Binding of alpha-actinin to F-actin or to tropomyosin F-actin is a function of both alpha-actinin concentration and gel structure.

We have studied by electron microscopy as well as by measurements of low shear viscosity, rigidity and binding, the effect of alpha-actinin on the gel formed at 37 degrees C with F-actin and with tropomyosin-decorated F-actin. Contrary to previous reports in the literature, alpha-actinin at nanomolar concentrations is an efficient actin gelling protein, even at 37 degrees C, provided that the concentration of actin (or of tropomyosin-decorated F-actin) is low (1.2-2.4 microM). The binding of alpha-actinin to F-actin, as a function of actin concentration, is anomalous. The amount of bound alpha-actinin increases when actin concentration increases from 0 to 1.2 microM but does not change significantly when actin concentration is further increased up to 48 microM. A similar result is obtained with tropomyosin-decorated F-actin. These observations can be explained by an hypothesis that binding is a function of the alpha-actinin - F-actin association constant as well as of the rigidity of the gel. When the concentration of actin increases, the rigidity of the gel also increases and more work is required to bring two actin filaments to the reaction distance with alpha-actinin and, consequently, a larger alpha-actinin concentration is required to attain the same ratio of bound alpha-actinin to actin monomers in the filaments.