Part of a checkpoint which monitors spindle integrity and prevents premature exit from mitosis. This cell-cycle arrest depends upon inhibition of the G-protein TEM1 by the BFA1/BUB2 complex. Interacts with BUB2. Multiply phosphorylated in a cell-cycle-dependent manner with the major phosphorylation occurring in mitosis. Present with 1380 molecules/cell in log phase SD medium. To S.pombe byr4.