome 1q24-q25, a region of linkage to T2DM in the Amish and several other populations. VAMP4 is expressed in liver and skeletal muscle and participates in intracellular trafficking of secreted and membrane-associated proteins.

DESIGN AND METHODS

We sequenced VAMP4 in 20 Amish subjects. Polymorphisms in and around VAMP4 were genotyped in 65 Amish subjects with T2DM, 64 subjects with impaired glucose homeostasis (IGH), and 126 normal glucose tolerant controls, as well as in an expanded set of 749 participants of the Amish Family Diabetes Study for whom glucose and insulin levels during an oral glucose tolerance test (OGTT) and other quantitative traits related to diabetes were available. Case-control and quantitative trait association analyses were performed.

RESULTS

We found three common non-coding intragenic polymorphisms: a 23bp insertion/deletion (I/D) in the 5' untranslated region (UTR) in exon 1 at position 73127, and G35319T and C335296T single nucleotide polymorphisms (SNPs) in the 3' UTR (NCBI Accession No. Z98751). The two 3' UTR SNPs were in complete linkage disequilibrium (LD) and both were in strong LD with the exon 1 I/D polymorphism (|D'|=0.82). Similarly, three extragenic flanking SNPs (rs978985, rs203255, and rs1023479) showed moderate LD with the neighboring intragenic SNPs (|D'|=0.23-0.69). None of the SNPs individually nor any of the 2-, 3-, 4-, or 5-polymorphism haplotypes were associated with T2DM or IGH. The exon 1 I/D polymorphism was not associated with significant differences in mean fasting or stimulated glucose or insulin levels during an OGTT or other diabetes-related quantitative traits in the expanded set of 749 subjects.

CONCLUSION

Variation in VAMP4 does not significantly influence risk of T2DM or IGH in the Amish.

Secretory carrier membrane proteins interact and regulate trafficking of the organellar (Na+,K+)/H+ exchanger NHE7

The mammalian (Na(+),K(+))/H(+) exchanger NHE7 resides chiefly in the trans-Golgi network (TGN) and post-Golgi vesicles where it is thought to contribute to organellar pH homeostasis. However, the mechanisms that underlie the targeting and regulation of NHE7 are unknown. To gain insight into these processes, yeast two-hybrid methodology was used to screen a human brain cDNA library for proteins that interact with the cytoplasmic C-terminus of NHE7. One binding partner we identified was SCAMP2, a member of the secretory carrier membrane protein (SCAMP) gene family. Direct association of these two proteins was further supported by co-immunolocalization and co-immunoprecipitation analyses using transfected cells, by their co-sedimentation in membrane fractions resolved on sucrose density gradients, and by in vitro protein binding assays. Other members of the SCAMP family, such as SCAMP1 and SCAMP5, also associated with NHE7. The majority of the NHE7-SCAMP complexes accumulated at the TGN, but a minor fraction also resided in recycling vesicles. Biochemical analyses indicated that the C-terminal cytoplasmic tail of NHE7 bound preferentially to a highly conserved cytoplasmic loop between the second and the third transmembrane segments (TM2-TM3 loop) of SCAMP2. A deletion mutant of SCAMP2 lacking this region (SCAMP2/Delta184-208) bound weakly to NHE7, but caused a significant fraction of NHE7 and wild-type SCAMP2 to redistribute to a pool of scattered recycling vesicles without noticeably affecting the location of other resident TGN (syntaxin 6) or Golgi cisternae (GM130) proteins. Conversely, a GFP-tagged TM2-TM3 construct of SCAMP2 interacted with NHE7, but also led to the redistribution of NHE7 to dispersed vesicular structures. We propose a model wherein SCAMPs participate in the shuttling of NHE7 between recycling vesicles and the TGN.

MARCH-II is a syntaxin-6-binding protein involved in endosomal trafficking

Membrane-associated RING-CH (MARCH) is a recently identified member of the mammalian E3 ubiquitin ligase family, some members of which down-regulate the expression of immune recognition molecules. Here, we have identified MARCH-II, which is ubiquitously expressed and localized to endosomal vesicles and the plasma membrane. Immunoprecipitation and in vitro binding studies established that MARCH-II directly associates with syntaxin 6. Overexpression of MARCH-II resulted in redistribution of syntaxin 6 as well as some syntaxin-6-interacting soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) into the MARCH-II-positive vesicles. In addition, the retrograde transport of TGN38 and a chimeric version of furin to trans-Golgi network (TGN) was perturbed--without affecting the endocytic degradative and biosynthetic secretory pathways--similar to effects caused by a syntaxin 6 mutant lacking the transmembrane domain. MARCH-II overexpression markedly reduced the cell surface expression of transferrin (Tf) receptor and Tf uptake and interfered with delivery of internalized Tf to perinuclear recycling endosomes. Depletion of MARCH-II by small interfering RNA perturbed the TGN localization of syntaxin 6 and TGN38/46. MARCH-II is thus likely a regulator of trafficking between the TGN and endosomes, which is a novel function for the MARCH family.

PDZ proteins retain and regulate membrane transporters in polarized epithelial cell membranes

The plasma membrane of epithelial cells is subdivided into two physically separated compartments known as the apical and basolateral membranes. To obtain directional transepithelial solute transport, membrane transporters (i.e., ion channels, cotransporters, exchangers, and ion pumps) need to be targeted selectively to either of these membrane domains. In addition, the transport properties of an epithelial cell will be maintained only if these membrane transporters are retained and properly regulated in their specific membrane compartments. Recent reports have indicated that PDZ domain-containing proteins play a dual role in these processes and, in addition, that different apical and basolateral PDZ proteins perform similar tasks in their respective membrane domains. First, although PDZ-based interactions are dispensable for the biosynthetic targeting to the proper membrane domain, the PDZ network ensures that the membrane proteins are efficiently retained at the cell surface. Second, the close spatial positioning of functionally related proteins (e.g., receptors, kinases, channels) into a signal transduction complex (transducisome) allows fast and efficient control of membrane transport processes.

