Dissecting the roles of human BUB1 in the spindle assembly checkpoint.

Mitotic chromosome segregation is initiated by the anaphase promoting complex/cyclosome (APC/C) and its co-activator CDC20 (forming APC/C(CDC20)). APC/C(CDC20) is inhibited by the spindle assembly checkpoint (SAC) when chromosomes have not attached to spindle microtubules. Unattached kinetochores catalyze the formation of a diffusible APC/C(CDC20) inhibitor that comprises BUBR1 (also known as BUB1B), BUB3, MAD2 (also known as MAD2L1) and a second molecule of CDC20. Recruitment of these proteins to the kinetochore, as well as SAC activation, rely on the mitotic kinase BUB1, but the molecular mechanism by which BUB1 accomplishes this in human cells is unknown. We show that kinetochore recruitment of BUBR1 and BUB3 by BUB1 is dispensable for SAC activation. Unlike its yeast and nematode orthologs, human BUB1 does not associate stably with the MAD2 activator MAD1 (also known as MAD1L1) and, although required for accelerating the loading of MAD1 onto kinetochores, BUB1 is dispensable for the maintenance of steady-state levels of MAD1 there. Instead, we identify a 50-amino-acid segment that harbors the recently reported ABBA motif close to a KEN box as being crucial for the role of BUB1 in SAC signaling. The presence of this segment correlates with SAC activity and efficient binding of CDC20 but not of MAD1 to kinetochores.

Plk1 and Mps1 Cooperatively Regulate the Spindle Assembly Checkpoint in Human Cells.

Equal mitotic chromosome segregation is critical for genome integrity and is monitored by the spindle assembly checkpoint (SAC). We have previously shown that the consensus phosphorylation motif of the essential SAC kinase Monopolar spindle 1 (Mps1) is very similar to that of Polo-like kinase 1 (Plk1). This prompted us to ask whether human Plk1 cooperates with Mps1 in SAC signaling. Here, we demonstrate that Plk1 promotes checkpoint signaling at kinetochores through the phosphorylation of at least two Mps1 substrates, including KNL-1 and Mps1 itself. As a result, Plk1 activity enhances Mps1 catalytic activity as well as the recruitment of the SAC components Mad1:C-Mad2 and Bub3:BubR1 to kinetochores. We conclude that Plk1 strengthens the robustness of SAC establishment at the onset of mitosis and supports SAC maintenance during prolonged mitotic arrest.

Release of Mps1 from kinetochores is crucial for timely anaphase onset.

Mps1 kinase activity is required for proper chromosome segregation during mitosis through its involvements in microtubule-chromosome attachment error correction and the mitotic checkpoint. Mps1 dynamically exchanges on unattached kinetochores but is largely removed from kinetochores in metaphase. Here we show that Mps1 promotes its own turnover at kinetochores and that removal of Mps1 upon chromosome biorientation is a prerequisite for mitotic checkpoint silencing. Inhibition of Mps1 activity increases its half-time of recovery at unattached kinetochores and causes accumulation of Mps1 protein at these sites. Strikingly, preventing dissociation of active Mps1 from kinetochores delays anaphase onset despite normal chromosome attachment and alignment, and high interkinetochore tension. This delay is marked by continued recruitment of Mad1 and Mad2 to bioriented chromosomes and is attenuated by Mad2 depletion, indicating chronic engagement of the mitotic checkpoint in metaphase. We propose that release of Mps1 from kinetochores is essential for mitotic checkpoint silencing and a fast metaphase-to-anaphase transition.

Chemical genetic inhibition of Mps1 in stable human cell lines reveals novel aspects of Mps1 function in mitosis.

BACKGROUND: Proper execution of chromosome segregation relies on tight control of attachment of chromosomes to spindle microtubules. This is monitored by the mitotic checkpoint that allows chromosome segregation only when all chromosomes are stably attached. Proper functioning of the attachment and checkpoint processes is thus important to prevent chromosomal instability. Both processes rely on the mitotic kinase Mps1. PRINCIPAL FINDING: We present here two cell lines in which endogenous Mps1 has been stably replaced with a mutant kinase (Mps1-as) that is specifically inhibited by bulky PP1 analogs. Mps1 inhibition in these cell lines is highly penetrant and reversible. Timed inhibition during bipolar spindle assembly shows that Mps1 is critical for attachment error-correction and confirms its role in Aurora B regulation. We furthermore show that Mps1 has multiple controls over mitotic checkpoint activity. Mps1 inhibition precludes Mad1 localization to unattached kinetochores but also accelerates mitosis. This acceleration correlates with absence of detectable mitotic checkpoint complex after Mps1 inhibition. Finally, we show that short-term inhibition of Mps1 catalytic activity is sufficient to kill cells. CONCLUSIONS/SIGNIFICANCE: Mps1 is involved in the regulation of multiple key processes that ensure correct chromosome segregation and is a promising target for inhibition in anti-cancer strategies. We report here two cell lines that allow specific and highly penetrant inhibition of Mps1 in a reproducible manner through the use of chemical genetics. Using these cell lines we confirm previously suggested roles for Mps1 activity in mitosis, present evidence for novel functions and examine cell viability after short and prolonged Mps1 inhibition. These cell lines present the best cellular model system to date for investigations into Mps1 biology and the effects of penetrance and duration of Mps1 inhibition on cell viability.