Cell cycle control of kinetochore assembly

Abstract

The kinetochore is a large proteinaceous structure assembled on the centromeres of chromosomes. The complex machinery links chromosomes to the mitotic spindle and is essential for accurate chromosome segregation during cell division. The kinetochore is composed of two submodules: the inner and outer kinetochore. The inner kinetochore is assembled on centromeric chromatin and persists with centromeres throughout the cell cycle. The outer kinetochore attaches microtubules to the inner kinetochore, and assembles only during mitosis. The review focuses on recent advances in our understanding of the mechanisms governing the proper assembly of the outer kinetochore during mitosis and highlights open questions for future investigation.

Keywords: CCAN; CENP-A; CENP-C; CENP-T; KMN; Kinetochore assembly; cell cycle; centromere; phosphorylation.

Figure 1.

A schematic of a canonical kinetochore. The main structure of the kinetochore consists of constitutive centromere-associated network (CCAN), which includes five subgroups (CENP-L-N, CENP-H-I-K-M, CENP-O-P-Q-R-U, CENP-T-W-S-X and CENP-C), and the KMN (Knl1, Mis12, and Ndc80 complexes) network. Kinetochore position is specified by CENP-A-containing nucleosomes, upon which CCAN assembles. CCAN recruits KMN, which directly binds microtubules, during mitosis. Two independent pathways, CENP-C and CENP-T, link KMN to CCAN.

Figure 2.

Phosphorylation-mediated competitive exclusion between Ccp1 and Ndc80 at the N-terminus of CENP-T regulates the recruitment of KMN. The Ccp1-binding domain of CENP-T is localized adjacent to the Ndc80-binding domain at the N-terminal region of CENP-T. When cells enter mitosis, the Ccp1-binding domain of CENP-T is phosphorylated by the Cdk1 kinase. Phosphorylation of the Ccp1-binding domain dissociates Ccp1 from CENP-T, allowing Ndc80C to bind to the Ndc80-binding domain. Ndc80C then directly interacts with microtubules to facilitate chromosome segregation. When cells exit from mitosis, the Ccp1-binding domain is dephosphorylated, which recruits Ccp1. Reassociation of Ccp1 with the Ccp1-binding domain blocks the binding of Ndc80C to CENP-T during interphase. P: phosphorylation.