Calponin.

Calponin is a troponin-T like protein purified from chicken gizzard smooth muscle. It binds to actin, myosin, Ca(2+)-binding proteins and tropomyosin and inhibits the actomyosin ATPase as well as the movement of actin filaments over myosin in vitro. These properties have led to the proposal that calponin may be involved in the Ca(2+)-dependent regulation of actin-myosin interaction and consequently of smooth muscle contraction. Calponin is localized in both the contractile and the cytoskeletal parts of the smooth muscle cell and may have a structural function in smooth muscle cells. It may also regulate the pool of free actin available for cytoskeleton organization. In vitro calponin function is modulated by its interaction with a Ca(2+)-binding protein and/or by its phosphorylation. This suggests that calponin may play an important role in signal transduction from the membrane receptor to the contractile proteins in smooth muscle.