The influence of caldesmon on papain proteolysis of monomeric smooth muscle myosin.

The influence of caldesmon on papain digestion of chicken gizzard monomeric myosin in folded (10S) conformation depends on its phosphorylation. Caldesmon exposes the head/rod junction of myosin in phosphorylated form to proteolytic attack (particularly in the presence of Ca2+) and slightly screens it in unphosphorylated form. In both folded forms RLCs are protected by caldesmon, more in unphosphorylated than in phosphorylated myosin. The results indicate that the conformations of folded unphosphorylated and phosphorylated myosin are distinct and suggest that caldesmon destabilizes the regulatory domain in folded conformation of phosphorylated myosin.