5.16:
Histone Variants at the Centromere
The centromere is a constricted region on each chromosome where the kinetochore and microtubules assemble to ensure faithful separation of the chromatids during anaphase of cell division.
In most eukaryotes, the centromere is partitioned into two chromatin domains- the centromere core and the pericentric heterochromatin region.
The centromere core in all eukaryotes contains a variant of core histone H3, called Centromere Protein A or CENP-A. Together with the three other core histones H2A, H2B, and H4, the CENP-A forms the centromere-specific nucleosome.
The functional C-terminal fold domain of the CENP-A is highly conserved among eukaryotes. However, the N-terminal tail of the protein shows significant variations both in size and sequence.
The centromere cores in the yeast Saccharomyces cerevisiae contain a single CENP-A nucleosome attached to a single mitotic spindle; hence, these are referred to as point centromeres.
This core centromere region is flanked by approximately 125 bps of AT-rich pericentric DNA sequences. This region is characterized by nucleosomes containing methylated histone H3.
The centromere chromatin forms a three-dimensional structure to expose CENP-A containing nucleosomes for interaction with the kinetochore and microtubules.
In contrast, the centromere core of human chromosomes has alternatively arranged CENP-A nucleosomes and regular H3 nucleosomes. These regular H3 nucleosomes are dimethylated.
These alternating blocks can span up to 5 Mbs long and largely contain short, repeated AT-rich DNA sequences called alpha satellite DNA, flanked by pericentric heterochromatin DNA.
The timing of CENP-A loading to nucleosomes also differs between species. For example, while in humans, it's loaded between the anaphase and G1 phases; in plants, the loading occurs in the late G2 phase.
5.16:
Histone Variants at the Centromere
Histone variants are the histone proteins with structural and sequence variations. These variants may be regarded as “mutant” forms that replace their canonical histone counterparts in the nucleosomes. Specific post-translational modifications on the histone variants enable further chromatin complexity and regulate tissue-specific gene expression. The most common histone variants are from histone H2A, H2B, and linker histone H1 families. However, several variants of histone H3 variants are also increasingly being studied.
Histone H3 variant: CENP-A
Centrosomal chromatin contains a specialized histone protein called CENP-A which shares 60% similarity with canonical histone H3. It is essential for kinetochore assembly and subsequent binding of microtubules. It is also hypothesized that CENP-A acts as an epigenetic mark to maintain centromere identity. The CENP-A is recognized and targeted to centromere via the CENP-A targeting domain (CATD).
In vitro studies have shown that centromeric nucleosomes form octomers with two copies of CENP-A along with two copies of H2A, H2B and H4 histones. Nevertheless, new studies in different eukaryotes have led to several competing models for the structure of CENP-A containing nucleosomes. According to the hemisome model, the nucleosome contains only one copy of each histone, forming a tetramer. In addition, recent studies have shown that CENP-A nucleosomes are cell cycle-regulated. They exist as octamers in S-phase and as hemisomes during other phases of the cell cycle.
Deregulation of CENP-A functions is linked to chromosome instability and cancer. Several pieces of evidence indicate overexpression of CENP-A in colon cancer, adenocarcinoma, testicular germ cell tumors, breast cancer and hepatocellular carcinoma.
Suggested Reading
- Molecular Biology of Cell, Alberts, 6th edition, Pages 203-204
- Vardabasso, Chiara, Dan Hasson, Kajan Ratnakumar, Chi-Yeh Chung, Luis F. Duarte, and Emily Bernstein. "Histone variants: emerging players in cancer biology." Cellular and molecular life sciences 71, no. 3 (2014): 379-404.