The cyclin-dependent kinase inhibitor Roughex is involved in mitotic exit in Drosophila

Background: Exit from mitosis is a tightly regulated event. This process ha s been studied in greatest detail in budding yeast, where several activitie s have been identified that cooperate to downregulate activity of the cycli n-dependent kinase (CDK) Cdc28 and force an exit from mitosis. Cdc28 is ina ctivated through proteolysis of B-type cyclins by the multisubunit ubiquiti n ligase termed the anaphase promoting complex/cyclosome (APC/C) and inhibi tion by the cyclin-dependent kinase inhibitor (CKI) Sic1. In contrast, the only mechanism known to be essential for CDK inactivation during mitosis in higher eukaryotes is cyclin destruction. Results: We now present evidence that the Drosophila CKI Rougher (Rux) cont ributes to exit from mitosis, Observations of fixed and living embryos show that metaphase is significantly longer in rux mutants than in wild-type em bryos. In addition, Rux overexpression is sufficient to drive cells experim entally arrested in metaphase into interphase. Furthermore, rux mutant embr yos are impaired in their ability to overcome a transient metaphase arrest induced by expression of a stable cyclin A. Rux has numerous functional sim ilarities with Sic1. While these proteins share no sequence similarity, we show that Sic1 inhibits mitotic Cdk1-cyclin complexes from Drosophila in vi tro and in vivo, Conclusions: Rux inhibits Cdk1-cyclin A kinase activity during metaphase, t hereby contributing to exit from mitosis, To our knowledge, this is the fir st mitotic function ascribed to a CKI in a multicellular organism and indic ates the existence of a novel regulatory mechanism for the metaphase to ana phase transition during development.