Myosin VI Lever Arm Rotation: Fixed or Variable?

Abstract

Two recent articles 1,2 addressed the power-stroke of myosin VI molecules during stepping. Although both groups measured the angles of fluorescent probes attached on the myosin VI molecule lever arm using polarized fluorescence techniques, they differ about whether the myosin VI lever arm rotation is fixed 1 or variable 2 . Here we discuss the causes of the discrepancy between the two studies and the implications for myosin VI processive motility.Introduction: Myosin VI, an unconventional myosin motor that walks toward the minus ends of actin filaments, has received great attention recently. Reifenberger et al. 1 have presented evidence from Defocused Orientation and Positional Imaging (DOPI) assays that myosin VI lever arms tilt by an angle very close to 180° on each step. This contrasts with results from Sun et al. 2 wherein we applied single molecule Polarized Total Internal Reflection Fluorescence (polTIRF) microscopy and found variable degrees of tilting both in the axial and azimuthal directions relative to actin. This variability is consistent with the known variable step size of myosin VI. There are several technical differences in the two papers that interested researchers should understand, and more importantly, substantive differences in the conclusions.

Molecular Machines Like Myosin Use Randomness to Behave Predictably

“…This property is especially true in myosin VI because of its elastic arms, which allow it to bind to sites other than the 13th subunit without increasing energy demands significantly. 106,185,186 Binding sites on an actin filament are separated by about 28°. While 36 nm steps translate into myosin VI binding at every 13th subunit, the high variability in its step size indicates that the 11th subunit is equally preferable, especially at high loads when the step size slightly lessens.…”