The function of two heads of myosin in muscle contraction

Myosin has two heads which can bind with F-actin and react with ATP. The skeletal muscle myosin forms each 1 mol of the myosin-phosphate-ADP complex (M-P-ADP) and the myosin-ATP complex (M-ATP). The actomyosin ATPase reaction which is coupled with muscle contraction is catalyzed only by the head which forms M-P-ADP. However, the function of M-ATP forming head in muscle contraction has not been elucidated. We studied the binding of S-1 and HMM with F-actin and the dissociation of acto-S-1 or acto-HMM by ATP or AMPPNP using the change in light-scattering and fluorescence of pyrene bound to F-actin. S-1 and HMM bound with actin at 1:1 and 1:2 molar ratio, respectively. Acto-S-1 dissociated by one mole of ATP per mole of S-1 but acto-HMM dissociated by 1 mol ATP per mol of HMM (0.5 mol/mol head). Acto-HMM dissociates by AMPPNP (or ADP) via a ternally complex. Acto-HMM bound two mole of AMPPNP, but acto-HMM dissociated by a function of (AMPPNP) but not (AMPPNP)2. These results suggested that the affinity of HMM with F-actin decreased by the binding of one mole of AMPPNP. The result presented here showed that binding of M-ATP forming head with F-actin is controlled by the ATPase reaction of the M-P-ADP forming head. It is suggested that during muscle contraction two heads react cooperatively with thin filament.