Full-Length Myosin VI Dimerizes and Moves Processively along Actin Filaments upon Monomer Clustering

Abstract

Myosin VI is a reverse direction actin-based motor capable of taking large steps (30-36 nm) when dimerized. However, all dimeric myosin VI molecules so far examined have included non-native coiled-coil sequences, and reports on full-length myosin VI have failed to demonstrate the existence of dimers. Herein, we demonstrate that full-length myosin VI is capable of forming stable, processive dimers when monomers are clustered, which move up to 1-2 mum in approximately 30 nm, hand-over-hand steps. Furthermore, we present data consistent with the monomers being prevented from dimerizing unless they are held in close proximity and that dimerization is somewhat inhibited by the cargo binding tail. A model thus emerges that cargo binding likely clusters and initiates dimerization of full-length myosin VI molecules. Although this mechanism has not been previously described for members of the myosin superfamily, it is somewhat analogous to the proposed mechanism of dimerization for the kinesin Unc104.

Myosin VI Lever Arm Rotation: Fixed or Variable?

“…This point is confirmed by ours and earlier data [7][8][9] . Thus, the geometrical relationship between the two lever arms should depend on the distance between the two heads after any given step, a point made previously by both groups 2,9 .…”

Powering membrane traffic in endocytosis and recycling

“…Although myosin VI is a monomer in solution, it nevertheless can move processively along actin filaments with surprisingly long strokes 33 . Dimerization was found to be regulated by the clustering of myosin VI on the surface of its vesicular cargo 34 , reminiscent of the mechanism that was proposed for the kinesin UNC104 (REF. 35).…”

Cargo binding induces dimerization of myosin VI

“…Thus myosin VI can potentially fulfill a number of specialized cell biological functions (8-10). Paradoxically, although these functional features suggest that myosin VI is designed to work in cells as a dimer, myosin VI as isolated from cells is a monomer, and expressed full-length myosin VI is also monomeric (4,5,11).A number of cargo adaptor proteins that recruit myosin VI have been identified (8). For example, it has been demonstrated that optineurin is essential for myosin VI localization to the Golgi complex (12), and binds to a site within the globular tail of myosin VI.…”

“…Thus myosin VI can potentially fulfill a number of specialized cell biological functions (8-10). Paradoxically, although these functional features suggest that myosin VI is designed to work in cells as a dimer, myosin VI as isolated from cells is a monomer, and expressed full-length myosin VI is also monomeric (4,5,11).…”

“…Myosin VI dimers take large and variable steps on actin (average of 30-36 nm) using a short lever arm and a unique lever arm extension (4)(5)(6). Not only is the myosin VI dimer capable of processive movement (i.e., can move as a single molecule) along an actin filament (4)(5)(6), it also functions as a load-dependent actin anchoring protein (7). Thus myosin VI can potentially fulfill a number of specialized cell biological functions (8)(9)(10).…”