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J Biol Chem, Vol. 273, Issue 12, 6944-6950, March 20, 1998
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,From the Membrane Biology Laboratory, Institute of Molecular and Cell Biology, Singapore 117609, Republic of Singapore
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ABSTRACT |
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We have cloned a new member of the syntaxin
family of proteins. The open reading frame encodes a polypeptide of 272 amino acids with potential coiled-coil domains and a C-terminal
hydrophobic tail. Northern blot analysis showed that the transcript is
fairly ubiquitous. A soluble recombinant form of the polypeptide
without the hydrophobic region binds to
-SNAP (soluble
N-ethylmaleimide-sensitive factor attachment protein) and
syndet/SNAP-23 in vitro. Polyclonal antibody raised against
the recombinant protein recognized a 39-kDa protein in the membrane
fraction of cell lysates. Indirect immunofluorescence studies using the
polyclonal antibody showed that the protein is localized to
intracellular membrane structures. Selective permeabilization studies
with digitonin and saponin indicate that the epitope(s) recognized by
the antibody is expose to the cytoplasm, consistent with the predicted
orientation characteristic of SNAP receptor molecules. Morphological
alterations of the staining pattern of the protein with brefeldin A and
wortmannin treatment indicate that the protein is localize to the
endosome. The cDNA we have cloned apparently corresponded to three
previously described expressed sequence tags named as syntaxins 12, 13, and 14, respectively. We therefore propose to retain the name syntaxin
12 for this protein.
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INTRODUCTION |
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A biochemical and biophysical understanding of vesicular transport at the molecular level has been facilitated by in vitro assays, which reconstitute transport processes in cell-free systems (1). Thus, molecular components required for both vesicle budding (2) and vesicle docking/fusion processes (3-4) have been isolated. The N-ethylmaleimide-sensitive factor (NSF),1 an ATPase whose activity regulates the formation and dissociation of fusion complexes, is the first cytosolic factor characterized as such. NSF works in conjunction with another soluble factor, the soluble NSF attachment protein (SNAP) in mediating vesicle docking (1). Membrane components that are responsible for determining the specificity of the docking and fusion of the right vesicles to the right membranes are eventually identified based on the ability to interact with SNAP (4). These are known as SNAP receptors (SNARE). SNAREs can be broadly divided into two classes. Those present on transport vesicles are the v-SNAREs, and those present on the target membranes are the t-SNAREs. Thus, the SNARE hypothesis, a working hypothesis proposed by Rothman and co-workers (3, 4), holds that a transport vesicle chooses its target for fusion when v-SNAREs pair with the cognate t-SNAREs at the target membrane. Genetic dissections of the yeast secretory pathway and the biochemical characterization of molecules involved in synaptic vesicle docking and fusion have resulted in the isolation and/or molecular cloning of putative SNARE molecules that are structurally related (5-6). However, isolation of SNAREs in the constitutive secretory and endocytotic pathway in mammalian cells remains difficult. This difficulty has been overcome in part by the availability of an expanding data base of expressed sequence tags (EST) and efficient data base search and sequence alignment programs (7).
The first member of the syntaxin family of proteins, syntaxin 1A, was first characterized as a neuronal-specific protein involved in the regulation of neurotransmitter release (8). Its localization to the plasma membrane and its interaction with the synaptic vesicle v-SNARE synaptobrevin point to its function as a t-SNARE. Subsequently, a family of syntaxin-related molecules that shares 23-84% amino acid identity has been identified (6, 9). These syntaxins are more ubiquitous in their expressions in various tissues, which is indicative of their possible functions in other vesicular transport steps in the cell. These syntaxins also display a variety of cellular localizations within the secretory pathway. Whereas syntaxins 2, 3, and 4 are apparently cell surface proteins (6, 10-11), syntaxin 5 and syntaxin 6 are localized to the Golgi region (6, 9).
