Tetrahymena p85 is localized to the presumptive division plane before the formation of contractile ring microfilaments. p85 binds to calmodulin in a Ca2+-dependent manner and both proteins colocalize to the division furrow. Inhibition of the binding of p85 and Ca2+/calmodulin prevents both the localization of p85 and calmodulin to the division plane and the formation of the contractile ring, suggesting that the interaction of p85 and Ca2+/calmodulin is important in the formation of the contractile ring. We investigated the mechanisms of the formation of contractile ring, and the relationship among p85, CaM, and actin using cosedimentation assay: p85 binds to G-actin in a Ca2+/ calmodulin-dependent manner, but does not bind to F-actin. Therefore, we propose that a Ca2+/calmodulin signal and its target protein p85 are cooperatively involved in the recruitment of G-actin to the division plane and the formation of the contractile ring. (C) 2002 Elsevier Science (USA).