Modes of spindle pole body inheritance and segregation of the Bfa1p-Bub2p checkpoint protein complex

Yeast spindle pole bodies (SPBs) duplicate once per cell cycle by a conserv ative mechanism resulting in a pre-existing 'old' and a newly formed SPB. T he two SPBs of yeast cells are functionally distinct. It is only the SPB th at migrates into the daughter cell, the bud, which carries the Bfa1p-Bub2p GTPase-activating protein (GAP) complex, a component of the spindle positio ning checkpoint. We investigated whether the functional difference of the t wo SPBs correlates with the time of their assembly. We describe that in unp erturbed cells the 'old' SPB always migrates into the bud. However, Bfa1p l ocalization is not determined by SPB inheritance. It is the differential in teraction of cytoplasmic microtubules with the mother and bud cortex that d irects the Bfa1p-Bub2p GAP to the bud-ward-localized SPB. In response to de fects of cytoplasmic microtubules to interact with the cell cortex, the Bfa 1p-Bub2p complex binds to both SPBs. This may provide a mechanism to delay cell cycle progression when cytoplasmic microtubules fail to orient the spi ndle. Thus, SPBs are able to sense cytoplasmic microtubule properties and r egulate the Bfa1p-Bub2p GAP accordingly.