Filamentous actin (F-actin) is a stiff biopolymer that is tightly crosslinked in vivo by actin-binding proteins (ABPs) to form stiff bundles that form major constituents of a multitude of slender cytoskeletal processes including stereocilia, filopodia, microvilli, neurosensory bristles, cytoskeletal stress fibers, and the acrosomal process of sperm cells (Fig. 1). The mechanical properties of these cytoskeletal processes play key roles in a broad range of cellular functions — the bending stiffness of stereocilia mediates the mechanochemical transduction of mechanical stimuli such as acoustic waves to detect sound, the critical buckling load of filopodia and acrosomal processes determines their ability to withstand compressive mechanical forces generated during cellular locomotion and fertilization, and the entropic stretching stiffness of cytoskeletal bundles mediates cytoskeletal mechanical resistance to cellular deformation. Thus, a detailed understanding of F-actin bundle mechanics is fundamental to gaining a mechanistic understanding of cytoskeletal function.

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