Identification of conserved amino acids N-terminal of the PKC epsilon C1b domain crucial for protein kinase C epsilon-mediated induction of neurite outgrowth

We have shown previously that protein kinase C (PKC) epsilon can induce neurite outgrowth independently of its catalytic activity via a region encompassing its C1 domains. In this study we aimed at identifying specific amino acids in this region crucial for induction of neurite outgrowth. Deletion studies demonstrated that only 4 amino acids N-terminal and 20 residues C-terminal of the C1 domains are necessary for neurite induction. The corresponding regions from all other novel isoforms but not from PKCalpha were also neuritogenic. Further mutation studies indicated that amino acids immediately N-terminal of the C1a domain are important for plasma membrane localization and thereby for neurite induction. Addition of phorbol ester made this construct neurite-inducing. However, mutation of amino acids flanking the C1b domain reduced the neurite-inducing capacity even in the presence of phorbol esters. Sequence alignment highlighted an 8-amino acid-long sequence N-terminal of the C1b domain that is conserved in all novel PKC isoforms. Specifically, we found that mutations of either Phe-237, Val-239, or Met-241 in PKCepsilon completely abolished the neurite-inducing capacity of PKCepsilon C1 domains. Phorbol ester treatment could not restore neurite induction but led to a plasma membrane translocation. Furthermore, if 12 amino acids were included N-terminal of the C1b domain, the C1a domain was dispensable for neurite induction. In conclusion, we have identified a highly conserved sequence N-terminal of the C1b domain that is crucial for neurite induction by PKCepsilon, indicating that this motif may be critical for some morphological effects of PKC.

Recycling of the dense-core vesicle membrane protein phogrin in Min6 beta-cells

Phogrin (IA2-beta) is an integral membrane protein of dense-core vesicles in neuroendocrine cells. We have examined the recycling of endogenous phogrin following exocytosis in insulin secreting Min6 beta-cells by monitoring stimulus dependent-uptake of antibodies directed against the lumenal domain of the protein. While low levels of internalized phogrin accumulated in LAMP1-positive lysosomes, more than 35% of internalized phogrin recycled back to an insulin-positive compartment and could return to the cell surface during a second exocytic stimulation. The recycling phogrin transited a syntaxin 6-positive compartment but did not appear to go through the TGN38-positive trans Golgi network. The results suggest a model in which secretory membrane components can recycle from the endosomal system to immature secretory granules without interaction with the major portion of the TGN.

Internalization and trafficking of neurotensin via NTS3 receptors in HT29 cells

The neurotensin receptor-3, originally identified as sortilin, is unique among neuropeptide receptors in that it is a single trans-membrane domain, type I receptor. To gain insight into the functionality of neurotensin receptor-3, we examined the neurotensin-induced intracellular trafficking of this receptor in the human carcinoma cell line HT29, which expresses both neurotensin receptor-1 and -3 sub-types. At steady state, neurotensin receptor-3 was found by sub-cellular fractionation and electron microscopic techniques to be predominantly associated with intracellular elements. A small proportion (approximately 10%) was associated with the plasma membrane, but a significant amount (approximately 25%) was observed inside the nucleus. Following stimulation with neurotensin (NT), neurotensin/neurotensin receptor-3 complexes were internalized via the endosomal pathway. This internalization entailed no detectable loss of cell surface receptors, suggesting compensation through either recycling or intracellular receptor recruitment mechanisms. Internalized ligand and receptors were both sorted to the pericentriolar recycling endosome/Trans-Golgi Network (TGN), indicating that internalized neurotensin is sorted to this compartment via neurotensin receptor-3. Furthermore, within the Trans-Golgi Network, neurotensin was bound to a lower molecular form of the receptor than at the cell surface or in early endosomes, suggesting that signaling and transport functions of neurotensin receptor-3 may be mediated through different molecular forms of the protein. In conclusion, the present work suggests that the neurotensin receptor-3 exists in two distinct forms in HT29 cells: a high molecular weight, membrane-associated form responsible for neurotensin endocytosis from the cell surface and a lower molecular weight, intracellular form responsible for the sorting of internalized neurotensin to the Trans-Golgi Network.

Role of the mammalian retromer in sorting of the cation-independent mannose 6-phosphate receptor

The cation-independent mannose 6-phosphate receptor (CI-MPR) mediates sorting of lysosomal hydrolase precursors from the TGN to endosomes. After releasing the hydrolase precursors into the endosomal lumen, the unoccupied receptor returns to the TGN for further rounds of sorting. Here, we show that the mammalian retromer complex participates in this retrieval pathway. The hVps35 subunit of retromer interacts with the cytosolic domain of the CI-MPR. This interaction probably occurs in an endosomal compartment, where most of the retromer is localized. In particular, retromer is associated with tubular–vesicular profiles that emanate from early endosomes or from intermediates in the maturation from early to late endosomes. Depletion of retromer by RNA interference increases the lysosomal turnover of the CI-MPR, decreases cellular levels of lysosomal hydrolases, and causes swelling of lysosomes. These observations indicate that retromer prevents the delivery of the CI-MPR to lysosomes, probably by sequestration into endosome-derived tubules from where the receptor returns to the TGN.

Association of gamma-secretase with lipid rafts in post-Golgi and endosome membranes

Alzheimer's disease-associated beta-amyloid peptides (Abeta) are generated by the sequential proteolytic processing of amyloid precursor protein (APP) by beta- and gamma-secretases. There is growing evidence that cholesterol- and sphingolipid-rich membrane microdomains are involved in regulating trafficking and processing of APP. BACE1, the major beta-secretase in neurons is a palmitoylated transmembrane protein that resides in lipid rafts. A subset of APP is subject to amyloidogenic processing by BACE1 in lipid rafts, and this process depends on the integrity of lipid rafts. Here we describe the association of all four components of the gamma-secretase complex, namely presenilin 1 (PS1)-derived fragments, mature nicastrin, APH-1, and PEN-2, with cholesterol-rich detergent insoluble membrane (DIM) domains of non-neuronal cells and neurons that fulfill the criteria of lipid rafts. In PS1(-/-)/PS2(-/-) and NCT(-/-) fibroblasts, gamma-secretase components that still remain fail to become detergent-resistant, suggesting that raft association requires gamma-secretase complex assembly. Biochemical evidence shows that subunits of the gamma-secretase complex and three TGN/endosome-resident SNAREs cofractionate in sucrose density gradients, and show similar solubility or insolubility characteristics in distinct non-ionic and zwitterionic detergents, indicative of their co-residence in membrane microdomains with similar protein-lipid composition. This notion is confirmed using magnetic immunoisolation of PS1- or syntaxin 6-positive membrane patches from a mixture of membranes with similar buoyant densities following Lubrol WX extraction or sonication, and gradient centrifugation. These findings are consistent with the localization of gamma-secretase in lipid raft microdomains of post-Golgi and endosomes, organelles previously implicated in amyloidogenic processing of APP.

