  We examine the mechanism of bundling of cytoskeletal actin filaments by two
representative bundling proteins, fascin and espin. Small-angle X-ray studies
show that increased binding from linkers drives a systematic \textit{overtwist}
of actin filaments from their native state, which occurs in a linker-dependent
fashion. Fascin bundles actin into a continuous spectrum of intermediate twist
states, while espin only allows for untwisted actin filaments and
fully-overtwisted bundles. Based on a coarse-grained, statistical model of
protein binding, we show that the interplay between binding geometry and the
intrinsic \textit{flexibility} of linkers mediates cooperative binding in the
bundle. We attribute the respective continuous/discontinous bundling mechanisms
of fascin/espin to differences in the stiffness of linker bonds themselves.
