title: Affinity of alpha-actinin for actin determines the structure and mechanical properties of actin filament gels.
abstract: Proteins that cross-link actin filaments can either form bundles of parallel filaments or isotropic networks of individual filaments. We have found that mixtures of actin filaments with alpha-actinin purified from either Acanthamoeba castellanii or chicken smooth muscle can form bundles or isotropic networks depending on their concentration. Low concentrations of alpha-actinin and actin filaments form networks indistinguishable in electron micrographs from gels of actin alone. Higher concentrations of alpha-actinin and actin filaments form bundles. The threshold for bundling depends on the affinity of the alpha-actinin for actin. The complex of Acanthamoeba alpha-actinin with actin filaments has a Kd of 4.7 microM and a bundling threshold of 0.1 microM; chicken smooth muscle has a Kd of 0.6 microM and a bundling threshold of 1 microM. The physical properties of isotropic networks of cross-linked actin filaments are very different from a gel of bundles: the network behaves like a solid because each actin filament is part of a single structure that encompasses all the filaments. Bundles of filaments behave more like a very viscous fluid because each bundle, while very long and stiff, can slip past other bundles. We have developed a computer model that predicts the bundling threshold based on four variables: the length of the actin filaments, the affinity of the alpha-actinin for actin, and the concentrations of actin and alpha-actinin.
authors.affiliation: Department of Cell Biology and Anatomy, Johns Hopkins University, Baltimore, Maryland 21205.
authors.forename: D H
authors.initials: DH
authors.lastname: Wachsstock
authors.forename: W H
authors.initials: WH
authors.lastname: Schwartz
authors.forename: T D
authors.initials: TD
authors.lastname: Pollard
mesh_terms: D000048:Acanthamoeba; D000185:Actinin / Q000378:metabolism* / Q000648:ultrastructure; D000199:Actins / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D000818:Animals; D001665:Binding Sites; D001696:Biomechanical Phenomena; D055592:Biophysical Phenomena; D001703:Biophysics; D002645:Chickens; D003198:Computer Simulation; D005782:Gels; D066298:In Vitro Techniques; D007700:Kinetics; D008854:Microscopy, Electron; D008954:Models, Biological; D015394:Molecular Structure; D011108:Polymers / Q000378:metabolism; D011817:Rabbits; D012212:Rheology
chemical_list: D000199:Actins; D005782:Gels; D011108:Polymers; D000185:Actinin