Participation of the syntaxin 5/Ykt6/GS28/GS15 SNARE complex in transport from the early/recycling endosome to the trans-Golgi network

An in vitro transport assay, established with a modified Shiga toxin B subunit (STxB) as a marker, has proved to be useful for the study of transport from the early/recycling endosome (EE/RE) to the trans-Golgi network (TGN). Here, we modified this assay to test antibodies to all known soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) that have been shown to localize in the Golgi and found that syntaxin 5, GS28, Ykt6, and GS15 antibodies specifically inhibited STxB transport. Because syntaxin 5, GS28, Ykt6, and GS15 exist as a unique SNARE complex, our observation indicates that these four SNAREs function as a complex in EE/RE-TGN transport. The importance of GS15 in EE/RE-TGN transport was further demonstrated by a block in recombinant STxB transport in HeLa cells when GS15 expression was knocked down by its small interfering iRNA. Morphological analyses showed that some GS15 and Ykt6 were redistributed from the Golgi to the endosomes when the recycling endosome was perturbed by SNX3-overexpression, suggesting that GS15 and Ykt6 might cycle between the endosomes and the Golgi apparatus. Further studies indicated that syntaxin 5 and syntaxin 16 exerted their role in EE/RE-TGN transport in an additive manner. The kinetics of inhibition exhibited by syntaxin 16 and syntaxin 5 antibodies is similar.

Identification of an amino acid residue in the protein kinase C C1b domain crucial for its localization to the Golgi network

Protein kinase C (PKC) isoforms have been reported to be targeted to the Golgi complex via their C1 domains. We have shown recently that the regulatory domain of PKC induces apoptosis in neuroblastoma cells and that this effect is correlated to Golgi localization via the C1b domain. This study was designed to identify specific residues in the C1 domains that mediate Golgi localization. We demonstrate that the isolated C1b domains from PKCalpha, -delta, -epsilon, -eta, and - are targeted to the Golgi complex, whereas the corresponding C1a domains localize throughout the cell. Sequence alignment showed that amino acid residues corresponding to Glu-246 and Met-267 in PKC are conserved among C1b but absent from C1a domains. Mutation of Met-267, but not of Glu-246, to glycine abolished the Golgi localization of the isolated C1b domain and the regulatory domain of PKC. The mutated PKC regulatory domain constructs lacking Golgi localization were unable to induce apoptosis, suggesting a direct correlation between Golgi localization and apoptotic activity of PKC regulatory domain. Mutation of analogous residues in the C1b domain of PKCepsilon abrogated its Golgi localization, demonstrating that this effect is not restricted to one PKC isoform. The abolished Golgi localization did not affect neurite induction by PKCepsilon. However, the PKCepsilon mutant did not relocate to the Golgi network in response to ceramide and ceramide did not suppress the neurite-inducing capacity of the protein. Thus, the specific mutations in the C1b domain influence both the localization and function of full-length PKCepsilon.

Retention and stimulus-dependent recycling of dense core vesicle content in neuroendocrine cells

We have used fluorescence imaging of individual exocytic events in combination with immunogold electron microscopy and FM1-43 photoconversion to study the stimulus-dependent recycling of dense core vesicle content in isolated rat pituitary lactotrophs. Secretory stimulation with high external [K+] resulted in 100 exocytic sites per cell that were labeled by extracellular antibodies against the peptide hormone prolactin. Morphological analysis demonstrated that the prolactin was retained and internalized in intact dense cores. Vesicles containing non-secreted, internalized prolactin did not colocalize with DiI-LDL that had been chased into lysosomes but did transiently colocalize with internalized transferrin. The recycling vesicles also trafficked through a syntaxin 6-positive compartment but not the TGN38-positive trans-Golgi. Recycling vesicles, which returned to the cell surface in a slow basal manner, could also be stimulated to undergo exocytosis with a high release probability during subsequent exocytic stimulation with external K+. These studies suggest a functional role for recycling vesicles that retain prolactin.

A Mutant Impaired in SNARE Complex Dissociation Identifies the Plasma Membrane as First Target of Synaptobrevin 2

Membrane fusion depends on the formation of a complex of four SNARE motifs, three that bear a central glutamine and are localized in the target membrane (t-SNARE) and one that bears an arginine and is localized in the donor vesicle (v-SNARE). We have characterized the arginine 56 to proline mutant (R56P) of synaptobrevin-2 (Sb). SbR56P was blocked at the plasma membrane in association with the endogenous plasma membrane t-SNARE due to an inhibition of SNARE complex dissociation, suggesting that the plasma membrane is its first target. Cell surface blockade of SbR56P could be rescued by coexpression of synaptophysin, a partner of Sb. Sb was blocked at the plasma membrane but SNARE complexes were unaffected in cells expressing defective dynamin, indicating that the phenotype of SbR56P was not due to an internalization defect. When expressed in neurons, SbR56P localized both to axonal and dendritic plasma membranes, showing that both domains are initial targets of Sb. The R56P mutation affects a highly conserved position in v-SNAREs, and might thus provide a general tool for identifying their first target membranes.

3-Methyladenine specifically inhibits retrograde transport of cation-independent mannose 6-phosphate/insulin-like growth factor II receptor from the early endosome to the TGN

3-Methyladenine (3-MA), a well-known inhibitor of autophagic sequestration, can also prevent class III phosphatidylinositide (PI) 3-kinase activity, which is required for many processes in endosomal membrane trafficking. Although much is known about the effects of other PI 3-kinase inhibitors, such as wortmannin and LY294002, on endosomal membrane trafficking, little is known about those of 3-MA. Here we show that the treatment of cells with 3-MA results in a specific redistribution of the cation-independent mannose 6-phosphate/insulin-like growth factor II receptor (MPR300) from the trans-Golgi network (TGN) to early/recycling endosomal compartments containing internalized transferrin. Importantly, in contrast to wortmannin and LY294002, 3-MA did not cause the enlargement of late endosomal/lysosomal compartments. The results suggest that the effect of 3-MA is restricted to the retrieval of MPR300 from early/recycling endosomes.

