Proposed modification of the Huxley-Simmons model for myosin head motion along an actin filament.

A model is proposed for myosin head motion along an actin filament which accommodates recent experimental data. The model includes three attached states of a myosin head and is thus similar to the classical Huxley & Simmons (1971) model, but differs in that an explicit expression is given for the spatial distribution of potential energy wells for the myosin head. Our model also differs from the classical model, in that it assumes that the proportion of myosin heads attached to actin filament is constant and independent of shortening velocity, as suggested by X-ray diffraction data. Furthermore, it posits that the crossbridge is string-like rather than spring-like. This modified model fits well to the experimental data in the following respects. (1) The calculated tension dependence of muscle stiffness agrees with the observation by Ford et al. (1985 J. Physiol. 361, 131-150). (2) A myosin head under low load can move as far as 60 nm along an actin filament during one ATP hydrolysis cycle in muscle, in agreement with the results by Yanagida et al. (1985 Nature 316, 366-369) and others. (3) The model predicts that such movements consist of a series of elementary steps of 11 nm. (4) A single myosin head hardly moves after the first step of 11 nm under the condition of in vitro experiment carried out by Finer et al. (1994 Nature 368, 113-119), in agreement with their observation. (5) The calculated energy liberation rate reproduces the characteristics of Hill's equation. (6) The "double-hyperbolic force-velocity relation" reported by Edman (1988 J. Physiol. 404, 301-321) can be understood in terms of a potential barrier against movement of a potential well in which a myosin head is trapped.