Reconstitution of Actomyosin Bundles

Abstract

Many vital processes among diverse cell types are dependent on proper actomyosin contraction. Despite muscle and non-muscle cells containing many different proteins believed to form the requisite structure necessary for contraction, the minimal requirements for effecting contractile motion and sustaining a tensile load are unknown. While myosin motors are known to be responsible for force generation, it is generally thought that long range force transmission requires accessory proteins to cross-link F-actin. We show that a minimal set of proteins, F-actin and smooth muscle myosin thick filaments, spontaneously assemble into contractile bundles that undergo dramatic reductions in contour length and generate contractile forces on the bundle end points. These contractile bundles are responsive to changes in boundary conditions, whereby dynamic remodeling seeks to minimize the distance and build tension between end points. The degree of contraction is sensitive to myosin/actin stoichiometries; at sufficiently low myosin concentration, contraction fails to commence. The rate of contractile is proportional to bundle length and, thus, these in vitro bundles show similar telescoping phenomenon observed in striated muscle. Contraction and further remodeling is abrogated at a critical tension, consistent with force-induced stalling of myosin motors within a bundle. Thus, we have found a two protein component system comprised of F-actin and thick filaments of smooth muscle myosin is sufficient to reconstitute a dynamic material that rapidly remodels through contraction when tension is low and forms a stable structure under high tension. This serves as a model to understand adaptive force sensing and transmission in the actomyosin cytoskeleton.