The unconventional structure of centromeric nucleosomes

Abstract

The centromere is a defining feature of the eukaryotic chromosome, required for attachment to spindle microtubules and segregation to the poles at both mitosis and meiosis. The fundamental unit of centromere identity is the centromere-specific nucleosome, in which the centromeric histone 3 (cenH3) variant takes the place of H3. The structure of the cenH3 nucleosome has been the subject of controversy, as mutually exclusive models have been proposed, including conventional and unconventional left-handed octamers (octasomes), hexamers with non-histone protein constituents, and right-handed heterotypic tetramers (hemisomes). Hemisomes have been isolated from native centromeric chromatin, but traditional nucleosome assembly protocols have generally yielded partially unwrapped left-handed octameric nucleosomes. In budding yeast, topology analysis and high-resolution mapping has revealed that a single right-handed cenH3 hemisome occupies the ~80-bp Centromere DNA Element II (CDEII) of each chromosome. Overproduction of cenH3 leads to promiscuous low-level incorporation of octasome-sized particles throughout the yeast genome. We propose that the right-handed cenH3 hemisome is the universal unit of centromeric chromatin, and that the inherent instability of partially unwrapped left-handed cenH3 octamers is an adaptation to prevent formation of neocentromeres on chromosome arms.

The CentO satellite confers translational and rotational phasing on cenH3 nucleosomes in rice centromeres

“…The protected sizes of DNA on all of these cenH3 nucleosomes are smaller than the measured sizes (125-150 bp) (40-42) of DNA protected by CENP-A octameric nucleosomes, including the 147 bp protected by octameric CENP-A nucleosomes assembled on human centromeric α-satellite (43). The composition of cenH3 nucleosomes has been controversial, with the proposal of tetrameric, hexameric, and octameric models of the arrangement of subunits (44). Our results do not directly address the composition of rice CENH3 nucleosomes but indicate that these nucleosomes protect only enough DNA for a single DNA wrap around a nucleosome core and protect less DNA than the measured sizes of DNA protected by octameric CENP-A nucleosomes.…”

The AAA-ATPase molecular chaperone Cdc48/p97 disassembles sumoylated centromeres, decondenses heterochromatin, and activates ribosomal RNA genes

“…If left unchecked, neocentromeres result in the loss of chromosome integrity, which leads to diseases, including cancer, in humans (1,38). Below a certain threshold of cellular CenH3 levels, cells cease mitosis in Arabidopsis (39).…”

“…It remains unclear how the function of the CDC48A NPL4 complex, which does not directly associate with H3K9me2, is coupled to the loss of this mark in the pollen vegetative cell. Stable centromeres are characterized by long arrays of CenH3 nucleosomes, which are conducive to kinetochore formation (1). It is possible that the removal of long CenH3 nucleosome arrays initiates a broader decondensation of centromeric heterochromatin.…”

“…Of central importance to centromeres are nucleosomes with the centromere-specific histone H3 variant (CenH3)/centromere protein A (CENP-A) (1). Centromeres are intimately associated with heterochromatin, which is frequently formed adjacent to an array of CenH3 nucleosomes, upon which the kinetochore is assembled, and heterochromatin is required to promote CenH3 assembly at centromeres (2).…”

The Composition, Functions, and Regulation of the Budding Yeast Kinetochore

“…Although Cse4 is an essential component of centromeric chromatin, the precise composition of the Cse4 nucleosome is controversial (Henikoff and Furuyama 2012). Cse4 is released from minichromosomes in 0.3 M NaCl, conditions that do not affect the binding of canonical H3 to DNA (Akiyoshi et al 2009b).…”