Ca²⁺/calmodulin and p85 cooperatively regulate an initiation of cytokinesis in Tetrahymena

Tetrahymena p85 differs in mobility in two-dimensional SDS-polyacrylamide gel electrophoresis between wild-type and temperature-sensitive cell-division-arrest mutant cdaA1 cell extracts, and is localized to the presumptive division plane before the formation of the division furrow. The p85 contained three identical sequences which show homology to the calmodulin binding site Ca²⁺/calmodulin dependent protein kinase Type II Saccharomyces cerevisiae. We found the p85 directly interacts with Tetrahymena calmodulin in a Ca²⁺-dependent manner, using a co-sedimentation assay. We next examined the localization of p85 and calmodulin during cytokinesis using indirect immunofluorescence. The results showed that both proteins colocalize in the division furrow. This is the first observation that calmodulin is localized in the division furrow. Moreover, the direct interaction between p85 and Ca²⁺/calmodulin was inhibited by Ca²⁺/calmodulin inhibitor N-(6-aminohexyl)5-chloro-1-naphthalenesulfonamide HCl. When the cells were treated with the drug just before the beginning of cytokinesis, the drug also inhibited the localization of p85 and calmodulin to the division plane, and the formation of the contractile ring and division furrow. Therefore, we propose that the Ca²⁺/calmodulin signal and its target protein p85 cooperatively regulate an initiation of cytokinesis and may be also concerned with the progression of cytokinesis in Tetrahymena.