Simultaneous interaction of actin with alpha-actinin and calponin

Interaction of calponin and alpha-actinin with actin was analyzed by means of cosedimentation and electron microscopy. G-actin was polymerized in the presence of calponin, alpha-actinin, or both of these actin-binding protein s (ABPs). The single and bundled actin filaments were separated, and the st oichiometry of ABPs and actin in both types of filaments was determined. Bi nding of calponin to the single or bundled actin filaments was not dependen t on the presence of alpha-actinin and did not displace alpha-actinin from actin. In the presence of calponin, however, less alpha-actinin was bound t o the bundled actin filaments, and the binding of alpha-actinin was accompa nied by a partial decrease in the calponin/actin stoichiometry in the bundl es of actin filaments. Calponin had no influence on the binding of alpha-ac tinin 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 o f either one singly. We conclude that calponin and alpha-actinin can coexis t on actin and that nearly each actin monomer can bind one of these ABPs.