Calponin is a smooth muscle-specific regulatory protein. The evidences of molecular cloning revealed the existence of 2 isoforms:α and β, composed of 292 and 252 residues respectively. Calponin binds to actin and inhibits actomyosin Mad²⁺-ATPase activity, thereby smooth muscle contraction. The actin-binding domain of calponin lies in 38 residues (145-182). Ser175 plays a critical role in the interaction of calponin with actin and the inhibition of the ATPase. It also binds to calmodulin in a Ca²⁺-dependent manner. The calmodulin-binding domain is between residue 52 and 144. Both binding and inhibition of the ATPase can be regulated by phosphorylation and dephosphorylation of calponin.