A NOVEL EF-HAND CA2-BINDING PROTEIN FROM ABDOMINAL MUSCLE OF CRUSTACEANS WITH SIMILARITY TO CALCYPHOSINE FROM DOG THYROIDEA()

The amino acid sequence of a novel EF-hand Ca2+-binding protein from t he abdominal muscle of the crayfish, Orconectes limosus, has been eluc idated by tandem mass spectrometry and automated Edman degradation. Th e name CCBP-23 (23-kDa crustacean Ca2+-binding protein) is proposed. T he protein can also exist as a disulfide-linked homodimer. The sequenc e of the monomeric form spans 200 residues with an acetylated N-termin al Ser and reveals four EF-hand domains. The 174-mass-unit difference between the calculated average molecular mass of 22669.6 Da deduced fr om the sequence and the obtained electrospray ionization mass spectros copy (ESI-MS) mass of 22844 Da has not yet been explained. Partial seq uence analysis (137 residues) of CCBP-23 from the lobster, Homarus ame ricanus, showed a sequence identity of 74% with the crayfish protein. Homology searches revealed a 44% sequence identity of CCBP-23 from cra yfish to calcyphosine, a Ca2+-binding protein from dog thyroidea (Lefo rt et al., 1989). Although CCBP-23 also shows a 44% identity to R2D5 a ntigen (Nemoto et al., 1993), we believe that both proteins represent two distinct subgroups within the family of EF-hand proteins.