Bub2 Background Information The mitotic checkpoint blocks cell cycle progression before anaphase in case of mistakes in the alignment of chromosomes on the mitotic spindle (1). In budding yeast, the Mad1-3, and Bub1-3 proteins mediate this arrest (1). Unlike the other Mad and Bub proteins, Bub2 localizes at the spindle pole body (SPB) throughout the cell cycle (1). In the presence of kinetochore or spindle damage, Bub2 cells initiate a preanaphase delay but do not maintain it (3). The continuous presence of Bub2 protein is required for maintaining spindle damage-induced arrest (3). Cell-cycle arrest depends upon inhibition of the G-protein Tem1 that appears to be regulated by Bfa1/Bub2, a two-component GTPase-activating protein, and the exchange factor Lte1 (4). Bub2 and Bfa1 physically associate across the entire cell cycle and bind to Tem1 during mitosis and early G1 (4). Bfa1 is multiply phosphorylated in a cell-cycle-dependent manner with the major phosphorylation occurring in mitosis and this Bfa1 phosphorylation is Bub2-dependent (4). Bfa1 also can play a role in the regulation of mitotic exit by directly inhibiting the interaction between Tem1 and Cdc15 even in the absence of Bub2 (5).
