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We construct a coarse-grained model of parallel actin bundles crosslinked bycompact, globular bundling proteins, such as fascin and espin, necessarycomponents of filapodial and mechanosensory bundles. Consistent with structuralobservations of bundles, we find that the optimal geometry for crosslinking isovertwisted, requiring a coherent structural change of the helical geometry ofthe filaments. We study the linker-dependent thermodynamic transition ofbundled actin filaments from their native state to the overtwisted state andmap out the "twist-state'' phase diagram in terms of the availability as wellas the flexibility of crosslinker proteins. We predict that the transition fromthe uncrosslinked to fully-crosslinked state is highly sensitive to linkerflexibility: flexible crosslinking smoothly distorts the twist-state of bundledfilaments, while rigidly crosslinked bundles undergo a phase transition,rapidly overtwisting filaments over a narrow range of free crosslinkerconcentrations. Additionally, we predict a rich spectrum of intermediatestructures, composed of alternating domains of sparsely-bound (untwisted) andstrongly-bound (overtwisted) filaments. This model reveals that subtledifferences in crosslinking agents themselves modify not only the detailedstructure of parallel actin bundles, but also the thermodynamic pathway bywhich they form.

Structural Reorganization of Parallel Actin Bundles by Crosslinking Proteins: Incommensurate States of Twist