The spindle checkpoint ensures that cells inherit the correct number of chromosomes at mitosis, delaying sister chromatid separation until chromosomes are correctly attached to the spindle. Checkpoint proteins, such as Mad2 and Bub1, probably monitor both attachment of kinetochores (the complexes that link chromosome centromeres to spindle microtubules) and tension, which signifies that kinetochore pairs have correctly attached to microtubules emanating from opposite poles. Claudio Sunkel and co-workers now identify which checkpoint proteins monitor what in flies (see ). They first clear up a case of mistaken identity by reclassifying the fly checkpoint proteins Bub1 and BubR1. They then go on to use novel antibodies to examine how checkpoint proteins respond to unattached kinetochores or loss of tension induced by low doses of the microtubule-depolymerizing drug taxol: they show that Mad2 and Bub1 bind only to unattached kinetochores whereas BubR1 and Bub3 can bind to attached kinetochores that are not under tension. The authors also demonstrate that recruitment of BubR1/Bub3 but not Bub1/Mad2 requires phosphorylation of a specific tension-sensing site on kinetochores. They therefore conclude that Mad2 and Bub1 monitor kinetochore attachment but BubR1 and Bub3 monitor tension.
