Influence of Ionic Strength, Actin State, and Caldesmon Construct Size on the Number of Actin Monomers in a Caldesmon Binding Site

There is no consensus on the mechanism of inhibition of actin−myosin ATPase activity by caldesmon. Various models are based on different assumptions for the number of actin monomers that constitute a caldesmon binding site. Differences in binding behavior may be due to variations in the assay, the range of caldesmon concentrations, the type of caldesmon, and the method of data analysis used. We have evaluated these factors by measuring binding in the presence and absence of tropomyosin with both intact caldesmon and a recombinant 35 kDa actin binding fragment and with actin initially in the polymerized state or monomeric state. In all cases caldesmon binding could be simulated with a model having one class of binding sites. However, the number of actin monomers constituting a site was variable. Binding to F-actin at 165 mM ionic strength was best described with 7 actin monomers per site. When caldesmon bound to actin during the polymerization of G-actin, the size of the binding site was 3. Binding of the expressed truncated fragment, Cad35, could be described with 3 monomers per site. A simple interpretation of the data is that caldesmon binds tightly to 2−3 actin monomers. Additional parts of caldesmon bind less tightly to actin, causing caldesmon to cover approximately 7 actin monomers. The appendix contains an analysis of several binding curves with multiple binding site models. There is no compelling evidence for two classes of binding sites.