Twirling motion of actin filaments in gliding assays with non-processive myosin motors

TL;DR

This study models how actin filaments exhibit a left-handed twirling motion in gliding assays driven by non-processive myosin motors, revealing that filament structure and target zones influence the direction and pitch of rotation.

Contribution

The paper introduces a stochastic model explaining the twirling motion of actin filaments with non-processive myosin motors, highlighting the role of target zones and filament velocity.

Findings

Filaments rotate counter to their structural handedness.

Twirling pitch varies with filament velocity, reaching ~400nm at low speeds.

The model predicts the dependence of twirling on ATP concentration.

Abstract

We present a model study of gliding assays in which actin filaments are moved by non-processive myosin motors. We show that even if the power stroke of the motor protein has no lateral component, the filaments will rotate around their axis while moving over the surface. Notably, the handedness of this twirling motion is opposite from that of the actin filament structure. It stems from the fact that the gliding actin filament has "target zones" where its subunits point towards the surface and are therefore more accessible for myosin heads. Each myosin head has a higher binding probability before it reaches the center of the target zone than afterwards, which results in a left-handed twirling. We present a stochastic simulation and an approximative analytical solution. The calculated pitch of the twirling motion depends on the filament velocity (ATP concentration). It reaches about 400nm for low speeds and increases with higher speeds.