Conformation-dependent proteolysis of smooth-muscle myosin

The folded 10 S conformation of turkey gizzard myosin is more resistant to proteolysis by papain than the extended 6 S conformation. These findings confirm those of Onishi and Watanabe (Onishi, H., and Watanabe, S. (1984) J. Biochem. (Tokyo) 95, 899-902). In addition, we suggest that the effect of phosphorylation on heavy-chain digestion by papain is related to the dependence of conformation on phosphorylation and not to a direct effect of phosphorylation itself. Proteolysis by Staphylococcus aureus protease and trypsin also is slower for 10 S compared to the 6 S conformation. Heavy chain hydrolysis by alpha-chymotrypsin is not dependent on myosin conformation. Filamentous myosin and heavy meromyosin are more resistant to papain proteolysis in the dephosphorylated compared to the phosphorylated states. The different sensitivities to proteolysis probably are caused by changes in the subfragment 1-subfragment 2 region of the molecule rather than at the heavy meromyosin-light meromyosin junction. These changes are induced as part of the 6 S-10 S transition and occur in monomeric and filamentous myosin and in heavy meromyosin. These more subtle alterations in the head-neck junctions of the molecule may be more important in modifying myosin enzymatic activity than the actual interaction of the tail and neck regions of the molecule.