NOVEL ISOFORMS OF SYNEXIN IN XENOPUS-LAEVIS - MULTIPLE TANDEM PGQM REPEATS DISTINGUISH MESSENGER-RNAS IN SPECIFIC ADULT TISSUES AND EMBRYONIC STAGES

Synexin (annexin VII) is a calcium-dependent, phospholipid-binding and membrane fusion protein in the annexin gene family, which forms calci um channels and may play a role in exocytotic secretion. We report her e the cloning and characterization of five novel isoforms of cDNAs enc oding 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 5 4 kDa protein. This Xenopus protein is 6 kDa larger than the previousl y reported human and mouse synexins with which it shares approx. 73% i dentity in the C-terminal region and approx. 44% identity in the N-ter minal 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 differen t forms of synexin due to the varying numbers of tandem PGQM repeats t hat are expressed differently in different adult tissues and embryonic stages. For these reasons we have labelled this set of unique isoform s annexin VIIb, referring to mammalian forms, which lack the PGQM tand em repeats, as annexin VIIa. In spite of these differences from annexi n VIIa, the form of recombinant annexin VIIb with three PGQM repeats w as found to be catalytically active. We interpret these results to ind icate that the actual calcium and phospholipid binding sites are conse rved in Xenopus, and that the variations observed between members of t he synexin gene family in the regulatory domain clearly point towards the tissue- and stage-specific roles of individual members, possibly i nvolving the exocytotic process.