Mammalian mad2 and bub1/bubR1 recognize distinct spindle-attachment and kinetochore-tension checkpoints

Metaphase checkpoint controls sense abnormalities of chromosome alignment d uring mitosis and prevent progression to anaphase until proper alignment ha s been attained. A number of proteins, including mad2, bub1, and bubR1, hav e been implicated in the metaphase checkpoint control in mammalian cells. M etaphase checkpoints have been shown, in various systems, to read loss of e ither spindle tension or microtubule attachment at the kinetochore. Charact eristically, HeLa cells arrest in metaphase in response to low levels of mi crotubule inhibitors that leave an intact spindle and a metaphase plate. He re we show that the arrest induced by nanomolar vinblastine correlates with loss of tension at the kinetochore, and that in response the checkpoint pr oteins bub1 and bubR1 are recruited to the kinetochore but mad2 is not, mad 2 remains competent to respond and is recruited at higher drug doses that d isrupt spindle association with the kinetochores. Further, although mad2 fo rms a complex with cdc20, it does not associate with bub1 or bubR1. We conc lude that mammalian bub1/bubR1 and mad2 operate as elements of distinct pat hways sensing tension and attachment, respectively.