Defects in chromosome-microtubule attachment can cause chromosomal instability (CIN), frequently associated with infertility and aggressive cancers. Chromosome-microtubule attachment is mediated by a large macromolecular structure, the kinetochore. Sister kinetochores of each chromosome are pulled by microtubules from opposing spindle-poles, a state called biorientation which prevents chromosome missegregation. Kinetochore-microtubule attachments that lack the opposing-pull are detached by Aurora-B/Ipl1. It is unclear how mono-oriented attachments that precede biorientation are spared despite the lack of opposing-pull. Using an RNAi-screen, we uncover a unique role for the Astrin-SKAP complex in protecting mono-oriented attachments. We provide evidence of domains in the microtubule-end associated protein that sense changes specific to end-on kinetochore-microtubule attachments and assemble an outer-kinetochore crescent to stabilise attachments. We find that Astrin-PP1 and Cyclin-B-CDK1 pathways counteract each other to preserve mono-oriented attachments. Thus, CIN prevention pathways are not only surveying attachment defects but also actively recognising and stabilising mature attachments independent of biorientation.

染色体-微管附着缺陷可导致染色体不稳定 (CIN)，通常与不孕症和侵袭性癌症有关。染色体-微管附着由大分子结构——动粒介导。每条染色体的姐妹动粒被微管从相对的纺锤极拉出，这种状态称为生物定向，可防止染色体错误分离。缺乏反向拉力的动粒微管附件被 Aurora-B/Ipl1 分离。尽管缺乏反向拉动，但尚不清楚在生物定向之前的单定向附件是如何幸免的。使用 RNAi 筛选，我们发现 Astrin-SKAP 复合物在保护单向附件方面的独特作用。我们提供了微管末端相关蛋白中域的证据，这些域感知末端动粒-微管附件的特定变化，并组装外部动粒新月体以稳定附件。我们发现 Astrin-PP1 和 Cyclin-B-CDK1 通路相互抵消以保持单向附件。因此，CIN 预防途径不仅是调查附件缺陷，而且还积极识别和稳定独立于生物定向的成熟附件。