Syntaxin-6 SNARE involvement in secretory and endocytic pathways of cultured pancreatic beta-cells

In pancreatic beta-cells, the syntaxin 6 (Syn6) soluble N-ethylmaleimide-sensitive factor attachment protein receptor is distributed in the trans-Golgi network (TGN) (with spillover into immature secretory granules) and endosomes. A possible Syn6 requirement has been suggested in secretory granule biogenesis, but the role of Syn6 in live regulated secretory cells remains unexplored. We have created an ecdysone-inducible gene expression system in the INS-1 beta-cell line and find that induced expression of a membrane-anchorless, cytosolic Syn6 (called Syn6t), but not full-length Syn6, causes a prominent defect in endosomal delivery to lysosomes, and the TGN, in these cells. The defect occurs downstream of the endosomal branchpoint involved in transferrin recycling, and upstream of the steady-state distribution of mannose 6-phosphate receptors. By contrast, neither acquisition of stimulus competence nor the ultimate size of beta-granules is affected. Biosynthetic effects of dominant-interfering Syn6 seem limited to slowed intragranular processing to insulin (achieving normal levels within 2 h) and minor perturbation of sorting of newly synthesized lysosomal proenzymes. We conclude that expression of the Syn6t mutant slows a rate-limiting step in endosomal maturation but provides only modest and potentially indirect interference with regulated and constitutive secretory pathways, and in TGN sorting of lysosomal enzymes.

Localization of the AP-3 adaptor complex defines a novel endosomal exit site for lysosomal membrane proteins

The adaptor protein (AP) 3 adaptor complex has been implicated in the transport of lysosomal membrane proteins, but its precise site of action has remained controversial. Here, we show by immuno-electron microscopy that AP-3 is associated with budding profiles evolving from a tubular endosomal compartment that also exhibits budding profiles positive for AP-1. AP-3 colocalizes with clathrin, but to a lesser extent than does AP-1. The AP-3- and AP-1-bearing tubular compartments contain endocytosed transferrin, transferrin receptor, asialoglycoprotein receptor, and low amounts of the cation-independent mannose 6-phosphate receptor and the lysosome-associated membrane proteins (LAMPs) 1 and 2. Quantitative analysis revealed that of these distinct cargo proteins, only LAMP-1 and LAMP-2 are concentrated in the AP-3-positive membrane domains. Moreover, recycling of endocytosed LAMP-1 and CD63 back to the cell surface is greatly increased in AP-3-deficient cells. Based on these data, we propose that AP-3 defines a novel pathway by which lysosomal membrane proteins are transported from tubular sorting endosomes to lysosomes.

Role of acetylated human AP-endonuclease (APE1/Ref-1) in regulation of the parathyroid hormone gene

The human AP-endonuclease (APE1/Ref-1), a multifunctional protein central to repairing abasic sites and single-strand breaks in DNA, also plays a role in transcriptional regulation. Besides activating some transcription factors, APE1 is directly involved in Ca2+-dependent downregulation of parathyroid hormone (PTH) expression by binding to negative calcium response elements (nCaREs) present in the PTH promoter. Here we show that APE1 is acetylated both in vivo and in vitro by the transcriptional co-activator p300 which is activated by Ca2+. Acetylation at Lys6 or Lys7 enhances binding of APE1 to nCaRE. APE1 stably interacts with class I histone deacetylases (HDACs) in vivo. An increase in extracellular calcium enhances the level of acetylated APE1 which acts as a repressor for the PTH promoter. Moreover, chromatin immunoprecipitation (ChIP) assay revealed that acetylation of APE1 enhanced binding of the APE1-HDACs complex to the PTH promoter. These results indicate that acetylation of APE1 plays an important role in this key repair protein's action in transcriptional regulation.

Role of Amphiphysin II in Somatostatin Receptor Trafficking in Neuroendocrine Cells

Amphiphysins are SH3 domain-containing proteins thought to function in clathrin-mediated endocytosis. To investigate the potential role of amphiphysin II in cellular trafficking of G protein-coupled somatostatin (SRIF) receptors, we generated an AtT-20 cell line stably overexpressing amphiphysin IIb, a splice variant that does not bind clathrin. Endocytosis of (125)I-[d-Trp(8)]SRIF was not affected by amphiphysin IIb overexpression. However, the maximal binding capacity (B(max)) of the ligand on intact cells was significantly lower in amphiphysin IIb overexpressing than in non-transfected cells. This difference was no longer apparent when the experiments were performed on crude cell homogenates, suggesting that amphiphysin IIb overexpression interferes with SRIF receptor targeting to the cell surface and not with receptor synthesis. Accordingly, immunofluorescence experiments demonstrated that, in amphiphysin overexpressing cells, sst(2A) and sst(5) receptors were segregated in a juxtanuclear compartment identified as the trans-Golgi network. Amphiphysin IIb overexpression had no effect on corticotrophin-releasing factor 41-stimulated adrenocorticotropic hormone secretion, suggesting that it is not involved in the regulated secretory pathway. Taken together, these results suggest that amphiphysin II is not necessary for SRIF receptor endocytosis but is critical for its constitutive targeting to the plasma membrane. Therefore, amphiphysin IIb may be an important component of the constitutive secretory pathway.

