Thin filament-associated protein that is implicated in the regulation and modulation of smooth muscle contraction. It is capable of binding to actin, calmodulin and tropomyosin. The interaction of calponin with actin inhibits the actomyosin Mg-ATPase activity. Smooth muscle, and tissues containing significant amounts of smooth muscle. Phosphorylation by PKC or CaM kinase II reduces the binding of calponin to F-actin and tropomyosin. Belongs to the calponin family.