Structural changes induced in scallop heavy meromyosin molecules by Ca2+ and ATP

We have used physicochemical and ultrastructural methods to investigate the effects of Ca2+ and ATP on the structure of purified heavy meromyosin (HMM) from the striated adductor muscle of the scallop, a species with myosin-linked regulation. Using papain as a structural probe, we found that, in the presence of ATP, the head/tail junction was five times more susceptible to digestion at high levels of Ca2+ than at low levels. By HPLC gel filtration, two fractions of scallop HMM with different Stokes radii were detected in the presence of ATP at low Ca2+, while at high Ca2+ a single peak with the larger Stokes radius predominated. Electron microscopy of rotary-shadowed HMM suggested that molecules with the smaller Stokes radius had their heads bent back towards their tails, while those with the larger radius had heads pointing away from the tail. The number of molecules with their heads bent back decreased at high Ca2+ levels. The data also showed that in the absence of ATP or at high salt, HMM molecules behaved similarly to those in the presence of ATP at high Ca2+. These results suggest that scallop myosin heads can exist in two conformations (heads down towards the tail and heads up away from the tail) and that the equilibrium between these two conformations is altered by the concentrations of salt, ATP and Ca2+. However, the equilibrium between the two forms appears to be too slow to be involved in regulating contraction. The 'heads-down' configuration may instead be related to the inactive, folded (10S) form of scallop myosin and possibly involved in filament assembly during development.