The calcium binding loops of the cytosolic phospholipase A2 C2 domain specify targeting to Golgi and ER in live cells

Translocation of cytosolic phospholipase A2 (cPLA2) to Golgi and ER in response to intracellular calcium mobilization is regulated by its calcium-dependent lipid-binding, or C2, domain. Although well studied in vitro, the biochemical characteristics of the cPLA2C2 domain offer no predictive value in determining its intracellular targeting. To understand the molecular basis for cPLA2C2 targeting in vivo, the intracellular targets of the synaptotagmin 1 C2A (Syt1C2A) and protein kinase Calpha C2 (PKCalphaC2) domains were identified in Madin-Darby canine kidney cells and compared with that of hybrid C2 domains containing the calcium binding loops from cPLA2C2 on Syt1C2A and PKCalphaC2 domain backbones. In response to an intracellular calcium increase, PKCalphaC2 targeted plasma membrane regions rich in phosphatidylinositol-4,5-bisphosphate, and Syt1C2A displayed a biphasic targeting pattern, first targeting phosphatidylinositol-4,5-bisphosphate-rich regions in the plasma membrane and then the trans-Golgi network. In contrast, the Syt1C2A/cPLA2C2 and PKCalphaC2/cPLA2C2 hybrids targeted Golgi/ER and colocalized with cPLA2C2. The electrostatic properties of these hybrids suggested that the membrane binding mechanism was similar to cPLA2C2, but not PKCalphaC2 or Syt1C2A. These results suggest that primarily calcium binding loops 1 and 3 encode structural information specifying Golgi/ER targeting of cPLA2C2 and the hybrid domains.

Metabolic cholesterol depletion hinders cell-surface trafficking of the nicotinic acetylcholine receptor

The effects of metabolic inhibition of cholesterol biosynthesis on the trafficking of the nicotinic acetylcholine receptor (AChR) to the cell membrane were studied in living CHO-K1/A5, a Chinese hamster ovary clonal line that heterologously expresses adult alpha2betadeltaepsilon mouse AChR. To this end, we submitted CHO-K1/A5 cells to long-term cholesterol deprivation, elicited by Mevinolin, a potent inhibitor of 3-hydroxy-3-methyl-glutaryl-CoA reductase and applied a combination of biochemical, pharmacological and fluorescence microscopy techniques to follow the fate of the AChR. When CHO-K1/A5 cells were grown for 48 h in lipid-deficient medium supplemented with 0.5 microM Mevinolin, total cholesterol was significantly reduced (40%). Concomitantly, the maximum number of binding sites (Bmax) of the cell-surface AChR for the competitive antagonist alpha-bungarotoxin was reduced from 647+/-30 to 352+/-34 fmol/mg protein, i.e. by 46%. The apparent dissociation constant (Kdapp) for alpha-bungarotoxin of the AChRs remaining at the cell surface was not modified by cholesterol depletion. Similarly, the half-concentration inhibiting the specific binding of the radioligand (IC50) for another competitive antagonist, d-tubocurarine, did not differ from that in control cells. The decrease in cell-surface AChR was paralleled by an increase in intracellular AChR levels, which rose from 44+/-2.1% in control cells to 74+/-3.3% in Mevinolin-treated cells. When analyzed by wide-field fluorescence microscopy, the fluorescence signal arising from alpha-bungarotoxin labeled cell-surface AChRs was reduced by approximately 70% in Mevinolin-treated cells. The distribution of intracellular AChR also changed: Alexa594-alpha-bungarotoxin-labeled AChR exhibited a highly compartmentalized pattern, concentrating at the perinuclear and Golgi-like regions. Temperature-arrest of protein trafficking magnified this effect, emphasizing the Golgi localization of the AChR. Colocalization studies using the transiently expressed fluorescent trans-Golgi/trans-Golgi network marker pEYFP/human beta1,4-galactosyltransferase and the trans-Golgi network marker syntaxin 6 provided additional support for the Golgi localization of intracellular AChRs. The low AChR cell-surface expression and the increase in intracellular AChR pools in cholesterol-depleted cells raise the possibility that cholesterol participates in the trafficking of the receptor protein to the plasmalemma and its stability at this surface location.

Calcium–myristoyl switch, subcellular localization, and calcium-dependent translocation of the neuronal calcium sensor protein VILIP-3, and comparison with VILIP-1 in hippocampal neurons☆

Neuronal calcium sensor (NCS) proteins including the subfamily of visinin-like-proteins (VILIPs) are involved in regulation of various signaling cascades. One molecular regulation mechanism is the calcium-myristoyl switch. VILIPs show a calcium-dependent membrane association in brain homogenates; however, differences in calcium-induced conformation changes and degree of membrane association are reported. Little is known about differences in the calcium-myristoyl switch in living cells leading to localization of VILIPs to distinct subcellular compartments. Therefore, we studied the calcium-dependent localization of green fluorescent protein (GFP)-tagged VILIP-3 in living cell lines and hippocampal neurons and compared it with that of GFP-VILIP-1. Interestingly, the observed fast and reversible calcium-myristoyl switch of VILIP-3-GFP and VILIP-1-GFP differed, e.g., in calcium-dependent translocation to Golgi membranes. Similarily, the calcium-dependent localization of endogenously expressed VILIP-3 and -1 in dendrites differed. Thus, VILIPs co-expressed in the same neuron show clear differences in calcium-dependent localization which may allow neurons a highly selective response to various calcium stimuli.

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.

Immunoglobulin mu heavy chains do not mediate tyrosine phosphorylation of Ig alpha from the ER-cis-Golgi

Signals delivered by Ig receptors guide the development of functional B lymphocytes. For example, clonal expansion of early mu heavy chain ( mu HC)-positive pre-B cells requires the assembly of a signal-competent pre-B cell receptor complex (pre-BCR) consisting of a mu HC, a surrogate L chain, and the signal dimer Ig alpha beta. However, only a small fraction of the pre-BCR is transported to the cell surface, suggesting that pre-BCR signaling initiates already from an intracellular compartment, e.g., the endoplasmic reticulum (ER). The finding that differentiation of pre-B cells and allelic exclusion at the IgH locus take place in surrogate L chain-deficient mice further supports the presence of a mu HC-mediated intracellular signal pathway. To determine whether a signal-competent Ig complex can already be assembled in the ER, we analyzed the consequence of pervanadate on tyrosine phosphorylation of Ig alpha in J558L plasmacytoma and 38B9 pre-B cells transfected with either a transport-competent IgL chain-pairing or an ER-retained nonpairing micro HC. Flow cytometry, combined Western blot-immunoprecipitation-kinase assays, and confocal microscopy revealed that both the nonpairing and pairing mu HC assembled with the Ig alpha beta dimer; however, in contrast to a pairing mu HC, the nonpairing mu HC was retained in the ER-cis-Golgi compartment, and neither colocalized with the src kinase lyn nor induced tyrosine phosphorylation of Ig alpha after pervanadate treatment of cells. On the basis of these findings, we propose that a signal-competent Ig complex consisting of mu HC, Ig alpha beta, and associated kinases is assembled in a post-ER compartment, thereby supporting the idea that a pre-BCR must be transported to the cell surface to initiate pre-BCR signaling.

