Amino acid sequence of a low molecular weight, high affinity calcium-binding protein from the optic lobe of the squid Loligo pealei.

The amino acid sequence was determined for squid calcium-binding protein (CaBP), a low molecular weight, high affinity calcium-binding protein from squid optic lobe. The sequence shows this protein to consist of 149 amino acids with an N-acetylated N terminus. The protein has a molecular weight of 16,894 and is homologous to calmodulin, which is also present in squid optic lobe (Head, J. F., Spielberg, S., and Kaminer, B. (1983) Biochem. J. 209, 797-802). When the sequences of squid CaBP and bovine brain calmodulin are appropriately aligned, the proteins are found to share 68% identity, with a single residue insertion in squid CaBP, between domains III and IV. The four-domain structure of calmodulin appears to be retained in squid CaBP, which is consistent with the previously reported presence of four calcium-binding sites per molecule (Sheldon, A., and Head, J. F. (1988) J. Biol. Chem. 263, 14384-14389). The two tyrosines of squid CaBP are located in different halves of the molecule, one at the position corresponding to Tyr-138 in calmodulin, the other in an equivalent position in domain II. In addition, squid CaBP exhibits several differences in the region corresponding to the long central helix of calmodulin. These differences include the replacement of Lys-77 by glycine, Asp-78 by proline, and Ser-81 by proline. The sequence of this portion of the squid CaBP molecule suggests the protein is unlikely to possess the continuous long central helix found in calmodulin.