Mechanics of Kinesin-Crosslinked Microtubule Networks

Abstract

We have recently shown that the mechanical properties of chemically crosslinked microtubule networks depend sensitively on the single-molecule properties of the crosslinking molecules they contain. In particular, for networks that are rigidly crosslinked by biotin-streptavidin, a small number of crosslinkers bear stress, and their force-induced detachment from the microtubule determines the time-dependent network rearrangements. Interestingly, the network retains its elastic modulus even under conditions of significant plastic flow, suggesting that crosslinker breakage is balanced by the formation of new bonds. This leads to a remarkable resilience under repeated loading, as long as a sufficient number of the original crosslinkers are preserved per loading cycle. In this current work, we have expanded our study to include compliant crosslinkers, formed by multimeric complexes of kinesin-1. A variety of biophysical approaches are used, including electron and optical microscopy to measure network architecture under different crosslinking conditions, and magnetic tweezer-based microrheology to determine the creep response of the networks under controlled loading. We find that protein compliance and filament bundling enables load splitting among adjacent crosslinkers, thereby enhancing material strength and diminishing the importance of single crosslinker unbinding in determining the mesoscopic rheology. Our results are important to understanding how network architecture and crosslinker properties influence network mechanics, and are particularly important to understanding the role of kinesin proteins in forming stress-transmitting microtubule structures, such as the mitotic spindle.