Without its GAP activity, Bub2 (white) remains on both SPBs and blocks mitotic exit.

To prevent chromosome segregation errors, cytokinesis in Saccharomyces cerevisiae awaits the arrival of a spindle pole body (SPB) in the bud. On page 335, Fraschini et al. show that this arrival is announced by the mother-bound SPB, which heralds the onset of cytokinesis and mitotic exit.

Mitotic exit relies on a G-protein called Tem1, which activates the destruction of mitotic cyclins.

As Tem1 is activated by Lte1, which is found in the bud, Tem1's arrival in the bud on the SPB was thought to trigger timely mitotic exit. But another mechanism for Tem1 activation is revealed in the new study, thus partly explaining why Lte1 is not essential.

Whereas Lte1 activates Tem1, a SPB-localized complex of Bub2 and Bfa1 is known to keep Tem1 inactive during spindle assembly and orientation. Timing of Bub2/Bfa1 loss from the mother-bound SPB coincides with mitotic exit and is prevented by activation of the spindle position checkpoint. The authors find that a mutant Bub2 that remains on the mother-bound SPB during telophase inhibits mitotic exit in some cells. The specific loss of Bub2 from the mother-bound pole thus contributes to mitotic exit.

Bub2's GTPase-activating protein (GAP) activity was needed for its removal from the mother-bound SPB. Septin and kinases at the bud neck were also necessary, suggesting that the passage of one SPB through the bud neck creates a diffusible signal that travels to the mother SPB to activate the Bub2 GAP.

The authors imagine that microtubule motors or binding proteins might transmit a Bub2-activating signal along the spindle and are now testing this idea. They have already shown that loss of the microtubule plus-end binding protein Bim1 slightly decreases Bub2 loss from the mother-bound SPB, but other proteins must contribute.

Although required for correct localization, the GAP activity of Bub2 is not necessary to inactivate Tem1 in vitro. The authors feel that Bfa1 might do the actual Tem1 inactivation, whereas Bub2 recruits Bfa1 to the SPB.