Role of Essential Light Chain EF Hand Domains in Calcium Binding and Regulation of Scallop Myosin

The specific Ca2+binding site that triggers contraction of molluscan muscle requires the presence of an essential light chain (ELC) from a Ca2+binding myosin. Of the four EF hand-like domains in molluscan ELCs, only domain III has an amino acid sequence predicted to be capable of binding Ca2+. In this report, we have used mutant ELCs to locate the Ca2+binding site in scallop myosin and to probe the role of the ELC in regulation. Point mutations in domain III of scallop ELC have no effect on Ca2+binding. Interestingly, scallop and rat cardiac ELC chimeras support Ca2+binding only if domain I is scallop. These results are nevertheless in agreement with structural studies on a proteolytic fragment of scallop myosin, the regulatory domain. Furthermore, Ca2+sensitivity of the scallop myosin ATPase requires scallop ELC domain I: ELCs containing cardiac domain I convert scallop myosin to an unregulated molecule whose activity is no longer repressed in the absence of Ca2+. Despite its unusual EF hand domain sequence, our data indicate that the unique and required contribution of molluscan ELCs to Ca2+binding and regulation of molluscan myosins resides exclusively in domain I.