Hybrid formation between scallop myofibrils and foreign regulatory light-chains

Scallop myofibrils (Placopecten magellanicus) from which regulatory light-chains had been completely removed by EDTA treatment at 30 °C were hybridized with regulatory light-chains of different myosins. Pure hybrids, containing only foreign regulatory light-chains with a stoichiometry of two moles per myosin, were readily formed with all the light-chains tested. Some of the regulatory light-chains restored regulatory functions to desensitized myofibrils by selectively inhibiting the actin activated Mg-ATPase in the absence of calcium. Light-chains from Mercenaria, Spisula Loligo and Urechis behaved as scallop regulatory light-chains, were inhibitory in the absence of calcium, and restored high-affinity calcium binding sites. Regulatory light-chains of Limulus, cricket, chicken gizzard and platelet were also inhibitory; however, calcium binding was restored with a lowered affinity and the hybrids required higher calcium concentrations for ATPase activation. Hybrids formed with the regulatory light-chains of vertebrate striated (rabbit, chicken, skate), bovine cardiac and lobster tail and claw muscles remained insensitive to calcium, their ATPase activity was not selectively depressed in the absence of calcium and specific high-affinity calcium binding sites were not restored. Phosphorylation of the light-chains (rabbit, cardiac and gizzard) has no effect on ATPase activity. The behaviour of the hybrids supports the interpretation that in vertebrate striated muscles myosin does not function as a regulatory switch.Foreign regulatory light-chains (Spisula, Loligo, Mercenaria, rabbit) bind to desensitized myofibrils with a similar or slightly higher affinity as scallop regulatory light-chains. The two light-chain binding sites of myosin are equivalent and differences in affinity appear to be the result of an interaction between the two halves of the myosin molecules.