5.16
Histone Variants at the Centromere
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Q1: What is CENP-A and why is it important at the centromere?
CENP-A, or Centromere Protein A, is a histone H3 variant that replaces canonical histone H3 in centromeric nucleosomes. It shares 60% similarity with regular H3 and is essential for kinetochore assembly and microtubule binding. CENP-A also acts as an epigenetic mark to maintain centromere identity throughout cell divisions.
Q2: How do centromeres differ between yeast and human chromosomes?
Yeast centromeres contain a single CENP-A nucleosome called point centromeres, attached to one mitotic spindle. Human centromeres have alternating CENP-A and regular H3 nucleosomes spanning up to 5 megabases. Human centromeres also contain alpha satellite DNA sequences, while yeast centromeres are flanked by AT-rich pericentric DNA with methylated histone H3.
Q3: What structural domains of CENP-A are conserved across eukaryotes?
The functional C-terminal fold domain of CENP-A is highly conserved among all eukaryotes, ensuring consistent centromere function. However, the N-terminal tail shows significant variations in both size and sequence across species. This conservation of the C-terminal domain reflects its critical role in nucleosome formation and kinetochore interaction.
Q4: What are the competing models for CENP-A nucleosome structure?
In vitro studies initially showed centromeric nucleosomes form octomers with two copies of CENP-A plus two copies each of H2A, H2B, and H4. However, the hemisome model proposes nucleosomes contain only one copy of each histone, forming a tetramer. Recent evidence indicates CENP-A nucleosomes are cell cycle-regulated, existing as octamers in S-phase and hemisomes during other phases.
Q5: When is CENP-A loaded onto centromeric nucleosomes during the cell cycle?
CENP-A loading timing varies between species. In humans, CENP-A is loaded between anaphase and G1 phases of the cell cycle. In plants, loading occurs during the late G2 phase. This species-specific timing reflects different strategies for maintaining centromere identity and ensuring proper inheritance of chromatin structures epigenetic inheritance.
Q6: How does CENP-A overexpression relate to cancer development?
Deregulation of CENP-A functions is linked to chromosome instability and cancer progression. Overexpression of CENP-A has been documented in colon cancer, adenocarcinoma, testicular germ cell tumors, breast cancer, and hepatocellular carcinoma. This suggests CENP-A dysregulation contributes to genomic instability underlying malignant transformation.
Q7: How is CENP-A recognized and targeted to the centromere?
CENP-A is recognized and targeted to centromeric regions through the CENP-A targeting domain, or CATD. This specialized targeting mechanism ensures CENP-A nucleosomes are deposited specifically at centromeres rather than elsewhere on chromosomes. The CATD is essential for maintaining proper centromere identity and function across cell divisions.