A dual mechanism controlling the localization and function of exocytic v-SNAREs

SNARE [soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein receptor] proteins are essential for membrane fusion but their regulation is not yet fully understood. We have previously shown that the amino-terminal Longin domain of the v-SNARE TI-VAMP (tetanus neurotoxin-insensitive vesicle-associated membrane protein)/VAMP7 plays an inhibitory role in neurite outgrowth. The goal of this study was to investigate the regulation of TI-VAMP as a model of v-SNARE regulation. We show here that the Longin domain (LD) plays a dual role. First, it negatively regulates the ability of TI-VAMP and of a Longin/Synaptobrevin chimera to participate in SNARE complexes. Second, it interacts with the adaptor complex AP-3 and this interaction targets TI-VAMP to late endosomes. Accordingly, in mocha cells lacking AP-3 delta, TI-VAMP is retained in an early endosomal compartment. Furthermore, TI-VAMPc, an isoform of TI-VAMP lacking part of the LD, does not interact with AP-3, and therefore is not targeted to late endosomes; however, this shorter LD still inhibits SNARE-complex formation. These findings support a mechanism controlling both localization and function of TI-VAMP through the LD and clathrin adaptors. Moreover, they point to the amino-terminal domains of SNARE proteins as multifunctional modules responsible for the fine tuning of SNARE function.

The inhibition of early N-glycan processing targets TRP-2 to degradation in B16 melanoma cells

Tyrosinase-related protein-2 (TRP-2) is a DOPAchrome tautomerase catalyzing a distal step in the melanin synthesis pathway. Similar to the other two melanogenic enzymes belonging to the TRP gene family, tyrosinase and TRP-1, TRP-2 is expressed in melanocytes and melanoma cells. Despite the increasing evidence of its efficiency as a melanoma antigen, little is known about the maturation and intracellular trafficking of TRP-2. Here we show that TRP-2 is mainly distributed in the TGN of melanoma cells instead of being confined solely to melanosomes. This, together with the plasma membrane occasional localization observed by immunofluorescence, suggest the TRP-2 participation in a recycling pathway, which could include or not the melanosomes. Using pulse-chase experiments we show that the TRP-2 polypeptide folds in the endoplasmic reticulum (ER) in the presence of calnexin, until it reaches a dithiothreitol-resistant conformation enabling its ER exit to the Golgi. If N-glycosylation inhibitors prevent the association with calnexin, the TRP-2 nascent chain undergoes an accelerated degradation process. This process is delayed in the presence of proteasomal inhibitors, indicating that the misfolded chain is retro-translocated from the ER into the cytosol and degraded in proteasomes. This is a rare example in which calnexin although indispensable for the nascent chain folding is not required for its targeting to degradation. Therefore TRP-2 may prove to be a good model to document the calnexin-independent retro-translocation process of proteasomally degraded proteins. Clearly, TRP-2 has a distinct maturation pathway from tyrosinase and TRP-1 and possibly a second regulatory function within the cell.

Islet cell autoantigen of 69 kDa is an arfaptin-related protein associated with the Golgi complex of insulinoma INS-1 cells

Islet cell autoantigen of 69 kDa (ICA69) is a cytosolic protein of still unknown function. Involvement of ICA69 in neurosecretion has been suggested by the impairment of acetylcholine release at neuromuscular junctions upon mutation of its homologue gene ric-19 in C. elegans. In this study, we have further investigated the localization of ICA69 in neurons and insulinoma INS-1 cells. ICA69 was enriched in the perinuclear region, whereas it did not co-localize with markers of synaptic vesicles/synaptic-like microvesicles. Confocal microscopy and subcellular fractionation in INS-1 cells showed co-localization of ICA69 with markers of the Golgi complex and, to a minor extent, with immature insulin-containing secretory granules. The association of ICA69 with these organelles was confirmed by immunoelectron microscopy. Virtually no ICA69 immunogold labeling was observed on secretory granules near the plasma membrane, suggesting that ICA69 dissociates from secretory granule membranes during their maturation. In silico sequence and structural analyses revealed that the N-terminal region of ICA69 is similar to the region of arfaptins that interacts with ARF1, a small GTPase involved in vesicle budding at the Golgi complex and immature secretory granules. ICA69 is therefore a novel arfaptin-related protein that is likely to play a role in membrane trafficking at the Golgi complex and immature secretory granules in neurosecretory cells.

The regulatory domain of protein kinase Ctheta localises to the Golgi complex and induces apoptosis in neuroblastoma and Jurkat cells

This study investigates apoptotic effects of protein kinase C (PKC) delta and theta in neuroblastoma cells. 12-O-tetradecanoylphorbol-13-acetate induces apoptosis in SK-N-BE(2) neuroblastoma cells overexpressing PKCdelta or PKCtheta, but not PKC epsilon. The PKC inhibitor GF109203X does not suppress this apoptotic effect, suggesting that it is independent of the catalytic activity of PKC. The isolated catalytic domains of PKCdelta and PKCtheta or the regulatory domain (RD) of PKCtheta also induce apoptosis in neuroblastoma cells. The apoptotic responses are suppressed by caspase inhibition and by Bcl-2 overexpression. The PKCtheta RD induced apoptosis also in Jurkat cells. Colocalisation analysis revealed that the PKCtheta RD primarily localises to the Golgi complex. The C1b domain is required for this localisation and removal of the C1b domain results in a PKCtheta construct that does not induce apoptosis. This suggests that the PKCtheta RD has apoptotic activity and that Golgi localisation may be important for this effect.

Mycobacterium tuberculosis glycosylated phosphatidylinositol causes phagosome maturation arrest

The tubercle bacillus parasitizes macrophages by inhibiting phagosome maturation into the phagolysosome. This phenomenon underlies the tuberculosis pandemic involving 2 billion people. We report here how Mycobacterium tuberculosis causes phagosome maturation arrest. A glycosylated M. tuberculosis phosphatidylinositol [mannose-capped lipoarabinomannan (ManLAM)] interfered with the phagosomal acquisition of the lysosomal cargo and syntaxin 6 from the trans-Golgi network. ManLAM specifically inhibited the pathway dependent on phosphatidylinositol 3-kinase activity and phosphatidylinositol 3-phosphate-binding effectors. These findings identify ManLAM as the M. tuberculosis product responsible for the inhibition of phagosomal maturation.

