Nucleosomal composition at the centromere: a numbers game

Abstract

The Centromere is a unique chromosomal locus where the kinetochore is formed to mediate faithful chromosome partitioning, thus maintaining ploidy during cell division. Centromere identity is inherited via an epigenetic mechanism involving a histone H3 variant, called centromere protein A (CENP-A) which replaces H3 in centromeric chromatin. In spite of extensive efforts in field of centromere biology during the past decade, controversy persists over the structural nature of the CENP-A-containing epigenetic mark, both at nucleosomal and chromatin levels. Here, we review recent findings and hypotheses regarding the structure of CENP-A-containing complexes.

Fig. 1

Proposed mechanism for the formation of putative CENP-A octameric nucleosomes. Newly synthesized CENP-A along with histone H4 is suggested to bind HJURP to form a pre-nucleosomal trimeric complex. Next, in order to interact with DNA, HJURP has to be released leaving a (CENP-A:H4)₂ tetramer. Addition of H2A:H2B dimers then complete the octamer formation

Fig. 2

Different models for the composition of CENP-A nucleosomes. These models differ in size, DNA wrapping and number as well as identity of their components. For a more detailed description of the differences between each model, please see the text

Fig. 3

Proposed structural dynamics of centromeric nucleosomes throughout the cell cycle. a An octamer to tetramer transition model in which passage of the replication fork splits the pre-existing octameric CENP-A nucleosomes into tetramers allowing equal inheritance of the epigenetic mark to daughter strands. In this model, HJURP exclusively is found at the centromere in G1 and mediates the reconstruction of octamers by incorporation of new CENP-A. b The tetramer to octamer transition model. CENP-A:H4: HJURP complex is recruited to the centromeric chromatin in G1 and is assumed to associate to the pre-existing CENP-A tetramers extra-nucleosomally. Posttranslational modifications (PTMs) of tetrameric CENP-A nucleosomes along with the presence of HJURP impede the stable incorporation of new CENP-A:H4 dimers into nucleosomes. In late G1/early S, action certain proteins chaperones and remodeling complexes is proposed to facilitate the tetramer to octamer transition by affecting the PMTs of histones and chromatin structure resulting in tetramer to octamer transition. Likewise to the previous model, passage of the replication fork splits the octamers to the tetramers. The tetramers are further stabilized by the reassociation of HJURP in G2 and presumed to be maintained in mitosis to form the kinetochore plate. c. In an alternative model, where CENP-A exists in octameric nucleosomes throughout the cell cycle, gaps generated as a result of the passage of the replication fork will be either maintained or could be transiently filled with multiple possible placeholder structures. In this model, HJURP is also found at the centromere exclusively in G1 to participate in new CENP-A assembly

Fig. 4

Possible steps of the tetramer to octamer transition. Upon transition of the cell to S phase, HJURP is released from the centromeric chromatin and certain posttranslational modifications and of CENP-A tetramers are resolved by yet-to-be-identified factors. In the first scenario, H2A:H2B dimers are also temporarily removed allowing the tetrameric (CENP-A:H4)₂ complex to form and interact with DNA. This reaction is speculated to be mediated by NAP1, which is a chromatin remodeler. Addition of a pair of H2A:H2B dimers will complete the octamer formation. In the second scenario, however, incorporation of new CENP-A:H4 as well as H2B:H2B dimers does not require disassembly of the pre-existing CENP-A tetramers