Defects in chromosome-microtubule attachment can cause chromosomal instability, associated with infertility and aggressive cancers. Chromosome-microtubule attachment is mediated by a large macromolecular structure, the kinetochore. Kinetochore pairs are bioriented and pulled by microtubules from opposing spindle poles to ensure the equal segregation of chromosomes. Kinetochore-microtubule attachments lacking opposing-pull are detached by Aurora-B/Ipl1; yet, how mono-oriented attachments that are a prerequisite for biorientation, but lacking opposing-pull are spared is unclear. Using an RNAi-mediated screen, we uncover a unique role for the Astrin-SKAP complex in protecting mono-oriented attachments. We provide the first evidence for how a microtubule-end associated protein senses outer-kinetochore changes specific to end-on attachments and assembles into an outer kinetochore crescent to stabilise mature attachments. We find that Astrin-PP1 and Cyclin-B-CDK1 activities counteract each other to preserve mono-oriented attachments. Thus, cells are not only surveying chromosome-microtubule attachment errors, but they are also actively sensing and stabilising mature attachments independent of biorientation.
