Functions of the centromere and kinetochore in chromosome segregation

Abstract

Centromeres play essential roles in equal chromosome segregation by directing the assembly of the microtubule binding kinetochore and serving as the cohesion site between sister chromatids. Here, we review the significant recent progress in our understanding of centromere protein assembly and how centromere proteins form the foundation of the kinetochore.

Figure legend. Schematics of centromere and kinetochore organization

In all figure panels, individual capitalized letters indicate a ‘CENP’ protein, while smaller letters indicate N-termini and C-termini (N and C, respectively). Arrows with question marks indicate interesting observations that require further investigation. A) Schematic of direct interactions between CENP-A and the CCAN. In vitro, CENP-N binds CENP-A's CATD domain and CENP-C binds through CENP-A's distinct C-terminal tail. A fragment of human CENP-C consisting of amino acids 426-537 is sufficient to bind CENP-A. In cells, CENP-N can somehow be recruited to centromeres independently of CENP-A (?₁), most likely through interactions within the extended CCAN (see text for details). B) Recruitment of the KMN network by the core CCAN. Recent data suggests that CENP-T/W/S/X and CENP-C provide the foundation for distinct components of the KMN network. The Mis12 component Nnf1 directly interacts with CENP-C in Drosophila, and amino acids 1-21 of human CENP-C are sufficient for the binding of CENP-C to the Mis12 complex. In parallel, Cdk1-mediated phosphorylation (yellow ‘P’) of CENP-T's N-terminal tail facilitates the direct binding of CENP-T to Ndc80. How CENP-T depends on CENP-A for its centromere localization (?₁), and how CENP-T may compete with Mis12 for Ndc80 binding (?₂), remain unclear. C) Recruitment of the extended CCAN complex by the core CCAN and an overview of CCAN depletion phenotypes. CCAN complexes are recruited to centromeres through their interactions with the chromatin binding proteins CENP-N, -C, and –T as depicted here going from the bottom (CENP-N) to the top (CENP-R). Removal of any one CCAN component affects all CCAN proteins within its complex and all those above it (but not below). The exception is CENP-H, which is required for correct CENP-L recruitment. While considered part of the CENP-O complex, CENP-R depletion does not affect any other known CCAN component. The phenotypes of CCAN depleted cells suggest that the CENP-H and CENP-O complexes antagonize each other (?₁). How the extended CCAN influences the microtubule attachments remains unclear, but CENP-Q and CENP-U directly bind microtubules, CENP-U may interact with Ndc80 (Hec1) (?₂), and CENP-K has been shown to be partially responsible for Ndc80 recruitment (?₃). In addition to CENP-N, CENP-C and CENP-T are also required for extended CCAN assembly. CENP-C may directly interact with CENP-L, but how CENP-T and CENP- C mediate CCAN assembly remains largely unclear (?₄). As summarized in the table on the left, depletion (by RNAi or knockout (KO)) of any extended CCAN component results in chromosome congression defects, but distinct phenotypes have been observed. See text for details.