A Little Motor, a Big Switcher! Bidirectional Membrane Tube Movement Driven by Collections of Nonprocessive Motors

Abstract

Motors proteins are essential players in intracellular transport, often working in groups to move cargo across the cell. Yet how multiple motors coordinate to mediate cargo movement is still unclear. Inspired by the motor-driven network of the endoplasmic reticulum, we examine the organization and transport of membrane material by active motors in a minimal model system using Giant Unilamellar Vesicles (GUVs) as a membrane reservoir. We find that motors attached to the outside of a GUV, in the presence of microtubule (MT) tracks, collectively exert forces large enough to deform the GUV and extract membrane nanotubes. Processive kinesin motors (motors that take many steps before falling off a MT) had previously been shown to extract membrane tubes, but surprisingly, nonprocessive ncd motors (motors that only take a single step before falling from a MT) can also mediate membrane tube formation. Moreover, tubes formed by nonprocessive motors show distinct phases of persistent growth, retraction, and an intermediate phase characterized by dynamic switching between the two. We probe the physical mechanism by which nonprocessive motors collectively mediate membrane tube dynamics with image correlation spectroscopy and fluorescence recovery after photobleaching. Nonprocessive motors at the interface between the underlying MT track and the membrane tube cargo show a diffusive behavior with a diffusion constant 1000 times smaller than that of a freely-diffusing lipid-motor complex. We interpret the small diffusion constant as an indicator that nonprocessive motors dynamically bind and unbind to the MT in order to maintain a continuous interaction between the membrane tube and MT. We consequently develop a model that describes the membrane tube dynamics through a balance between motor density fluctuations and membrane tube tension [Shaklee et al PNAS 2008; Biophys J accepted].