Posttranslational modifications of CENP-A: marks of distinction

Abstract

Centromeres are specialized chromosome domain that serve as the site for kinetochore assembly and microtubule attachment during cell division, to ensure proper segregation of chromosomes. In higher eukaryotes, the identity of active centromeres is marked by the presence of CENP-A (centromeric protein-A), a histone H3 variant. CENP-A forms a centromere-specific nucleosome that acts as a foundation for centromere assembly and function. The posttranslational modification (PTM) of histone proteins is a major mechanism regulating the function of chromatin. While a few CENP-A site-specific modifications are shared with histone H3, the majority are specific to CENP-A-containing nucleosomes, indicating that modification of these residues contribute to centromere-specific function. CENP-A undergoes posttranslational modifications including phosphorylation, acetylation, methylation, and ubiquitylation. Work from many laboratories have uncovered the importance of these CENP-A modifications in its deposition at centromeres, protein stability, and recruitment of the CCAN (constitutive centromere-associated network). Here, we discuss the PTMs of CENP-A and their biological relevance.

Keywords: CENP-A; Centromere; Chromatin; Kinetochore; Mitosis; Posttranslational modification.

Fig. 1

Posttranslational modifications of CENP-A and histone H3. A comparison of domains and posttranslational modifications of CENP-A and histone H3, which include methylation, phosphorylation, acetylation, and ubiquitylation. * In original articles, the initiating M cleavage was not taken into consideration and the position of these residues is shifted by + 1. The correct position of these residues is one unit less than depicted the diagram

Fig. 2

N-terminal modifications of CENP-A facilitate CCAN recruitment. a NRMT1 trimethylates CENP-A at Gly-1 in interphase; the proportion of which is increased during mitosis. Aurora-A initiates the Ser-7 phosphorylation in prophase, the mark is then maintained by Aurora B. During anaphase, Ser-7 phosphorylation is reduced. b CENP-A trimethylation facilitates the recruitment of CCAN components CENP-T and CENP-I. Ser-7 phosphorylation mediates the recruitment of CENP-C through 14-3-3 proteins. The CENP-A C-terminal tail also directly recruits CENP-C independent of Aurora phosphorylation

Fig. 3

CENP-A posttranslational modifications during cell cycle. A diagram depicting modifications on CENP-A primarily involved in interaction with HJURP and centromere deposition. K124 undergoes multiple types of mutually exclusive modifications that include ubiquitylation, acetylation, and methylation. Phosphorylation of the CENP-A amino terminus at Ser-18 and Ser-68 are cell cycle regulated and may contribute to restricting the timing of HJURP binding

Fig. 4

Conservation of human CENP-A posttranslational modifications across species. Schematic representation of CENP-A proteins in different organisms. Percent identity with respect to human CENP-A was computed through pairwise alignment using EMBOSS Needle program. CENP-A protein sequences in these organisms are also aligned through Clustal Omega and zoomed to focus on posttranslationally modified sites

Fig. 5

Cse4 ubiquitylation and proteolysis. Four E3 ligases Psh1, Rcy1, Ubr1, and Slx5 independently regulate Cse4 proteolysis through ubiquitylation. Note that ubiquitylation by Slx5 requires Siz1/2-directed sumoylation