Amino acid sequence of myosin essential light chain from the scallop Aquipecten irradians

The amino acid sequence of the scallop myosin essential light chain (SELC) was determined from analysis of the intact, S-carboxymethylated protein and peptides produced by cleavage at its four methionine residues by cyanogen bromide digestion and at its six arginine residues by citraconylation and tryptic digestion. SELC contains 156 amino acid residues, including three cysteines, four tyrosines, one tryptophan, two histidines, and an unblocked amino-terminal proline. The protein has a calculated Mr of 17,616. SELC is an acidic protein, with a net charge of 18- at physiological pH. Comparative analysis reveals four homologous domains (I-IV), which arose by reduplication of a gene for a small, ancestral calcium binding protein. Each domain has a helix-loop-helix structure, with all the ligands for calcium binding located within a 12-residue segment that spans the loop and the first turn of the following helix. Potential calcium binding sequences were found in the ancestral sites III (residues 94-105) and IV (residues 132-143). Mutations in critical positions in domains I and II seem to preclude the possibility of calcium binding in the amino-terminal half of SELC. An unexpected third potential calcium binding segment (at residues 119-130, predicted to be in helical conformation) was found in domain IV. A reactive thiol group (Cys-78) that is involved in binding of regulatory light chains was tentatively located in an extended "linker region", which connects the two halves of the molecule.