Organization and Dynamics of Crosslinked Actin Filaments in Confined Environments

Abstract

ABSTRACTThe organization of the actin cytoskeleton is impacted by the interplay between physical confinement, features of crosslinking proteins, and deformations of semiflexible actin filaments. Some crosslinking proteins preferentially bind filaments in parallel, while others bind more indiscriminately. However, a quantitative understanding of how the mode of binding influences the assembly of actin networks in confined environments is lacking. Here we employ coarse-grained computer simulations to study the dynamics and organization of semiflexible actin filaments in confined regions upon the addition of crosslinkers. We characterize how the emergent behavior is influenced by the system shape, the number and type of crosslinking proteins, and the length of filaments. Structures include isolated clusters of filaments, highly connected filament bundles, and networks of interconnected bundles and loops. Elongation of one dimension of the system promotes the formation of long bundles that align with the elongated axis. Dynamics are governed by rapid crosslinking into aggregates, followed by a slower change in their shape and connectivity. Crosslinking decreases the average bending energy of short or sparsely connected filaments by suppressing shape fluctuations. However, it increases the average bending energy in highly connected networks because filament bundles become deformed and small numbers of filaments exhibit long-lived, highly unfavorable configurations. Indiscriminate crosslinking promotes the formation of high-energy configurations due to the increased likelihood of unfavorable, difficult-to-relax configurations at early times. Taken together, this work demonstrates physical mechanisms by which crosslinker binding and physical confinement impact the emergent behavior of actin networks, which is relevant both in cells and in synthetic environments.SIGNIFICANCEThe actin cytoskeleton is vital for intracellular transport, yet it remains challenging to understand how its organization is impacted by the interplay between physical confinement and the crosslinking of semiflexible actin filaments. In this study, we explore how the mode of crosslinker binding and the shape of the confining region impact the assembly and organization of actin filaments. The dynamics are governed by rapid crosslinking of spatially proximal filaments into aggregates, followed by slower relaxation of their shape and connectivity. Indiscriminate crosslinking promotes more highly connected networks, greater curvature of long filament bundles, and a subset of filaments in highly unfavorable configurations. The results provide insight into mechanisms influencing the cytoskeleton in cells and in reconstituted systems.