Bimodal activation of BubR1 by Bub3 sustains mitotic checkpoint signaling

Abstract

Significance The mitotic checkpoint (or the spindle assembly checkpoint) ensures genome integrity by preventing premature chromosome segregation. The pathway is triggered locally by kinetochores, multiprotein complexes assembled onto centromeres. Unattached kinetochores produce Mad2 bound to Cdc20, the mitotic activator of the E3 ubiquitin ligase APC/C. The initial Mad2–Cdc20 complex is then converted into the final mitotic checkpoint inhibitor Bub3–BubR1–Cdc20 that blocks APC/C (anaphase promoting complex or cyclosome)-dependent ubiquitination of cyclin B and securin, thereby stabilizing them and preventing an advance to anaphase. In this study, we identify dual mechanisms by which Bub3 promotes mitotic checkpoint signaling. Bub3 binding to BubR1 promotes two distinct BubR1–Cdc20 interactions, one acting at unattached kinetochores and the other cytoplasmically to facilitate production of the mitotic checkpoint inhibitor.

Functional and Structural Characterization of Bub3·BubR1 Interactions Required for Spindle Assembly Checkpoint Signaling in Human Cells

“…In any case these results confirmed in human cells earlier observations with murine counterparts (31,49), indicating that blockage of the interaction between Bub3 and their binding partners Bub1 and BubR1 is sufficient for checkpoint inhibition. The latter is in line with very recent publications (18,39) showing that expression of wild type, but not mutated or truncated BubR1 GLEBS, could rescue the SAC in BubR1-depleted cells, although the p53 status might influence the SAC response (50).…”

“…In both complexes, the GLEBS peptide binds across the top surface of the ␤-propeller, forming an extensive interface (17). Mutations in either protein that disrupt the interface cause checkpoint deficiency and chromosome instability in yeast and human cells (17,18). Bub3-bound BubR1 in turn can interact with Cdc20 in two fashions; on one hand, at unattached kinetochores, and on the other, cytoplasmically, resulting in the production of the mitotic checkpoint inhibitor (18).…”

“…Mutations in either protein that disrupt the interface cause checkpoint deficiency and chromosome instability in yeast and human cells (17,18). Bub3-bound BubR1 in turn can interact with Cdc20 in two fashions; on one hand, at unattached kinetochores, and on the other, cytoplasmically, resulting in the production of the mitotic checkpoint inhibitor (18). Bub3 Ϫ/Ϫ mice accumulate mitotic errors and do not survive beyond days 6.5-7.5 post coitus (5).…”

The Cdc20-binding Phe Box of the Spindle Checkpoint Protein BubR1 Maintains the Mitotic Checkpoint Complex During Mitosis

“…In fact, a role for the BubR1 middle region in Cdc20 kinetochore targeting was recently reported (37). We first tested whether BubR1 was required for the kinetochore localization of Cdc20.…”

Bub1/BubR1: swiss army knives at kinetochores

“…3,5,6 The role of the BubR1 ABBA motif appears to be dual in that it also contributes to SAC silencing by an unknown mechanism. 5 As part of the MCC, BubR1 binds stably to a Cdc20 molecule through its N-terminus so one possibility is that the ABBA motif weakens this interaction to allow efficient MCC disassembly upon checkpoint satisfaction.…”