A summary of the available binding site data is shown in Figure 6 and should be compared with the predicted structures in Figure 5. The blocks of amino acid sequence were those known at the time the cDNA sequencing was being done; they now include the entire head region. The phosphorylation site, indicated here, is a potential site, a consensus sequence, -AXYS(T)-, for the multifunctional calmodulin-dependent kinase. It has not been established that this site is utilized, and it is not clearly related to any of the established binding domains. Exactly how caldesmon regulates the actomyosin interaction and whether it contributes to the latch state is unclear. The organization of the molecule suggests, however, that it could function as a bridge between thick and thin filaments. In support of this idea, our recent efforts to map the smaller non-muscle form of caldesmon indicate that it retains the myosin and the calmodulin approximately actin binding domains, but is missing the central repeated region, arguing this region may serve to space the N and C terminal-binding domains in smooth muscle. Work from several laboratories has demonstrated that smooth muscle caldesmon is an elongated molecule with a calmodulin, tropomyosin, and actin-binding region at the C-terminus and a myosin-binding domain at the N-terminus. Sequence determination has shown that smooth muscle caldesmon is smaller than previously believed, has demonstrated similarities between caldesmon and troponin T, and has suggested possible calmodulin-binding peptides. The available sequence and domain mapping studies on smooth muscle caldesmon are reviewed.