The Drosophila Bub3 protein is required for the mitotic checkpoint and for normal accumulation of cyclins during G2 and early stages of mitosis.

During mitosis, a checkpoint mechanism delays metaphase-anaphase transition in the presence of unattached and/or unaligned chromosomes. This delay is achieved through inhibition of the anaphase promoting complex/cyclosome (APC/C) preventing sister chromatid separation and cyclin degradation. In the present study, we show that Bub3 is an essential protein required during normal mitotic progression to prevent premature sister chromatid separation, missegreation and aneuploidy. We also found that Bub3 is required during G2 and early stages of mitosis to promote normal mitotic entry. We show that loss of Bub3 function by mutation or RNAi depletion causes cells to progress slowly through prophase, a delay that appears to result from a failure to accumulate mitotic cyclins A and B. Defective accumulation of mitotic cyclins results from inappropriate APC/C activity, as mutations in the gene encoding the APC/C subunit cdc27 partially rescue this phenotype. Furthermore, analysis of mitotic progression in cells carrying mutations for cdc27 and bub3 suggest the existence of differentially activated APC/C complexes. Altogether, our data support the hypothesis that the mitotic checkpoint protein Bub3 is also required to regulate entry and progression through early stages of mitosis.