Interaction of smooth muscle caldesmon with S‐100 protein

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Abstract
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 caldesmondependent regulation of actin and myosin interaction.