Membrane-induced Lever Arm Expansion Allows Myosin VI to Walk with Large and Variable Step Sizes

Cong Yu,Jizhong Lou,Jingjing Wu,Lifeng Pan,Wei Feng,Mingjie Zhang

doi:10.1074/jbc.m111.328781

Abstract

Myosin VI, the only known minus-ended actin filament-dependent motor, plays diverse cellular roles both as a processive motor and as a mechanical anchor. Although myosin VI has a short lever arm containing only one "IQ-motif" and a unique insertion for CaM binding, the motor walks with large and variable step sizes of ∼30-36 nm. Here, we show that the previously predicted coiled-coil domain immediately following the IQ-motifs (referred to as the lever arm extension (LAE)) adopts a stable monomeric, three-helix bundle fold in solution. Importantly, the LAE can undergo reversible, lipid membrane-dependent conformational changes. Upon exposure to lipid membranes, the LAE adopts a partially extended rod shape, and the removal of lipids from the LAE converts it back into the compact helix bundle structure. Molecular dynamics simulations indicate that lipid membrane binding may initiate unfolding and thereby trigger the LAE expansion. This reversible, lipid membrane-dependent expansion of the LAE provides a mechanistic base for myosin VI to walk with large and variable step sizes.

Highlights

Myosin VI plays diverse cellular roles ranging from intracellular transport to mechanical anchor
The reversible, cargo vesicle-dependent expansion of the LAE, together with the previously identified IQ-motifs and possibly the SAH rod, provides sufficient space and flexibility for myosin VI to walk on actin filaments with a large and variable step size of ϳ30 –36 nm
We found that the LAE C-terminal to the IQmotif adopts a stable monomeric, three-helix bundle structure with a length of ϳ3 nm

Summary

Background

Myosin VI plays diverse cellular roles ranging from intracellular transport to mechanical anchor. Upon binding to CaM, the IQ-motifs of each motor form rigid extended ␣-helices, and these extended ␣-helices function as the two “legs” that each myosin dimer uses to walk along actin filaments in a hand-over-hand manner (the so-called lever arm hypothesis) [22] Both the length and the swing angle of the lever arms are directly correlated to the step size of each myosin motor. The reversible, cargo vesicle-dependent expansion of the LAE (termed the expandable “knee joint” of the myosin VI “leg”), together with the previously identified IQ-motifs and possibly the SAH rod, provides sufficient space and flexibility for myosin VI to walk on actin filaments with a large and variable step size of ϳ30 –36 nm

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION