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 Ca(2+)-dependent manner, and p85 and CaM colocalize in the division furrow. A Ca(2+)/CaM inhibitor N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide HCl (W7) inhibits the direct interaction between p85 and Ca(2+)/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-1alpha), 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 Ca(2+)-binding motif, suggesting that Tetrahymena fimbrin probably cross-links actin filaments in a Ca(2+)- insensitive manner during cytokinesis. The evidence also indicates that Ca(2+)/CaM inhibits the F-actin-bundling activity of EF-1alpha; and EF-1alpha and CaM colocalize in the division furrow. In this review, we propose that the Ca(2+)/CaM signal and its target protein p85 cooperatively regulate the determination of the division plane, and that a Ca(2+)-insensitive actin-bundling protein, Tetrahymena fimbrin, and a Ca(2+)/CaM-sensitive actin-bundling protein, EF-1alpha, play pivotal roles in regulating the organization of the contractile ring microfilaments.
Copyright 2000 Wiley-Liss, Inc.