Spatial separation provides the force-coupling mechanism for the spindle assembly checkpoint

The spindle assembly checkpoint (SAC) prevents anaphase until chromosomes have formed correct bipolar attachments to the mitotic spindle. Spatial separation of pericentromeric Aurora B from outer-kinetochore substrates explains how tension-dependent biorientation stabilises microtubule-kinetochore attachments. How this geometry controls the SAC and is ultimately sensed by the key checkpoint kinase MPS1 has remained unclear. Here, we show that MPS1 recruitment is regulated by the spatial relationship between pericentromeric Aurora B-INCENP and the outer kinetochore. Shortening INCENP reduces MPS1 recruitment, whereas extending INCENP delays MPS1 removal and SAC silencing. Partial NDC80 depletion prevents normal Aurora B-kinetochore separation while retaining MPS1-dependent checkpoint signalling. Conversely, acute recruitment of Aurora B to microtubule-attached outer kinetochores under tension bypasses spatial separation and rapidly restores MPS1 and downstream SAC signalling. We propose that error correction and MPS1-dependent SAC signalling share an upstream spatial-sensing mechanism in which force-dependent separation from Aurora B favours attachment stabilisation while limiting MPS1 recruitment.