Srivastava M, Zhang-Keck Z Y, Caohuy H, McPhie P, Pollard H B
Laboratory of Cell Biology and Genetics, National Institute of Diabetes, Digestive and Kidney Diseases, NIH, Bethesda, MD 20892, USA.
Biochem J. 1996 Jun 15;316 ( Pt 3)(Pt 3):729-35. doi: 10.1042/bj3160729.
Synexin (annexin VII) is a calcium-dependent, phospholipid-binding and membrane fusion protein in the annexin gene family, which forms calcium channels and may play a role in exocytotic secretion. We report here the cloning and characterization of five novel isoforms of cDNAs encoding Xenopus synexin from brain, oocyte and stage 24 cDNA libraries. The most prevalent Xenopus synexin has 1976 bp of cDNA sequence, which contains a 1539 bp open reading frame of 512 amino acids encoding a 54 kDa protein. This Xenopus protein is 6 kDa larger than the previously reported human and mouse synexins with which it shares approx. 73% identity in the C-terminal region and approx. 44% identity in the N-terminal region. Further studies with PCR revealed the molecular basis of the substantial divergence in the Xenopus synexin's N-terminal domain. The domain equivalent to the mammalian tissue-specific cassette exon occurs at a different position and is variable in size and sequence. The most interesting observation relates to the occurrence of different forms of synexin due to the varying numbers of tandem PGQM repeats that are expressed differently in different adult tissues and embryonic stages. For these reasons we have labelled this set of unique isoforms annexin VIIb, referring to mammalian forms, which lack the PGQM tandem repeats, as annexin VIIa. In spite of these differences from annexin VIIa, the form of recombinant annexin VIIb with three PGQM repeats was found to be catalytically active. We interpret these results to indicate that the actual calcium and phospholipid binding sites are conserved in Xenopus, and that the variations observed between members of the synexin gene family in the regulatory domain clearly point towards the tissue- and stage-specific roles of individual members, possibly involving the exocytotic process.
联会蛋白（膜联蛋白VII）是膜联蛋白基因家族中一种钙依赖性、磷脂结合和膜融合蛋白，它形成钙通道并可能在胞吐分泌中发挥作用。我们在此报告了从脑、卵母细胞和24期cDNA文库中克隆和鉴定编码非洲爪蟾联会蛋白的五个新的cDNA同工型。最常见的非洲爪蟾联会蛋白具有1976 bp的cDNA序列，其中包含一个1539 bp的开放阅读框，编码512个氨基酸，产生一个54 kDa的蛋白质。这种非洲爪蟾蛋白比先前报道的人类和小鼠联会蛋白大6 kDa，它与人类和小鼠联会蛋白在C端区域约有73%的同一性，在N端区域约有44%的同一性。通过PCR的进一步研究揭示了非洲爪蟾联会蛋白N端结构域存在显著差异的分子基础。与哺乳动物组织特异性盒式外显子等效的结构域位于不同位置，大小和序列可变。最有趣的发现是，由于串联PGQM重复序列数量不同，会出现不同形式的联会蛋白，这些重复序列在不同的成年组织和胚胎阶段表达不同。出于这些原因，我们将这组独特的同工型标记为膜联蛋白VIIb，将缺乏PGQM串联重复序列的哺乳动物形式称为膜联蛋白VIIa。尽管与膜联蛋白VIIa存在这些差异，但发现具有三个PGQM重复序列的重组膜联蛋白VIIb具有催化活性。我们认为这些结果表明非洲爪蟾中实际的钙和磷脂结合位点是保守的，并且在联会蛋白基因家族成员之间在调节结构域中观察到的差异清楚地表明了各个成员在组织和阶段特异性的作用，可能涉及胞吐过程。
