A two-step mechanism for epigenetic specification of centromere identity and function

Abstract

Summary The basic determinant of chromosome inheritance, the centromere, is specified in many eukaryotes by an epigenetic mark. Using gene targeting in human cells and fission yeast, chromatin containing the centromere-specific histone H3 variant CENP-A is demonstrated to be the epigenetic mark that acts through a two-step mechanism to identify, maintain and propagate centromere function indefinitely. Initially, centromere position is replicated and maintained by chromatin assembled with the centromere-targeting domain (CATD) of CENP-A substituted into H3. Subsequently, nucleation of kinetochore assembly onto CATD-containing chromatin is shown to require either CENP-A’s amino- or carboxy-terminal tails for recruitment of inner kinetochore proteins, including stabilizing CENP-B binding to human centromeres or direct recruitment of CENP-C, respectively.

The Centromere: Epigenetic Control of Chromosome Segregation during Mitosis

“…Indeed, before identification of the CENP-T/W/S/X tetramer, CENP-T/W was suggested to associate primarily with histone H3 rather than CENP-A (Hori et al 2008a;Ribeiro et al 2010). Consistent with this, although CENP-T is lost from centromeres completely lacking CENP-A, its centromere levels are largely unaffected by a 90% CENP-A reduction (Fachinetti et al 2013). Therefore, CENP-T/W/S/X, either as a complex or separately, may interact specifically with non-CENP-A centromeric DNA in a non-nucleosome-like manner.…”

“…However, when CENP-A is completely abolished, CENP-B centromere levels drop by around 50%, revealing a previously uncharacterized dependence of CENP-B on CENP-A nucleosomes (Fachinetti et al 2013). Mutation of centromeric CENP-A revealed CENP-B centromere recruitment requires the CENP-A amino-terminal tail through an unknown mechanism, and CENP-B:CENP-A amino-terminal cooperation may contribute to sustaining localization of centromere proteins (Fachinetti et al 2013). Thus, CENP-B may represent a redundant pathway for CENP-A nucleosome stabilization, positioning, and/or recruitment of other centromerespecific components.…”

“…Thus, CENP-B may represent a redundant pathway for CENP-A nucleosome stabilization, positioning, and/or recruitment of other centromerespecific components. It is interesting to place this in the context of studies that find that human centromeres can remain functional even after extensive, but not complete, depletion of CENP-A (Liu et al 2006;Fachinetti et al 2013). CENP-B may specify the minimal sites of CENP-A nucleosomes sufficient for functional centromere formation, in this way acting as a backup to guard against CENP-A loss.…”

“…Partial reduction in CENP-A levels has no effect on CENP-B (Fachinetti et al 2013). However, when CENP-A is completely abolished, CENP-B centromere levels drop by around 50%, revealing a previously uncharacterized dependence of CENP-B on CENP-A nucleosomes (Fachinetti et al 2013).…”

“…Importantly, a central region (residues 75 -114 in humans) constitutes the CENP-A targeting domain (CATD) (Black et al 2004) that is essential for numerous aspects of CENP-A function and structure, including binding of the CENP-A chaperone Holliday junction recognition protein (HJURP) and the centromere protein CENP-N (discussed below). Finally, CENP-A has an extended hydrophobic carboxy-terminal tail of six amino acids that binds the essential centromere protein CENP-C (Carroll et al 2010;Guse et al 2011;Fachinetti et al 2013). The primary sequences of CENP-A homologs are quite divergent, suggesting the mode of CENP-A-mediated specification of centromere formation may be distinct between organisms.…”