Proteolysis and actin-binding properties of 10S and 6S smooth muscle myosin: identification of a site protected from proteolysis in the 10S conformation and by the binding of actin.

It was shown previously [Ikebe, M., & Hartshorne, D. J. (1985) Biochemistry 24, 2380-2387] that the conformation of gizzard myosin, either 10S or 6S, influences proteolysis of myosin at two regions designated sites A and B. The studies reported here are focused on site A, which is located approximately 68,000 daltons from the N-terminus of the myosin heavy chain. With papain, Staphylococcus aureus protease, and actinidin, it is shown that the formation of 10S myosin reduces proteolysis at site A. Binding of actin to 6S myosin also hinders cleavage at site A for each of these proteases. To investigate binding of actin to 6S and 10S myosins, adenosine 5'-(beta,gamma-imidotriphosphate) (AMPPNP) is used as a substitute for ATP. In the presence of AMPPNP, it is shown that the 6S to 10S transition occurs and that 10S myosin binds actin with lower affinity than 6S myosin. For 6S myosin at high salt (0.35 M KCl) the dissociation constant of actin from the actin-myosin-nucleotide complex (K3) is approximately the same for phosphorylated (1.9 mol of P/mol of myosin) and dephosphorylated myosin, i.e., 1.3-2.4 microM, respectively. At lower ionic strength (0.17 M KCl) K3 for dephosphorylated myosin (10S myosin) is 42 microM and K3 for phosphorylated myosin (6S myosin) is 0.3 microM. These data show that the conformation of myosin influences the actin-myosin interaction. The constant (K4) for the dissociation of nucleotide from the actin-myosin-nucleotide complex varies slightly (in the range of 0.2-1.3 mM), but there is no marked change as a result of either a conformational change or phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)