Specific genetic interactions between spindle assembly checkpoint proteins and B-Type cyclins in Saccharomyces cerevisiae.

The B-type cyclin Clb5 is involved primarily in control of DNA replication in Saccharomyces cerevisiae. We conducted a synthetic genetic array (SGA) analysis, testing for synthetic lethality between the clb5 deletion and a selected 87 deletions related to diverse aspects of cell cycle control based on GO annotations. Deletion of the spindle checkpoint genes BUB1 and BUB3 caused synthetic lethality with clb5. The spindle checkpoint monitors the attachment of spindles to the kinetochore or spindle tension during early mitosis. However, another spindle checkpoint gene, MAD2, could be deleted without ill effects in the absence of CLB5, suggesting that the bub1/3 clb5 synthetic lethality reflected some function other than the spindle checkpoint of Bub1 and Bub3. To characterize the lethality of bub3 clb5 cells, we constructed a temperature-sensitive clb5 allele. At nonpermissive temperature, bub3 clb5-ts cells showed defects in spindle elongation and cytokinesis. High-copy plasmid suppression of bub3 clb5 lethality identified the C-terminal fragment of BIR1, the yeast homolog of survivin; cytologically, the BIR1 fragment rescued the growth and cytokinesis defects. Bir1 interacts with IplI (Aurora B homolog), and the addition of bub3 clb5-ts significantly enhanced the lethality of the temperature-sensitive ipl1-321. Overall, we conclude that the synthetic lethality between clb5 and bub1 or bub3 is likely related to functions of Bub1/3 unrelated to their spindle checkpoint function. We tested requirements for other B-type cyclins in the absence of spindle checkpoint components. In the absence of the related CLB3 and CLB4 cyclins, the spindle integrity checkpoint becomes essential, since bub3 or mad2 deletion is lethal in a clb3 clb4 background. clb3 clb4 mad2 cells accumulated with unseparated spindle pole bodies. Thus, different B-type cyclins are required for distinct aspects of spindle morphogenesis and function, as revealed by differential genetic interactions with spindle checkpoint components.