Kinetochore-microtubule attachments are strengthened by Cnn1 stabilization of Stu2

Accurate chromosome segregation requires kinetochores to form robust, load-bearing attachments to dynamic spindle microtubules, mediated primarily by the Ndc80 complex. Two receptors, Dsn1 and Cnn1 (CENP-T), recruit multiple copies of Ndc80c to the kinetochore. We previously demonstrated that kinetochore components co-purifying with the yeast Dsn1 protein can maintain persistent load-bearing attachments that track with microtubule tip growth and shortening, but the properties of the Cnn1 receptor have not been explored. Using an optical trapping-based assay, we show here that Cnn1 purifications also sustain dynamic microtubule attachments under load. Mutation of a conserved region within the disordered N-terminal tail of Cnn1 weakened attachment strength in vitro and caused a growth defect when Dsn1 function was impaired. The Cnn1 mutation reduced Stu2 kinetochore levels without altering other kinetochore proteins. Restoring Stu2, either by direct addition in vitro or by tethering it to Ndc80c in vivo, rescued both attachment strength and cellular viability. These findings reveal a biophysical role for Cnn1 in enabling Stu2-dependent stabilization of kinetochore-microtubule attachments.