Myosin head interactions in Ca2+-activated skinned rabbit skeletal muscle fibers.

Interactions between the two myosin heads were studied in skinned rabbit slow-twitch muscle fibers activated in the presence of vanadate (Vi), a phosphate analog. The strong complex between Vi, MgADP, and myosin trapped the myosin in an inactivated myosin x MgADP x Vi state. Electron paramagnetic resonance spectroscopy was used to quantitate the fraction of myosin heads trapped in the presence of a spin labeled analog of ATP (SLATP). Force was found to depend directly on the fraction of untrapped heads. At high [Vi] (low force), most untrapped heads would have a trapped partner. The equivalence of force with the proportion of untrapped heads shows that the isometric force produced by a single untrapped myosin head on a molecule with a trapped partner is equivalent to that produced by either head of a myosin molecule with neither head trapped. The actin-activated MgATPase activities of one-headed and two-headed skeletal myosin species were inhibited similarly by Vi, suggesting that trapping one head did not preclude trapping its partner. These data indicate that the two skeletal muscle myosin heads can function without interacting during maximal Ca2+-activated force generation.