A mixed-kinetic model describes unloaded velocities of smooth, skeletal, and cardiac muscle myosin filaments in vitro.

In vitro motility assays, where purified myosin and actin move relative to one another, are used to better understand the mechanochemistry of the actomyosin adenosine triphosphatase (ATPase) cycle. We examined the relationship between the relative velocity () of actin and myosin and the number of available myosin heads () or [ATP] for smooth (SMM), skeletal (SKM), and cardiac (CMM) muscle myosin filaments moving over actin as well as from actin filaments moving over a bed of monomeric SKM. These data do not fit well to a widely accepted model that predicts that is limited by myosin detachment from actin (/), where equals step size and equals time a myosin head remains attached to actin. To account for these data, we have developed a mixed-kinetic model where is influenced by both attachment and detachment kinetics. The relative contributions at a given vary with the probability that a head will remain attached to actin long enough to reach the end of its flexible S2 tether. Detachment kinetics are affected by /, where is related to the tether length. We show that is relatively long for SMM, SKM, and CMM filaments (59 ± 3 nm, 22 ± 9 nm, and 22 ± 2 nm, respectively). In contrast, is shorter (8 ± 3 nm) when myosin monomers are attached to a surface. This suggests that the behavior of the S2 domain may be an important mechanical feature of myosin filaments that influences unloaded shortening velocities of muscle.