ACTOMYOSIN INTERACTIONS IN THE PRESENCE OF ATP AND THE N-TERMINAL SEGMENT OF ACTIN

The binding of myosin subfragment 1 (S-1) to actin in the presence of ATP and the acto-S-1 ATPase activities of acto-S-1 complexes were determined at 5-degrees-C under conditions of partial saturation of actin, up to 90%, by antibodies against the first seven N-terminal residues on actin. The antibodies [F(ab)(1-7)] inhibited strongly the acto-S-1 ATPase and the binding of S-1 to actin in the presence of ATP at low concentrations of S-1, up to 25-mu-M. Further increases in S-1 concentration resulted in a partial and cooperative recovery of both the binding of S-1 to actin and the acto-S-1 ATPase while causing only limited displacement of F(ab)(1-7) from actin. The extent to which the binding and the ATPase activity were recovered depended on the saturation of actin by F(ab)(1-7). The combined amounts of S-1 and F(ab) binding to actin suggested that the activation of the myosin ATPase activity was due to actin free of F(ab). Examination of the acto-S-1 ATPase activities as a function of S-1 bound to actin at different levels of actin saturation by F(ab)(1-7) revealed that the antibodies inhibited the activation of the bound myosin. Thus, the binding of antibodies to the N-terminal segment of actin can act to inhibit both the binding of S-1 to actin in the presence of ATP and a catalytic step in ATP hydrolysis by actomyosin. The implications of these results to the regulation of actomyosin interaction are discussed.