MYOSIN AND ACTIN: FUNCTIONAL TUNING VIA MOTOR DIFFUSION AND COMPLIANCE

P. Bryant Chase
Departments of Radiology and Physiology & Biophysics, University of Washington, Seattle, WA 98195-7115

The most abundant biological motor protein of vertebrates, myosin, provides an exceptional model system for learning about the function and mechanism of molecular motors. Myosin moves along actin filaments and this motion is generally regulated by calcium acting via additional proteins. Myosin typically performs in an ensemble which, combined with calcium regulation and also the compliance of proteins, can yield interesting cooperative behavior.

Since myosin function involves molecular motions, we tested the hypothesis that solvent viscosity influences motility. The maximum speed for translocation of actin filaments by myosin varied inversely with solution viscosity both in the muscle sarcomere (single, "skinned" fibers from skeletal muscle) and also for individual actin filaments (in vitro motility assay). Since the former observations were made at maximum calcium-activation and the latter were obtained with (rhodamine-phalloidin labeled) unregulated actin filaments, the effect of viscosity does not depend on the regulatory proteins. Myosin ATPase activity, measured in the absence of actin, decreased only when viscosity was elevated substantially above the range which slowed actin sliding speed. These effects of mono-, di- and poly-saccharides were not related to altered solution osmolarity. At experimentally observed speeds on the order of 1 - 10 micrometers per second, the viscous force on an actin filament is small relative to the force exerted by even an individual motor. Furthermore, in the muscle fiber preparation, the kinetics of tension development (essentially in the absence of filament sliding) also varied inversely with solution viscosity. Therefore we suggest that the primary effect of viscosity is slower diffusion of myosin motor domain to binding sites on actin filaments.

Modeling studies which attempt to incorporate myosin head diffusion to actin (which is related to the flexibility of the myosin "head" and "neck" regions) as part of the chemomechanical ATPase cycle as well as accounting for compliances of proteins in the sarcomere demonstrate significant tuning of the entire system. As the compliances for both myosin crossbridges and the filament lattice were systematically varied, we found that isometric force reached a maximum--due to compliant realignment of binding sites--when model compliances were similar to the best estimates of physiological values. Therefore compliance can enhance some functional aspects of ensembles of molecular motors.

Taken together with our previous work, these results illustrate: (1) significant design considerations for nanomachines, and features of the actomyosin system in muscle; and (2) interesting cooperative aspects of the function of molecular motors.

Supported by National Institutes of Health grant HL52558.