The Structural Basis for Kinetochore Stabilization by Cnn1/CENP-T.

Chromosome segregation depends on a regulated connection between spindle microtubules and centromeric DNA. The kinetochore mediates this connection and ensures it persists during anaphase, when sister chromatids must transit into daughter cells uninterrupted. The Ctf19 complex (Ctf19c) forms the centromeric base of the kinetochore in budding yeast. Biochemical experiments show that Ctf19c members associate hierarchically when purified from cell extract [1], an observation that is mostly explained by the structure of the complex [2]. The Ctf3 complex (Ctf3c), which is not required for the assembly of most other Ctf19c factors, disobeys the biochemical assembly hierarchy when observed in dividing cells that lack more basal components [3]. Thus, the biochemical experiments do not completely recapitulate the logic of centromeric Ctf19c assembly. We now present a high-resolution structure of the Ctf3c bound to the Cnn1-Wip1 heterodimer. Associated live-cell imaging experiments provide a mechanism for Ctf3c and Cnn1-Wip1 recruitment to the kinetochore. The mechanism suggests feedback regulation of Ctf19c assembly and unanticipated similarities in kinetochore organization between yeast and vertebrates.