The transport from the early endosome to the late endosome and lysosome is a major route of intracellular membrane trafficking. However, little is known at the molecular level about the mechanisms regulating membrane interactions in the endocytic pathway beyond early endosomes. Recently, it has been shown that COP1 coat components participate in endosomal transport (12). Using an in vitro transport assay to study the biochemical properties of endosome docking and fusion events, Robinson et al. (13) have shown that NSF and SNAPs are required for several steps of endosomal membrane transport. Of the known SNAREs, only the v-SNARE cellubrevin has been shown to have an endosomal localization (14). Should the mechanism of vesicular transport in the endosomes not differ too drastically from its exocytic counterpart, one might expect to find members of the syntaxin family functioning as t-SNAREs in the endosomal membranes. In this report, we present such a molecule.
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EXPERIMENTAL PROCEDURES |
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Materials-- Cell lines were primarily from the American Type Culture Collection. Monoclonal antibody against trans-Golgi network 38 (TGN38) was kindly provided by Dr George Banting (University of Bristol, United Kingdom). Expressed sequence tag clones were generated by the Washington University MERCK EST project and were obtained from the IMAGE consortium. Syndet cDNA (15) was kindly provided by Dr. G. Baldini (Columbia University, New York). Syntaxin 1A cDNA (6) was kindly provided by Dr. R. Scheller (Stanford University, CA). mSEC13 cDNA (16) was kindly provided by Dr. Anand Swaroop (University of Michigan).
Methods-- Data base searches were performed with the various BLAST algorithms available at the National Center for Biotechnology (NCBI) World Wide Web server. Library screening, cloning, and DNA sequencing were performed using standard methods as described (17). Northern blot analysis was performed using a rat multiple tissue Northern blot from CLONTECH.
In vitro translation of various constructs was performed using in vitro translation kits from Promega according to the manufacturer's protocol. For in vitro binding assays (18), the cytoplasmic domain of syntaxin 12 (amino acids 1-248) was generated by the polymerase chain reaction and cloned into the the plasmid pBSK (Stratagene). [35S]Methionine-labeled translation product was incubated with glutathione-Sepharose beads coated with either glutathione S-transferase (GST), GST-
SNAP, or GST-syndet in binding buffer (20 mM Hepes,
pH 7.5, 25 mM NaCl, 3% glycerol, 7 mM
MgCl2, 1 mM CaCl2, and 1 mM EDTA) with 0.1% bovine serum albumin and 0.5% Nonidet
P40 at 4 °C for 3 h. The beads were washed twice with the
complete incubation buffer, twice in buffer without bovine serum
albumin, and twice in buffer without bovine serum albumin and Nonidet
P-40. SDS sample buffer was then added, and the SDS eluates were
analyzed by SDS-polyacrylamide gel electrophoresis.
The cytoplasmic domain of the protein is expressed either as
hexahistidine-tagged or GST fusion proteins in bacteria. The fusion
proteins were also used to immunize rabbits. Polyclonal antibodies were
affinity-purified from serum harvested after several booster injections
by the fusion proteins immobilized on nitrocellulose strips.
Cells were maintained in RPMI medium supplemented with 10% fetal
bovine serum. Immunofluorescence microscopy was performed as described
previously (19-20). Cells plated on coverslips and subjected to
various treatments were fixed with 3% paraformaldehyde followed by
sequential incubation with the primary antibodies and fluorescein
isothiocyanate or rhodamine-conjugated secondary antibodies.
Fluorescence labeling was visualized using an Axiophot microscope (Carl
Zeiss, Inc., Thornwood, NY) with epifluorescence optics or MRC600
(Bio-Rad) confocal laser optics.
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RESULTS |
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Molecular Cloning and Sequencing of a Novel Member of the Syntaxin Family-- Data base searches have allowed us to identified human ESTs (accession numbers R21569 and N99549) potentially coding for a syntaxin-like molecule. A complete cDNA was isolated from a rat brain cDNA library, and sequencing revealed a 272-amino acid open reading frame as shown in Fig. 1A. The predicted amino acid sequence has a stretch of 22 hydrophobic residues at the C terminus, as illustrated by a Kyte-Doolittle hydrophobicity plot (Fig. 1B). This primary structure is characteristic of a hydrophobic tail anchor. The polypeptide has several potential regions that may form coiled-coil structures, as revealed by the Coils version 2.1 program (Fig. 1C).
