Ca2+-dependent binding of calcyclin to muscle tropomyosin.

The interaction of calcyclin with tropomyosin and tropomyosin-actin was studied with fluorescence titrations and photo-reactive crosslinking experiments. Titrations of pyreneiodoacetamide-labeled tropomyosin alone or with actin showed binding of calcyclin to tropomyosin with muM dissociation constants when Ca2+ was present. UV irradiation of mixtures of calcyclin and gizzard beta beta-tropomyosin labeled with benzophenone-iodoacetamide at Cys36, with or without actin, produced crosslinks between tropomyosin chains and calcyclin monomers only in the presence of Ca2+. These data provide direct evidence for a Ca+2-dependent tropomyosin-calcyclin interaction at or near Cys36 of tropomyosin and indicate that calcyclin binding to tropomyosin-actin does not cause tropomyosin dissociation.