Regulatory domains of myosins: influence of heavy chain on Ca(2+)-binding

Light chain binding domains of rabbit skeletal, turkey gizzard and scallop myosin comprised of equimolar amounts of a short heavy chain fragment, essential light chain, and regulatory light chain have been obtained following extensive tryptic digestion. These complexes that are analogous to the regulatory domain prepared previously from scallop myosin by digestion with clostripain resist proteolysis due to the mutual protection of the heavy chain and the light chains, and are common structural features of the myosins studied. Specific Ca(2+)-binding by the regulatory domains reflects the behaviour of intact myosin; only scallop regulatory domain has a specific Ca(2+)-binding site. The heavy chain fragments of the different regulatory domains have been isolated under denaturing conditions and reconstituted with scallop essential light chain and scallop regulatory light chain or turkey gizzard regulatory light chain to yield regulatory domain hybrids. Hybrids containing the turkey gizzard regulatory light chain were used in Ca(2+)-binding studies since they were far more stable than their counterparts with the scallop regulatory light chain. The gizzard hybrid binds Ca2+ with a comparable specificity but somewhat lower affinity than native scallop regulatory domain. The rabbit regulatory domain hybrid also binds Ca2+, although with a reduced affinity and specificity. The results indicate that Ca(2+)-binding ability is determined by the light chains and modified by the heavy chains.