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Syntaxin 12 Is a SNAP Receptor Molecule Localized to the
Endosome--
The predicted primary structure of syntaxin 12 and its
homology to other syntaxins suggest that it is a SNARE molecule. We sought to confirm this by investigating if syntaxin 12 binds to
-SNAP in vitro. [35S]Methionine-labeled
translation product of the soluble cytoplasmic domain of both syntaxin
1A and syntaxin 12 was incubated with glutathione-Sepharose beads
coated with either GST or GST-
SNAP. As shown in Fig.
3, the binding of the syntaxin 1A
cytodomain to GST-
SNAP is significantly higher than to GST itself.
Such is also the case for syntaxin 12. On the other hand, mSEC13 (20), a protein with multiple
-transducin or WD-40 repeats known to participate in protein-protein interactions, did not exhibit
significant binding to GST-
SNAP. The ability of syntaxin 12 to bind
SNAP is in good agreement with its putative function as a SNARE.
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DISCUSSION |
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We have therefore cloned a novel member of the syntaxin family with a unique subcellular localization. None of the syntaxins 1-6 published to date are associated with the endosome. Exogenous expression of syntaxin 1A does result in intracellular localization in Madin-Darby canine kidney cells, and exogenous expression of syntaxin 3 does have an intracellular component (10). These intracellular stainings were, however, shown not to be endosomal in nature but rather colocalized with a lysosomal marker (10). Another novel syntaxin, known as syntaxin 7, has recently been cloned in our laboratory (34) as well as by Wang et al. (35). Based on its homology to yeast and plant vacuolar syntaxins, Wang et al. proposed that syntaxin 7 may have a role in trafficking between the Golgi apparatus and the lysosomes. Our immunolocalization data, however, suggest that syntaxin 7 is localized to the endosomes (34). In view of the localizations of syntaxin 7 and syntaxin 12, the transport machinery of the endocytic pathway, like its exocytic counterpart, also utilizes members of the syntaxin family.
What may the function of syntaxin 12 be? There are several possibilities. Judging by its compact, perinuclear staining, syntaxin 12 may well reside in a late endosomal compartment. Indeed it does not colocalize with transiently internalized transferin (not shown). However, we could not rule out that small amounts of syntaxin 12 may reside in the early endosomes. If solely localize to a late endosomal compartment, syntaxin 12 may function to receive vesicles either from the TGN or the early endosome or participate in the recycling of surface receptors. Elucidation of its exact role in transport awaits experiments involving effective functional disruption either by the introduction of negative dominant mutants, inhibitory antibodies, or targeted disruption of the gene.
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ACKNOWLEDGEMENTS |
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We thank Dr. George Banting for monoclonal
antibody against TGN38, Dr. G. Baldini for syndet cDNA, Dr. R. Scheller for syntaxin 1A cDNA, Dr. Anand Swaroop for mSEC13
cDNA, Dr. S. H. Wong for GST-
SNAP, and Mr. Robin Philps for
protein sequencing.
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FOOTNOTES |
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* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) AF035632.
The first two authors contributed equally to this work.
§ Supported by a grant from the Institute of Molecular and Cell Biology. To whom correspondence should be addressed: Membrane Biology Laboratory, Institute of Molecular and Cell Biology, 30 Medical Dr., Singapore 117609, Republic of Singapore. Tel.: 65-874-3762; Fax: 65-779-1117; E-mail: mcbhwj{at}leonis.nus.sg.
1 The abbreviations used are: NSF, N-ethylmaleimide-sensitive factor; RPMI medium, Rosewell Park Memorial Institute medium; SNAP, soluble NSF attachment proteins, SNARE, SNAP receptor; NRK, normal rat kidney; EST, expressed sequence tags; GST, glutathione S-transferase; TGN, trans-Golgi network; BFA, brefeldin A.
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