Negative feedback at kinetochores underlies a responsive spindle checkpoint signal

Abstract

. We show that this responsiveness arises because the SAC primes kinetochore phosphatases to induce negative feedback and silence its own signal. Active SAC signalling recruits PP2A-B56 to kinetochores where it antagonises Aurora B to promote PP1 recruitment. PP1 in turn silences the SAC and delocalises PP2A-B56. Preventing or bypassing key regulatory steps demonstrates that this spatiotemporal control of phosphatase feedback underlies rapid signal switching at the kinetochore by; 1) allowing the SAC to quickly transition to the ON state in the absence of antagonising phosphatase activity, and 2) ensuring phosphatases are then primed to rapidly switch the SAC signal OFF when kinetochore kinase activities are diminished by force-producing microtubule attachments.The spindle assembly checkpoint (SAC) is globally activated at mitotic entry and only extinguished when all kinetochores have established force-producing microtubule attachments 2,3 . At each individual kinetochore however, the SAC responses are much more dynamic. Here, localised SAC signalling switches rapidly between the ON and OFF states depending on microtubule occupancy [2][3][4][5][6] .Exactly how kinetochores manage to achieve this rapid signal switching remains unknown. To address this we initially focussed on characterising the kinetochore phosphatases responsible for SAC silencing in mammalian cells. We performed a targeted screen with siRNAs to 222 individual phosphatase subunits to identify those that regulate mitotic exit in mammalian cells. 48 hours after siRNA transfection, cells were synchronised in mitosis using the microtubule poison nocodazole, after which mitotic exit was forced by the small molecule MPS1 inhibitor reversine 7 for 1 hour. (Fig.1b). Live monitoring of endogenous Cyclin B1 levels 16 showed that B56 depletion prevented efficient APC/C activation following MPS1 inhibition (Fig.1c). This indicated that PP2A-B56 depletion did not simply delay mitotic exit, but in fact prevented SAC silencing. PP2A-B56 has recently been shown to localise to the outer kinetochore via interaction with a short phosphorylated motif in BUBR1 (termed KARD) [17][18][19] . We found that all B56 isoforms that we tested (B56, ,  1 ,  3 , ) localised to the centromere/kinetochores regions of mitotic chromosomes, with some more clearly enriched on kinetochores than others (B56 1 ,  3 , ; Supplementary Fig.1a). We next deleted the B56 binding motif from BUBR1 (BUBR1 ∆KARD ; Supplementary Fig.1b-d), which specifically abolished B56 kinetochore localisation (Fig.1d, e and supplementary fig.1e), and delayed mitotic exit following MPS1 inhibition with either reversine (Fig.1f) or the distinct inhibitor ( Supplementary Fig.1f). These delays were accentuated by concomitant B56 depletion, which even allowed cells to mount a prolonged arrest with a high dose of reversine or . This was unrelated to effects on centromeric PP2A-B56 because SGO1 depletion caused mitotic arrest due to reduced centromeric PP2A and loss of sister chromatid cohesion, as expe...

How the SAC gets the axe: Integrating kinetochore microtubule attachments with spindle assembly checkpoint signaling

“…49 Thus, PP2A bound to BUBR1 dephosphorylates Aurora B sites on Knl1 thereby allowing PP1 binding to Knl1 to mediate dephosphorylation of MELT motifs and transition from SAC "C" to SAC "-" state. 48 Recent study by Nijenhuis et al further suggests that the kinetochore localized PP1 promotes removal of kinetochore-PP2A-B56 by dephosphorylating MELT and KARD motifs. 48 Similarly, Mps1-mediated phosphorylation of MELT repeats activates SAC and at the same time sets the way for SAC silencing by recruiting PP2A-B56 via BUBR1.…”

“…48 Recent study by Nijenhuis et al further suggests that the kinetochore localized PP1 promotes removal of kinetochore-PP2A-B56 by dephosphorylating MELT and KARD motifs. 48 Similarly, Mps1-mediated phosphorylation of MELT repeats activates SAC and at the same time sets the way for SAC silencing by recruiting PP2A-B56 via BUBR1. PP2A therefore plays a key role in regulating 2 major events: controlling the formation of stable kMT attachments by opposing Aurora B activity and also initiating SAC silencing by opposing Mps1 activity.…”

“…PP2A therefore plays a key role in regulating 2 major events: controlling the formation of stable kMT attachments by opposing Aurora B activity and also initiating SAC silencing by opposing Mps1 activity. 48,50 Hence spatiotemporal juxtaposition should exist between the 2 phosphatases (PP1 and PP2A) and 2 kinases (Aurora B and Mps1) to elicit a robust SAC signal. The PP2A-B56 promotes PP1 recruitment to kinetochores (by inhibiting Aurora B activity) that subsequently quenches the SAC by delocalizing PP2A from kinetochores.…”

LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential

“…30 PP2A also participates in a negative feedback loop, antagonizing Aurora B and Plk1 kinases, thus maintaining the spindle assembly checkpoint until microtubules are properly attached to chromosome kinetichores. 31,32 PP2A As a Tumor Suppressor PP2A has been classically characterized as a tumor suppressor gene. Both reduced expression and loss of function in PP2A have been implicated in numerous cancers, including those of the lung, colon, skin, breast, ovary, endometrium, and brain.…”

Studying Kinetochore Kinases

“…Finally, BUB1 promotes Aurora B localization by phosphorylating histone H2A on T120. Therefore the error-correction and SAC pathways are extensively entwined, and this is further exemplified by the fact that the phosphatases PP1 and PP2A-B56 counteract both error-correction and the SAC to stabilize chromosome spindle attachments and allow anaphase onset (9)(10)(11)(12)(13)(14).…”