p31comet acts to ensure timely spindle checkpoint silencing subsequent to kinetochore attachment

Abstract

The spindle assembly checkpoint links the onset of anaphase to completion of chromosome-microtubule attachment and is mediated by the binding of Mad and Bub proteins to kinetochores of unattached or maloriented chromosomes. Mad2 and BubR1 traffic between kinetochores and the cytosol, thereby transmitting a “wait anaphase” signal to the anaphase-promoting complex. It is generally assumed that this signal dissipates automatically upon kinetochore-microtubule binding, but it has been shown that under conditions of nocodazole-induced arrest p31comet, a Mad2-binding protein, is required for mitotic progression. In this article we investigate the localization and function of p31comet during normal, unperturbed mitosis in human and marsupial cells. We find that, like Mad2, p31comet traffics on and off kinetochores and is also present in the cytosol. Cells depleted of p31comet arrest in metaphase with mature bipolar kinetochore-microtubule attachments, a satisfied checkpoint, and high cyclin B levels. Thus p31comet is required for timely mitotic exit. We propose that p31comet is an essential component of the machinery that silences the checkpoint during each cell cycle.