Material properties of actin filament bundles

Abstract

Actin is an ubiquitous structural protein fundamental to such biological processes as cell motility and muscle contraction. Our model system is the acrosomal process of the Limulus sperm which extends a 60 ýtm long actin bundle during reproduction. It is an example of a biological spring where the force of elongation derives from twist energy stored within the bundle during spermatogenesis. In addition to actin the acrosome comprises only one other protein: scruin, an actin-binding protein specific to Limulus that decorates and crosslinks actin filaments into a crystalline bundle. Our goal is to reconstitute the structure of the acrosome using these two proteins in order to further elucidate the role of scruin in actin bundle crosslinking.A multi-scale approach is presented wherein the bending rigidity of scruin bundles and their constituent filaments are probed individually, then inter-related by simple mechanical models. Material properties of filaments and bundles are measured using a combination of optical tweezers, electron and fluorescence microscopy. We find that scruin bundles reconstituted from acrosome fragments display an ordered structure, with a bending rigidity orders of magnitude higher than their individual filaments. Actin bundles formed by depletion exhibit similar behavior, revealing an intrinsic regime of coupled actin bundle formation. Bundle elastic moduli are eight orders of magnitude stiffer than reconstituted networks and an order of magnitude softer than the native acrosome, highlighting scruin's ability to dictate a wide range of material properties depending on the formation conditions.