Phosphorylation-dependent regulation of Limulus myosin.

Myosin from Limulus, the horseshoe crab, is shown to be regulated by a calcium-calmodulin-dependent phosphorylation of its regulatory light chains. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis of a Limulus myosin preparation reveals three light chain bands. Two of these light chains have been termed regulatory light chains based on their ability to bind to light chain-denuded scallop myofibrils (Sellers, J. R., Chantler, P. D., and Szent-Györgyi, A. G. (1980) J. Mol. Biol. 144, 223-245). Ths other light chain does not bind to these myofibrils and is thus termed the essential light chain. Both Limulus regulatory light chains can be phosphorylated with a highly purified turkey gizzard myosin light chain kinase or with a partially purified myosin light chain kinase which can be isolated from Limulus muscle by affinity chromatography on a calmodulin-Sepharose column. Phosphorylation with both of these enzymes requires calcium and calmodulin. Limulus myosin is isolated in an unphosphorylated form. The MgATPase of this unphosphorylated myosin is only slightly activated by rabbit skeletal muscle actin plus tropomyosin. The calcium-dependent phosphorylation of the myosin results in an increase in the actin-activated MgATPase rate. Once phosphorylated, the actin-activated MgATPase rate is only slightly modified by calcium. This suggests that calcium operates mainly at the level of the myosin kinase-calmodulin system.