A sequential multi-target Mps1 phosphorylation cascade promotes spindle checkpoint signaling.

The master spindle checkpoint kinase Mps1 senses kinetochore-microtubule attachment and promotes checkpoint signaling to ensure accurate chromosome segregation. The kinetochore scaffold Knl1, when phosphorylated by Mps1, recruits checkpoint complexes Bub1-Bub3 and BubR1-Bub3 to unattached kinetochores. Active checkpoint signaling ultimately enhances the assembly of the mitotic checkpoint complex (MCC) consisting of BubR1-Bub3, Mad2, and Cdc20, which inhibits the anaphase-promoting complex or cyclosome bound to Cdc20 (APC/C) to delay anaphase onset.

The Bub1-Plk1 kinase complex promotes spindle checkpoint signalling through Cdc20 phosphorylation.

The spindle checkpoint senses unattached kinetochores and inhibits the Cdc20-bound anaphase-promoting complex or cyclosome (APC/C), to delay anaphase, thereby preventing aneuploidy. A critical checkpoint inhibitor of APC/C(Cdc20) is the mitotic checkpoint complex (MCC). It is unclear whether MCC suffices to inhibit all cellular APC/C.

Control of APC/C-dependent ubiquitin chain elongation by reversible phosphorylation.

Most metazoan E3 ligases contain a signature RING domain that promotes the transfer of ubiquitin from the active site of E2 conjugating enzymes to lysine residues in substrates. Although these RING-E3s depend on E2 enzymes for catalysis, how they turn on their E2s at the right time and place remains poorly understood. Here we report a phosphorylation-dependent mechanism that ensures timely activation of the E2 Ube2S by its RING-E3, the anaphase-promoting complex (APC/C); while phosphorylation of a specific serine residue in the APC/C coactivator Cdc20 prevents delivery of Ube2S to the APC/C, removal of this mark by PP2A(B56) allows Ube2S to bind the APC/C and catalyze ubiquitin chain elongation.

The Cdc20-binding Phe box of the spindle checkpoint protein BubR1 maintains the mitotic checkpoint complex during mitosis.

The spindle checkpoint ensures accurate chromosome segregation by monitoring kinetochore-microtubule attachment. Unattached or tensionless kinetochores activate the checkpoint and enhance the production of the mitotic checkpoint complex (MCC) consisting of BubR1, Bub3, Mad2, and Cdc20. MCC is a critical checkpoint inhibitor of the anaphase-promoting complex/cyclosome, a ubiquitin ligase required for anaphase onset.

Substrate-specific activation of the mitotic kinase Bub1 through intramolecular autophosphorylation and kinetochore targeting.

During mitosis of human cells, the kinase Bub1 orchestrates chromosome segregation through phosphorylating histone H2A and the anaphase-promoting complex/cyclosome activator Cdc20. Bub1-mediated H2A-T120 phosphorylation (H2A-pT120) at kinetochores promotes centromeric sister-chromatid cohesion, whereas Cdc20 phosphorylation by Bub1 contributes to spindle checkpoint signaling. Here, we show that phosphorylation at the P+1 substrate-binding loop of human Bub1 enhances its activity toward H2A but has no effect on its activity toward Cdc20.

Phospho-H2A and cohesin specify distinct tension-regulated Sgo1 pools at kinetochores and inner centromeres.

Accurate chromosome segregation requires coordination between the dissolution of sister-chromatid cohesion and the establishment of proper kinetochore-microtubule attachment. During mitosis, sister-chromatid cohesion at centromeres enables the biorientation of and tension across sister kinetochores. The complex between shugoshin and protein phosphatase 2A (Sgo1-PP2A) localizes to centromeres in mitosis, binds to cohesin in a reaction requiring Cdk-dependent phosphorylation of Sgo1, dephosphorylates cohesin-bound sororin, and protects a centromeric pool of cohesin from mitotic kinases and the cohesin inhibitor Wapl.

Tracking spindle checkpoint signals from kinetochores to APC/C.

Accurate chromosome segregation during mitosis is critical for maintaining genomic stability. The kinetochore--a large protein assembly on centromeric chromatin--functions as the docking site for spindle microtubules and a signaling hub for the spindle checkpoint. At metaphase, spindle microtubules from opposing spindle poles capture each pair of sister kinetochores, exert pulling forces, and create tension across sister kinetochores.

Cdh1 is a HECT of an activator.

Cdh1 is a well-established activator of APC/C, a RING-type ubiquitin ligase. In this issue of Molecular Cell, Wan et al. (2011) report an APC/C-independent role of Cdh1 during development as an activator for Smurf1, a HECT-type ubiquitin ligase.

Defining pathways of spindle checkpoint silencing: functional redundancy between Cdc20 ubiquitination and p31(comet).

The spindle checkpoint senses unattached or improperly attached kinetochores during mitosis, inhibits the anaphase-promoting complex or cyclosome (APC/C), and delays anaphase onset to prevent aneuploidy. The mitotic checkpoint complex (MCC) consisting of BubR1, Bub3, Mad2, and Cdc20 is a critical APC/C-inhibitory checkpoint complex in human cells. At the metaphase-anaphase transition, the spindle checkpoint turns off, and MCC disassembles to allow anaphase onset.