Simultaneous interaction of actin with α-actinin and calponin

Abstract

Interaction of calponin and α-actinin with actin was analyzed by means of cosedimentation and electron microscopy. G-actin was polymerized in the presence of calponin, α-actinin, or both of these actin-binding proteins (ABPs). The single and bundled actin filaments were separated, and the stoichiometry of ABPs and actin in both types of filaments was determined. Binding of calponin to the single or bundled actin filaments was not dependent on the presence of α-actinin and did not displace α-actinin from actin. In the presence of calponin, however, less α-actinin was bound to the bundled actin filaments, and the binding of α-actinin was accompanied by a partial decrease in the calponin/actin stoichiometry in the bundles of actin filaments. Calponin had no influence on the binding of αactinin to the single actin filaments. The structure of actin bundles formed in the presence of the two ABPs differed from that formed in the presence of either one singly. We conclude that calponin and α-actinin can coexist on actin and that nearly each actin monomer can bind one of these ABPs.