  F-actin bundles constitute principal components of a multitude of
cytoskeletal processes including stereocilia, filopodia, microvilli,
neurosensory bristles, cytoskeletal stress fibers, and the sperm acrosome. The
bending, buckling, and stretching behaviors of these processes play key roles
in cellular functions ranging from locomotion to mechanotransduction and
fertilization. Despite their central importance to cellular function, F-actin
bundle mechanics remain poorly understood. Here, we demonstrate that bundle
bending stiffness is a state-dependent quantity with three distinct regimes
that are mediated by bundle dimensions in addition to crosslink properties. We
calculate the complete state-dependence of the bending stiffness and elucidate
the mechanical origin of each. A generic set of design parameters delineating
the regimes in state-space is derived and used to predict the bending stiffness
of a variety of F-actin bundles found in cells. Finally, the broad and direct
implications that the isolated state-dependence of F-actin bundle stiffness has
on the interpretation of the bending, buckling, and stretching behavior of
cytoskeletal bundles is addressed.
