Xenopus annexin II (calpactin I) heavy chain has a distinct amino terminus

We have isolated cDNAs encoding annexin II (calpactin I) heavy chain homologues from a Xenopus oocyte cDNA library. Two of the clones are full length, while two appear to be derived from incompletely spliced mRNAs. The 1230- and 1240-base pair full length clones are 99% identical, and both have 84 bases of 5'-untranslated sequence, 1020-base open reading frames, and either 126- or 136-base 3'-untranslated domains. Northern blots show a 1.4-kilobase (kb) annexin II heavy chain transcript throughout oogenesis and in mature eggs. Xenopus annexin II mRNA levels are constant during early embryogenesis, but decrease at 8 h. After midblastula transition, the steady state level of the 1.4-kb transcript increases substantially, and a 3.3-kb transcript appears. Adult brain, heart, striated muscle, and liver contain moderate amounts of the 1.4-kb transcript, while skin has the 3.3-kb transcript and very high levels of the 1.4-kb transcript. Synthetic mRNA derived from the Xenopus annexin II cDNAs directs the synthesis of an apparent Mr = 36,500 polypeptide when microinjected into Xenopus oocytes. The predicted 339-amino acid protein products are 80% identical with murine annexin II heavy chain. Most of the differences are concentrated in the amino end from residues 15 to 24. The Xenopus annexin II heavy chain lacks the highly conserved tyrosine at position 23 which is the site of src oncogene tyrosine kinase phosphorylation in the murine protein. These results demonstrate that Xenopus oocytes contain an annexin II (calpactin I) heavy chain mRNA with a distinct amino terminus and suggest that multiple annexin II isoforms may be expressed during amphibian embryogenesis and development.