Structural Features of Myosin X that Favor Straddling more than One Actin Filament

Abstract

In the cell, myosin X (M10) is found almost exclusively at the tips of filopodia. The filopodial core contains a parallel and unidirectional arrangement of actin filaments that are bundled by fascin. We have discovered that M10 selects the fascin-actin bundle in the core of the filopodium for motility, and walks poorly on individual, unbundled actin filaments. Here, we have dissected the features that enhance the processivity of M10 on bundles. The structure of M10 may make it easier to walk by straddling two filaments, rather than placing both motor domains on a single filament in a strained configuration. We have measured the stepsize of M10 using both fluorescence imaging and optical traps, and have found ∼18 nm steps in both cases. This stepsize is difficult to achieve on a single actin filament, since the myosin would be forced to spin around the filament as it walked. However, the bundle presents additional options to the motor so that regular 18 nm steps are now possible. Moreover, we have completed a series of domain swaps between M10 and the nonselective myosin V motor, since we reasoned that these domain swaps would help to identify the structures required for bundle selection. Surprisingly, bundle selection is controlled by tail domains (including the SAH domain), a part of the myosin that is presumably far away from the actin track. This work reveals that the cellular cytoskeleton is not simply a collection of identical tracks pointing in every direction. The entire cellular system uses a set of simple and elegant interactions to sort cellular traffic to distinct locations; we expect that this theme will reappear many times as more myosins (and other motor families) are examined.