The contributions of the actin machinery to endocytic membrane bending and vesicle formation

Branched and crosslinked actin networks mediate cellular processes that move and shape membranes. To understand the mechanisms of force-generation by actin, we used a hybrid imaging approach that combines live imaging with correlative microscopy to analyse deletion mutants of yeast actin network components. Our data show that the actin network needs to be physically linked to the plasma membrane to overcome a force threshold for initiating membrane bending. Two force-generating mechanisms reshape the membrane in successive phases. First, nucleation and polymerization of new actin filaments pull the membrane invagination. Then, rearrangement and expansion of the actin network provide the driving force for vesicle formation and propulsion. The transition between the phases occurs 2 seconds before scission when the filament nucleation rate drops. Our findings suggest that actin-based motility can be uncoupled from actin network growth and can be driven by the release of stress accumulated by the crosslinked actin network.