The Saccharomyces cerevisiae checkpoint gene BUB1 encodes a novel protein kinase

Normal cell multiplication requires that the events of mitosis occur in a carefully ordered fashion. Cells employ checkpoints to prevent cycle progression until some prerequisite step has been completed. To explore the mechanisms of checkpoint enforcement, we previously screened for mutants of Saccharomyces cerevisiae which are unable to recover from a transient treatment with a benzimidazole-related microtubule inhibitor because they fail to inhibit subsequent cell cycle steps. Two of the identified genes, BUB2 and BUB3, have been cloned and described (M. A. Hoyt, L. Totis, and B. T. Roberts, Cell 66:507-517, 1991). Here we present the characterization of the BUB1 gene and its product. Genetic evidence was obtained suggesting that Bub1 and Bub3 are mutually dependent for function, and immunoprecipitation experiments demonstrated a physical association between the two. Sequence analysis of BUB1 revealed a domain with similarity to protein kinases. In vitro experiments confirmed that Bub1 possesses kinase activity; Bub1 was able to autophosphorylate and to catalyze phosphorylation of Bub3. In addition, overproduced Bub1 was found to localize to the cell nucleus.