Presence of Ca(2+)-sensitive and -insensitive SM22 alpha isoproteins in bovine aorta

Two proteins (25- and 22-kDa) that cross-react with anti-gizzard SM22 alpha antibody were isolated from bovine aorta. The former, identical to a SM22 alpha homolog reported previously (Kobayashi, R., Kubota, T., and Hidaka, H. (1994) Biochem. Biophys. Res. Commun. 198, 1275-1280), associates with the membrane fraction in the presence of Ca2+ and dissociates in the presence of EGTA, while the latter is insensitive to Ca2+. Peptide mapping and partial sequence analysis revealed that the 22-kDa protein is an isoform of the 25-kDa protein, without 22 amino acid residues from the C-terminus. Immunological analysis of the tissue distribution of these proteins showed that the 25-kDa protein exists in lung and spleen besides aorta, while the 22-kDa protein occurs specifically in the aorta and spleen.