The spindle assembly checkpoint (SAC) generates a diffusible protein complex that prevents anaphase until all chromosomes are properly attached to spindle microtubules. A key step in SAC initiation is the recruitment of MAD1 to kinetochores, which is generally thought to be governed by the microtubule-kinetochore (MT-KT) attachment status. However, we demonstrate that the recruitment of MAD1 via BUB1, a conserved kinetochore receptor, is not affected by MT-KT interactions in human cells. Instead, BUB1:MAD1 interaction depends on BUB1 phosphorylation, which is controlled by a biochemical timer that integrates counteracting kinase and phosphatase effects on BUB1 into a pulse-generating incoherent feedforward loop. We propose that this attachment-independent timer serves to rapidly activate the SAC at mitotic entry, before the attachment-sensing MAD1 receptors have become fully operational. The BUB1-centered timer is largely impervious to conventional anti-mitotic drugs, and it is, therefore, a promising therapeutic target to induce cell death through permanent SAC activation.

纺锤体装配检查点（SAC）产生可扩散的蛋白质复合物，该复合物可防止后期发育，直到所有染色体均正确连接到纺锤体微管上为止。SAC启动的关键步骤是将MAD1募集到动植物上，这通常被认为是由微管动粒（MT-KT）附着状态决定的。但是，我们证明了通过BUB1（一种保守的线粒体受体）来招募MAD1不受人类细胞中MT-KT相互作用的影响。取而代之的是，BUB1：MAD1的相互作用取决于BUB1的磷酸化，该磷酸化由生化计时器控制，该计时器将对BUB1的抵消激酶和磷酸酶作用整合到一个产生脉冲的非相干前馈环中。我们建议该独立于附件的计时器用于在有丝分裂进入时快速激活SAC，在附着敏感的MAD1受体完全发挥作用之前。以BUB1为中心的计时器在很大程度上不受常规抗有丝分裂药物的影响，因此，它是通过永久SAC激活诱导细胞死亡的有希望的治疗靶标。