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Novel isoform of syntaxin 1 is expressed in mammalian cells.

ABSTRACT: Syntaxin 1A has been identified previously as a neural-cell-specific, membrane-anchored receptor protein required for docking and fusion of synaptic vesicles with the presynaptic plasma membrane. Syntaxin 1A consists of 288 amino acid residues including a 265-residue N-terminal region exposed to the cytoplasm and a C-terminal hydrophobic stretch of 23 residues believed to anchor syntaxin to the plasma membrane. Using a human fat-cell library we have isolated a novel cDNA clone of syntaxin 1A containing an insert of 91 bp in codon 226. This insert and subsequent frame shift generated a cDNA that codes for a truncated protein of 260 residues without the C-terminal transmembrane domain characteristic of the syntaxin family. Analysis of the deduced amino acid sequence of the new cDNA clone, termed syntaxin 1C, showed that it was identical for the first 226 residues with the previously described neural syntaxin 1A, and diverged thereafter. The truncated protein lacked the botulinum neurotoxin C cleavage site (Lys253-Ala254), a feature of the syntaxin 1A protein, because of the novel C-terminal domain of 34 residues. The new C-terminal region contained a single cysteine residue and was moderately rich in proline, with three repeats of a PXP motif. The insert occurred within the region encoding the coiled-coil motifs required for interactions with synaptobrevin, alpha-SNAP (SNAP being soluble N-ethylmaleimide-sensitive factor attachment protein) and n-Sec1/Munc-18 (n-Sec1 being the rat brain homologue of yeast Sec1p and Munc-18 the mammalian homologue of Caenorhabditis elegans unc-18, but five residues outside the domain previously mapped as being required for binding SNAP-25. Interaction studies in vitro suggested that unlike syntaxin 1A, which binds to both Munc-18a and- 18b, syntaxin 1C binds only to Munc-18b. The new isoform syntaxin 1C, which might be generated by alternative splicing of the syntaxin 1 gene, was expressed in several human tissues, including brain. Immuno-precipitation and immunoblotting with the monoclonal antibody HPC-1 and a polyclonal antibody raised against a peptide corresponding to the unique C-terminal 35 residues of syntaxin 1C failed to detect syntaxin 1C at the protein level in extracts of muscle, fat or brain.

SUBMITTER: Jagadish MN

PROVIDER: S-EPMC1218049 | biostudies-other | 1997 Jan

REPOSITORIES: biostudies-other

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Project description:BACKGROUND:The murine discs large homolog 2 (DLG2; post synaptic density 93 (PSD-93); Chapsyn-110) is a member of the membrane-associated guanylate kinase (MAGUK) protein family involved in receptor assembly and associated with signaling enzymes on cell membranes. In neurons, DLG2 protein isoforms derived from alternatively spliced transcripts have been described to bind to NMDA (N-methyl-aspartate) receptors and K channels and to mediate clustering of these channels in the postsynaptic membrane. In myeloid cells of the immune system, such as dendritic cells (DCs), a lack of data exists on the expression or function of DLG2. In cDNA microarray transcriptome analyses, we found Dlg2 highly expressed in a subpopulation of plasmacytoid DCs (pDCs) stimulated to produce type I interferons (IFNs) such as IFN?. RESULTS:Using RACE- and RT-PCR as well as immunoprecipitation followed by Western blotting we characterised the differential expression of the Dlg2 splice variants in IFN?-producing pDCs. Besides Dlg2? this cell population expressed a novel short Dlg2? transcript we termed Dlg2?3. Our expression data were integrated into information from genome databases to obtain a novel and comprehensive overview of the mouse Dlg2 gene architecture. To elucidate the intracellular localisation pattern of protein isoforms, ectopical expression analysis of fluorescently tagged DLG2 splice variants was performed. Here we found an enrichment of the larger isoform DLG2?1 at the plasma membrane while the newly identified shorter (DLG2?) isoform as well as DLG2? were equally distributed throughout the cytoplasm. Additionally, DLG2? was also found in the nucleus. Analysis of Dlg2-knockout mice previously generated by deleting exon 9 surprisingly revealed that the protein for the novel DLG2? isoform was still expressed in the brain and in bone marrow-derived pDCs from mice carrying the homozygous deletion (Dlg2 ?E9/?E9 ). CONCLUSION:We describe a novel splice variant of the mouse Dlg2 gene termed Dlg2? and define the differential expression pattern of DLG2 isoforms in IFN?-producing pDCs. The presence of DLG2? protein in the CNS of Dlg2 ?E9/?E9 mice might influence the phenotype of these mice and has to be taken into account in the interpretation of results regarding the functional role of DLG2 in neuronal postsynaptic membranes.

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Novel isoform of syntaxin 1 is expressed in mammalian cells.

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