Amino acid sequence of troponin C from scallop striated adductor muscle.

Troponin and its Ca(2+)-binding subunit troponin C (TnC) of the Ezo-giant scallop, Patinopecten yessoensis, have been revealed to bind only 1 mol of Ca2+/mol irrespective of the presence and absence of Mg2+. The amino acid sequence of the TnC has been determined by the automated Edman degradation. TnC is composed of 152 residues including 3 tryptophans at positions 53, 70, and 109, 4 cysteines at positions 19, 31, 67, and 145, and no proline. The molecular weight is calculated as 17,410. The NH2 terminus of TnC is blocked by an acetyl group. The sequence of scallop TnC required deletion of three residues in the D/E linker region to maximize sequence homology to other TnCs and shows considerably lower homology to vertebrate skeletal TnCs (27-30%), ascidian TnC (26%), arthropoda TnCs (30-37%), and chicken calmodulin (37%). Further, we conclude that Ezo-giant scallop TnC binds Ca2+ at site IV, a site specific for Ca2+. The other sites I, II, and III appeared to lose the Ca2+ binding ability due to substitutions of some important residues.