Characterization of Three Regulatory States of the Striated Muscle Thin Filament

The troponin–tropomyosin-linked regulation of striated muscle contraction occurs through allosteric control by both Ca2+ and myosin. The thin filament fluctuates between two extreme states: the inactive “off” state and the active “on” state. Intermediate states have been proposed from structural studies and transient kinetic measurements. However, in contrast to the well-characterised, on and off states, the mechanochemical properties of the intermediate states are much less well understood because of the instability of those states. In the present study, we have characterized a myosin-induced intermediate that is stabilized by cross-linking myosin motor domains (S1) to actin filaments (with a maximum of one S1 molecule for 50 actin monomers). A single S1 molecule is known to interact with two adjacent actin monomers. A detailed analysis revealed that thin filaments containing S1 molecules cross-linked to just one actin monomer (actin1–S1 complexes) are regulated with a 79% inhibition of the ATPase in the absence of Ca2+. In contrast, filaments containing S1 molecules cross-linked at two positions, to two adjacent actin monomers (actin2–S1 complexes) totally lose their regulation in a highly cooperative manner. This loss of regulation was due both to an enhancement of the ATPase activity without calcium and an inhibition of the ATPase with calcium. Filaments containing actin2–S1 complexes, with significant ATPase activity in the absence of calcium (about 50%), did not move on a myosin-coated surface unless calcium was present. This partial uncoupling between the ATPase activity and in vitro motility in the absence of calcium demonstrates that the mechanical steps require actin–myosin contacts, which take place only in the on state and not in the off or intermediate states. These data provide new insights concerning the difference in cooperativity of Ca2+ regulation that exists between the biochemical and mechanical cycles of the actin–myosin motor.