Regulation by molluscan myosins

Molluscan myosins are regulated molecules that control muscle contraction b y the selective binding of calcium. The essential and the regulatory light chains are regulatory subunits. Scallop myosin is the favorite material for studying the interactions of the light chains with the myosin heavy chain since the regulatory light chains can be reversibly removed from it and its essential light chains can be exchanged, Mutational and structural studies show that the essential light chain binds calcium provided that the Ca-bin ding loop is stabilized by specific interactions with the regulatory light chain and the heavy chain. The regulatory light chains are inhibitory subun its. Regulation requires the presence of both myosin heads and an intact he adrod junction. Heavy meromyosin is regulated and shows cooperative feature s of activation while subfragment-l is non-cooperative. The myosin heavy ch ains of the functionally different phasic striated and the smooth catch mus cle myosins are products of a single gene, the isoforms arise from alternat ive splicing. The differences between residues of the isoforms are clustere d at surface loop-1 of the heavy chain and account for the different ATPase activity of the two muscle types. Catch muscles contain two regulatory lig ht chain isoforms, one phosphorylatable by gizzard myosin light chain kinas e. Phosphorylation of the light chain does not alter ATPase activity. We co uld not find evidence that light chain phosphorylation is responsible for t he catch state. (Mol Cell Biochem 190. 55-62, 1999).