Microtubules segregate chromosomes by attaching to macromolecular kinetochores. Only microtubule-end attached kinetochores can be pulled apart; how these end-on attachments are selectively recognised and stabilised is not known. Using the kinetochore and microtubule-associated protein, Astrin, as a molecular probe, we show that end-on attachments are rapidly stabilised by spatially-restricted delivery of PP1 near the C-terminus of Ndc80, a core kinetochore-microtubule linker. PP1 is delivered by the evolutionarily conserved tail of Astrin and this promotes Astrin’s own enrichment creating a highly-responsive positive feedback, independent of biorientation. Abrogating Astrin:PP1-delivery disrupts attachment stability, which is not rescued by inhibiting Aurora-B, an attachment destabiliser, but is reversed by artificially tethering PP1 near the C-terminus of Ndc80. Constitutive Astrin:PP1-delivery disrupts chromosome congression and segregation, revealing a dynamic mechanism for stabilising attachments. Thus, Astrin-PP1 mediates a dynamic ‘lock’ that selectively and rapidly stabilises end-on attachments, independent of biorientation, and ensures proper chromosome segregation.

During cell division, microtubules capture and pull chromosomes apart into two equal sets. Without the establishment of proper chromosome-microtubule attachment, microtubules cannot impart the pulling forces needed to separate sister chromatid pairs. How are chromosomes captured along microtubule walls? How is the attachment of chromosomes to dynamic microtubule-ends achieved and monitored? We discuss these key questions by considering the roles of kinetochore-bound microtubule regulating proteins and also the complex regulatory loops of kinases and phosphatases that control chromosome-microtubule attachment and ensure the accurate segregation of chromosomes.

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The vast majority of Chromosomal Instability (CIN) promoting mutations remain unknown. We assess the prevalence of Chromosomal Instability aiding Variants (CIVa) by collating Loss-of-Function (LoF) variants predicted in 135 chromosome segregation genes from over 150,000 humans, including consanguineous individuals. Surprisingly, we observe heterozygous and homozygous CIVa in Astrin and SKA3 genes that encode evolutionarily conserved microtubule-associated proteins essential for chromosome segregation. By combining high-resolution microscopy and controlled protein expression, we show the naturally occurring Astrin variant, p.Q1012*, as potentially harmful because it fails to localise normally, delays anaphase onset, induces chromosome misalignment and promotes chromosome missegregation. We show that N-terminal frameshift variants in Astrin and SKA3 are likely to generate shorter isoforms that do not compromise chromosome segregation revealing resilient mechanisms to cope with harmful variants. This study provides a framework to predict and stratify naturally occurring CIVa, an important step towards precision medicine for CIN syndromes.