Buckling of filamentous actin bundles in filopodial protrusions

The mechanical behavior of filamentous actin bundles plays an essential role in filopodial protrusions at the leading edge of crawling cells. These bundles consist of parallel actin filaments that are hexagonally packed and interconnected via cross-linking proteins including α-actinin, filamin, and fascin. When pushing against the plasma membrane and/or external barriers, actin bundles in filopodial protrusions inevitably encounter a compressive load. The bending stiffness and buckling stability of actin bundles are therefore important in determining the filopodial architecture and subsequent cell morphology. In this work, we employ a coarse-grained molecular dynamics model to investigate the buckling behavior of cross-linked actin bundles under compression, explicitly accounting for the properties of the constituent filaments and the mechanical description of the cross-linkers. The bending stiffness of actin bundles exhibits a generic size effect depending on the number of filaments in the bundle, explicitly depending on the degree of interfilament coupling. The distinct buckling modes are analyzed for bundles with different coupling states and crosslinker strengths. This study could clarify the stability and buckling mechanisms of parallel packed actin bundles and the structure–function relations of mechanical components in filopodial protrusions.