Involvement of caldesmon at the actin-myosin interface

Addition of myosin subfragment 1 (S-1) to the actin-caldesmon binary complex, which forms bundles of actin filaments resulted in the formation of actin/caldesmon-decorated filaments [Harricane, Bonet-Kerrache, Cavadore & Mornet (1991) Eur. J. Biochem. 196, 219-224]. The present data provide further evidence that caldesmon and S-1 compete for a common actin-binding region and demonstrate that a change occurs in the actin-myosin interface induced by caldesmon. S-1 digested by trypsin, which has an actin affinity 100-fold weaker than that of native S-1, was efficiently removed from actin by caldesmon, but not completely dissociated. This particular ternary complex was stabilized by chemical cross-linking with carbodi-imide, which does not have any spacer arm, and revealed contact interfaces between the different protein components. Cross-linking experiments showed that the presence of caldesmon had no effect on stabilization of actin-(20 kDa domain), whereas the actin-(50 kDa domain) covalent association was significantly decreased, to the point of being virtually abolished.