Myosin VI Dimerizes And Walks Processively Along Actin

Abstract

Myosin VI is an unconventional motor protein that can move processively along the actin filament in an opposite direction towards the minus-end, contrary to all other known myosins. Despite its short lever arm, represented by a single IQ domain, myosin VI demonstrates large step sizes (30-36nm), typically characteristic of motor proteins with longer lever arms, viz. myosin V with 6 IQ domains. In cells, myosin VI is involved in diverse functions including Golgi transport, endocytosis and stereocilia maintenance. Though it is possible that myosin VI can function either as a dimer or a monomer in cells, based on our studies on the functional properties of the protein, it is likely that a dimeric protein can undergo intramolecular strain to become a more efficient actin anchor which makes it more competent as a transporter. Previous studies from our lab have shown that both full-length as well HMM fragments are capable of forming stable, processive dimers upon clustering, indicating that myosin VI monomers need to be in close proximity to initiate dimerization. Our recent studies show dimerization of full-length myosin VI can be triggered by cargo binding and the cargo-bound motors walk processively on actin filaments with the expected step size. Following the IQ motif, the lever arm extension of about hundred amino acid residues contains the sequence sufficient for dimerization. However, the accurate location of dimer formation remains controversial since the putative dimerization domain in myosin VI has non-native coiled-coil sequences. Our working hypothesis is that dimerization triggers the unfolding of a 3-helix bundle creating the 12nm extension required for proper myosin VI walking. Based on a series of truncations, we are in the process of testing this hypothesis and defining the nature and sequence of the dimerization domain.