Helical motors and formins synergize to compact chiral filopodial bundles: a theoretical perspective.

Chiral actin bundles have been shown to play an important role in cell dynamics, but our understanding of the molecular mechanisms which combine to generate chirality remains incomplete. We numerically simulate a crosslinked filopodial bundle under the actions of helical myosin motors and/or formins and examine the collective buckling and twisting of the actin bundle. We find that the myosin spinning action effectively "braids" the bundle, compacting it, generating buckling, and enhancing crosslinking. Stochastic fluctuations of actin polymerization rates also contribute to filament buckling and bending of the bundle. Faster turnover of transient crosslinks attenuates the buckling and enhances coiling and compaction of the bundle. Formin twisting action by itself is not effective in inducing filopodial coiling and compaction, but co-rotating formins synergize with helical motors to coil and compact the actin bundle. We discuss implications of our findings for mechanisms of cytoskeletal chirality.