Cytokinesis in Tetrahymena: Determination of division plane and organization of contractile ring

A protein, Tetrahymena p85, is localized to the presumptive division plane before the formation of the contractile ring. p85 directly interacts with Tetrahymena calmodulin (CaM) in a Ca²⁺-dependent manner, and p85 and CaM colocalize in the division furrow. A Ca²⁺/CaM inhibitor N-(6- Aminohexyl)-5-chloro-1-naphthalenesulfonamide HCl (W7) inhibits the direct interaction between p85 and Ca²⁺/CaM. W7 also inhibits the localization of p85 and CaM to the division plane, and the formation of the contractile ring and division furrow. Tetrahymena fimbrin and elongation factor-1a (EF-1α), which induce bundling of Tetrahymena F-actin, are also localized to the division furrow during cytokinesis. The Tetrahymena fimbrin has two actin- binding domains, but lacks the EF-hand Ca²⁺-binding motif, suggesting that Tetrahymena fimbrin probably crosslinks actin filaments in a Ca²⁺- insensitive manner during cytokinesis. The evidence also indicates that Ca²⁺/CaM inhibits the F-actin-bundling activity of EF-1α; and EF-1α and CaM colocalize in the division furrow. In this review, we propose that the Ca²⁺/CaM signal and its target protein p85 cooperatively regulate the determination of the division plane, and that a Ca²⁺-insensitive actin- bundling protein, Tetrahymena fimbrin, and a Ca²⁺/CaM-sensitive actin- bundling protein, EF-1α, play pivotal roles in regulating the organization of the contractile ring microfilaments. (C) 2000 Wiley-Liss, Inc.