Octameric CENP-A Nucleosomes Are Present at Human Centromeres throughout the Cell Cycle.
Institute for Research in Immunology and Cancer (IRIC), Faculty of Medicine, University of Montreal, Montreal, QC H3T 1J4, Canada; Graduate programs in Molecular Biology-Systems Biology Option, Faculty of Medicine, University of Montreal, Montreal, QC H3T 1J4, Canada.
The presence of a single centromere on each chromosome that signals formation of a mitotic kinetochore is central to accurate chromosome segregation . The histone H3 variant centromere protein-A (CENP-A) is critical for centromere identity and function; CENP-A chromatin acts as an epigenetic mark to direct both centromere and kinetochore assembly [2-4]. Interpreting the centromere epigenetic mark ensures propagation of a single centromere per chromosome to maintain ploidy. Thus, understanding the nature of CENP-A chromatin is crucial for all cell divisions. However, there are ongoing debates over the fundamental composition of centromeric chromatin. Here we show that natively assembled human CENP-A nucleosomes are octameric throughout the cell cycle. Using total internal reflection fluorescence (TIRF)-coupled photobleaching-assisted copy-number counting of single nucleosomes obtained from cultured cells, we find that the majority of CENP-A nucleosomes contain CENP-A dimers. In addition, we detect the presence of H2B and H4 in these nucleosomes. Surprisingly, CENP-A associated with the chaperone HJURP can exist as either monomer or dimer, indicating possible assembly intermediates. Thus, our findings indicate that octameric CENP-A nucleosomes mark the centromeric region to ensure proper epigenetic inheritance and kinetochore assembly.
Curr. Biol. 2013;23(9):764-9.
Pubmed ID: 23623556