Interaction of smooth muscle caldesmon with S‐100 protein

The interaction of caldesmon with certain Ca-binding proteins was investigated by means of electrophoresis under non-denaturating conditions. In the presence of Ca2+ calmodulin, troponin C and S-100 protein form a complex with caldesmon. No complex formation takes place in the absence of Ca2+. Lactalbumin and pike parvalbumin (pI4.2) do not interact with caldesmon independently of Ca-concentration. Both S-100 protein and calmodulin effectively inhibit phosphorylation of caldesmon by Ca-phospholipid-dependent protein kinase. At low ionic strength S-100 protein reverses the inhibitory action of caldesmon on the skeletal muscle acto-heavy meromyosin ATPase more effectively than calmodulin. It is supposed that in certain tissues and cell compartments the proteins belonging to the S-100 family are able to substitute for calmodulin in the caldesmon-dependent regulation of actin and myosin interaction.