Studies of the Force-Dependent Motor Activity of Myosin I

Abstract

Class I myosins are implicated in membrane dynamics, cell structure, and mechanical signal transduction. Broadly speaking, these various roles can be divided into two categories, 1) trafficking function and 2) structural function. It is not clear how myosin I can achieve such disparate functions. One possibility is that the motor's function is regulated by interactions with other proteins and organelles in its cellular environment, a form of regulation that is observed with a class VI myosin. An alternate hypothesis is that the motor changes its activity in response to external forces. External forces selectively perturb mechanical transitions in a motor's mechanochemical cycle, and such perturbations can modulate the motor's kinetics and alter its function. This hypothesis is supported by optical trap studies of a myosin 1b construct, which demonstrated that myosin I is exquisitely sensitive to forces opposing its motion (Laasko et al., Science, 321 (5885):133-136). We tested this model by conducting single-molecule studies of the motor activity of Acanthamoeba myosin Ic (AM1C). Specifically, we used an optical trap assay that allows us to apply forces in various directions relative to the motor's motion along an actin filament.