The SNARE motif contributes to rbet1 intracellular targeting and dynamics independently of SNARE interactions

The endoplasmic reticulum/Golgi SNARE rbet1 cycles between the endoplasmic reticulum and Golgi and is essential for cargo transport in the secretory pathway. Although the quaternary SNARE complex containing rbet1 is known to function in membrane fusion, the structural role of rbet1 is unclear. Furthermore, the structural determinants for rbet1 targeting and its cyclical itinerary have not been investigated. We utilized protein interaction assays to demonstrate that the rbet1 SNARE motif plays a structural role similar to the carboxyl-terminal helix of SNAP-25 in the synaptic SNARE complex and demonstrated the importance to SNARE complex assembly of a conserved salt bridge between rbet1 and sec22b. We also examined the potential role of the rbet1 SNARE motif and SNARE interactions in rbet1 localization and dynamics. We found that, in contrast to what has been observed for syntaxin 5, the rbet1 SNARE motif was essential for proper targeting. To test whether SNARE interactions were important for the targeting function of the SNARE motif, we used charge repulsion mutations at the conserved salt bridge position that rendered rbet1 defective for binary, ternary, and quaternary SNARE interactions. We found that heteromeric SNARE interactions are not required at any step in rbet1 targeting or dynamics. Furthermore, the heteromeric state of the SNARE motif does not influence its interaction with the COPI coat or efficient recruitment onto transport vesicles. We conclude that protein targeting is a completely independent function of the rbet1 SNARE motif, which is capable of distinct classes of protein interactions.

Control of membrane fusion during spermiogenesis and the acrosome reaction

Membrane fusion is important to reproduction because it occurs in several steps during the process of fertilization. Many events of intracellular trafficking occur during both spermiogenesis and oogenesis. The acrosome reaction, a key feature during mammalian fertilization, is a secretory event involving the specific fusion of the outer acrosomal membrane and the sperm plasma membrane overlaying the principal piece of the acrosome. Once the sperm has crossed the zona pellucida, the gametes fuse, but in the case of the sperm this process takes place through a specific membrane domain in the head, the equatorial segment. The cortical reaction, a process that prevents polyspermy, involves the exocytosis of the cortical granules to the extracellular milieu. In lower vertebrates, the formation of the zygotic nucleus involves the fusion (syngamia) of the male pronucleus with the female pronucleus. Other undiscovered membrane trafficking processes may also be relevant for the formation of the zygotic centrosome or other zygotic structures. In this review, we focus on the recent discovery of molecular machinery components involved in intracellular trafficking during mammalian spermiogenesis, notably related to acrosome biogenesis. We also extend our discussion to the molecular mechanism of membrane fusion during the acrosome reaction. The data available so far suggest that proteins participating in the intracellular trafficking events leading to the formation of the acrosome during mammalian spermiogenesis are also involved in controlling the acrosome reaction during fertilization.

Mannose 6-phosphate receptors: new twists in the tale

The two mannose 6-phosphate (M6P) receptors were identified because of their ability to bind M6P-containing soluble acid hydrolases in the Golgi and transport them to the endosomal-lysosomal system. During the past decade, we have started to understand the structural features of these receptors that allow them to do this job, and how the receptors themselves are sorted as they pass through various membrane-bound compartments. But trafficking of acid hydrolases is only part of the story. Evidence is emerging that one of the receptors can regulate cell growth and motility, and that it functions as a tumour suppressor.

GLUT4 recycles via a trans-Golgi network (TGN) subdomain enriched in Syntaxins 6 and 16 but not TGN38: involvement of an acidic targeting motif

Insulin stimulates glucose transport in fat and muscle cells by triggering exocytosis of the glucose transporter GLUT4. To define the intracellular trafficking of GLUT4, we have studied the internalization of an epitope-tagged version of GLUT4 from the cell surface. GLUT4 rapidly traversed the endosomal system en route to a perinuclear location. This perinuclear GLUT4 compartment did not colocalize with endosomal markers (endosomal antigen 1 protein, transferrin) or TGN38, but showed significant overlap with the TGN target (t)-soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) Syntaxins 6 and 16. These results were confirmed by vesicle immunoisolation. Consistent with a role for Syntaxins 6 and 16 in GLUT4 trafficking we found that their expression was up-regulated significantly during adipocyte differentiation and insulin stimulated their movement to the cell surface. GLUT4 trafficking between endosomes and trans-Golgi network was regulated via an acidic targeting motif in the carboxy terminus of GLUT4, because a mutant lacking this motif was retained in endosomes. We conclude that GLUT4 is rapidly transported from the cell surface to a subdomain of the trans-Golgi network that is enriched in the t-SNAREs Syntaxins 6 and 16 and that an acidic targeting motif in the C-terminal tail of GLUT4 plays an important role in this process.

GRIP domain-mediated targeting of two new coiled-coil proteins, GCC88 and GCC185, to subcompartments of the trans-Golgi network

The GRIP domain is a targeting sequence found in a family of coiled-coil peripheral Golgi proteins. Previously we demonstrated that the GRIP domain of p230/golgin245 is specifically recruited to tubulovesicular structures of the trans-Golgi network (TGN). Here we have characterized two novel Golgi proteins with functional GRIP domains, designated GCC88 and GCC185. GCC88 cDNA encodes a protein of 88 kDa, and GCC185 cDNA encodes a protein of 185 kDa. Both molecules are brefeldin A-sensitive peripheral membrane proteins and are predicted to have extensive coiled-coil regions with the GRIP domain at the C terminus. By immunofluorescence and immunoelectron microscopy GCC88 and GCC185, and the GRIP protein golgin97, are all localized to the TGN of HeLa cells. Overexpression of full-length GCC88 leads to the formation of large electron dense structures that extend from the trans-Golgi. These de novo structures contain GCC88 and co-stain for the TGN markers syntaxin 6 and TGN38 but not for alpha2,6-sialyltransferase, beta-COP, or cis-Golgi GM130. The formation of these abnormal structures requires the N-terminal domain of GCC88. TGN38, which recycles between the TGN and plasma membrane, was transported into and out of the GCC88 decorated structures. These data introduce two new GRIP domain proteins and implicate a role for GCC88 in the organization of a specific TGN subcompartment involved with membrane transport.

Mammalian ykt6 is a neuronal SNARE targeted to a specialized compartment by its profilin-like amino terminal domain

SNAREs are required for specific membrane fusion throughout the endomembrane system. Here we report the characterization of rat ykt6, a prenylated SNARE selectively expressed in brain neurons. Immunofluorescence microscopy in neuronal and neuroendocrine cell lines revealed that membrane-associated ykt6 did not colocalize significantly with any conventional markers of endosomes, lysosomes, or the secretory pathway. However, ykt6-containing membranes displayed very minor overlaps with lysosomes and dense-core secretory granules and were similar to lysosomes in buoyant density. Thus, ykt6 appears to be specialized for the trafficking of a unique membrane compartment, perhaps related to lysosomes, involved in aspects of neuronal function. Targeting of this SNARE to the ykt6 compartment was mediated by its profilin-like amino-terminal domain, even in the absence of protein prenylation. Although several other R-SNAREs contain related amino-terminal domains, only the ykt6 version was able to confer the specialized localization. Rat ykt6, which contains an arginine in its SNARE motif zero-layer, was found to behave like other R-SNAREs in its SNARE assembly properties. Interestingly, cytosolic ykt6, constituting more than half of the total cellular pool, appeared to be conformationally inactive for SNARE complex assembly, perhaps indicative of a regulatory mechanism that prevents promiscuous and potentially deleterious SNARE interactions.

Visualization of TGN to endosome trafficking through fluorescently labeled MPR and AP-1 in living cells

We have stably expressed in HeLa cells a chimeric protein made of the green fluorescent protein (GFP) fused to the transmembrane and cytoplasmic domains of the mannose 6-phosphate/insulin like growth factor II receptor in order to study its dynamics in living cells. At steady state, the bulk of this chimeric protein (GFP-CI-MPR) localizes to the trans-Golgi network (TGN), but significant amounts are also detected in peripheral, tubulo-vesicular structures and early endosomes as well as at the plasma membrane. Time-lapse videomicroscopy shows that the GFP-CI-MPR is ubiquitously detected in tubular elements that detach from the TGN and move toward the cell periphery, sometimes breaking into smaller tubular fragments. The formation of the TGN-derived tubules is temperature dependent, requires the presence of intact microtubule and actin networks, and is regulated by the ARF-1 GTPase. The TGN-derived tubules fuse with peripheral, tubulo-vesicular structures also containing the GFP-CI-MPR. These structures are highly dynamic, fusing with each other as well as with early endosomes. Time-lapse videomicroscopy performed on HeLa cells coexpressing the CFP-CI-MPR and the AP-1 complex whose gamma-subunit was fused to YFP shows that AP-1 is present not only on the TGN and peripheral CFP-CI-MPR containing structures but also on TGN-derived tubules containing the CFP-CI-MPR. The data support the notion that tubular elements can mediate MPR transport from the TGN to a peripheral, tubulo-vesicular network dynamically connected with the endocytic pathway and that the AP-1 coat may facilitate MPR sorting in the TGN and endosomes.

Co-evolutionary analysis reveals insights into protein-protein interactions

Protein-protein interactions play crucial roles in biological processes. Experimental methods have been developed to survey the proteome for interacting partners and some computational approaches have been developed to extend the impact of these experimental methods. Computational methods are routinely applied to newly discovered genes to infer protein function and plausible protein-protein interactions. Here, we develop and extend a quantitative method that identifies interacting proteins based upon the correlated behavior of the evolutionary histories of protein ligands and their receptors. We have studied six families of ligand-receptor pairs including: the syntaxin/Unc-18 family, the GPCR/G-alpha's, the TGF-beta/TGF-beta receptor system, the immunity/colicin domain collection from bacteria, the chemokine/chemokine receptors, and the VEGF/VEGF receptor family. For correlation scores above a defined threshold, we were able to find an average of 79% of all known binding partners. We then applied this method to find plausible binding partners for proteins with uncharacterized binding specificities in the syntaxin/Unc-18 protein and TGF-beta/TGF-beta receptor families. Analysis of the results shows that co-evolutionary analysis of interacting protein families can reduce the search space for identifying binding partners by not only finding binding partners for uncharacterized proteins but also recognizing potentially new binding partners for previously characterized proteins. We believe that correlated evolutionary histories provide a route to exploit the wealth of whole genome sequences and recent systematic proteomic results to extend the impact of these studies and focus experimental efforts to categorize physiologically or pathologically relevant protein-protein interactions.

A role for the lysosomal membrane protein LGP85 in the biogenesis and maintenance of endosomal and lysosomal morphology

LGP85 (LIMP II) is a type III transmembrane glycoprotein that is located primarily in the limiting membranes of lysosomes and late endosomes. Despite being the abundant molecule of these compartments, whether LGP85 merely resides as one of the constituents of these membranes or plays a role in the regulation of endosome and lysosome biogenesis remains unclear. To elucidate these questions, we examined the effects of overexpression of LGP85 on the morphology and membrane traffic of the endosomal/lysosomal system. Here we demonstrate that overexpression of LGP85 causes an enlargement of early endosomes and late endosomes/lysosomes. Such a morphological alteration was not observed by overexpression of other lysosomal membrane proteins, LGP107 (LAMP-1) or LGP96 (LAMP-2), reflecting a LGP85-specific function. We further demonstrate that overexpression of LGP85 impairs the endocytic membrane traffic out of these enlarged compartments, which may be correlated with or account for the accumulation of cholesterol observed in these compartments. Interestingly, co-transfection of LGP85 and the dominant-negative form of Rab5b (Rab5bS34N) abolished the formation of large vacuoles, suggesting that the GTP-bound active form of Rab5b is involved in the enlargement of endosomal/lysosomal compartments induced by overexpression of LGP85. Thus, these findings provide important new insights into the role of LGP85 in the biogenesis and the maintenance of endosomes/lysosomes. We conclude that LGP85 may participate in reorganizing the endosomal/lysosomal compartments.