Replicating centromeric chromatin: spatial and temporal control of CENP-A assembly

Abstract

The centromere is the fundamental unit for insuring chromosome inheritance. This complex region has a distinct type of chromatin in which histone H3 is replaced by a structurally different homologue identified in humans as CENP-A. In metazoans, specific DNA sequences are neither required nor sufficient for centromere identity. Rather, an epigenetic mark comprised of CENP-A containing chromatin is thought to be the major determinant of centromere identity. In this view, CENP-A deposition and chromatin assembly are fundamental processes for the maintenance of centromeric identity across mitotic and meiotic divisions. Several lines of evidence support CENP-A deposition in metazoans occurring at only one time in the cell cycle. Such cell cycle-dependent loading of CENP-A is found in divergent species from human to fission yeast, albeit with differences in the cell cycle point at which CENP-A is assembled. Cell cycle dependent CENP-A deposition requires multiple assembly factors for its deposition and maintenance. This review discusses the regulation of new CENP-A deposition and its relevance to centromere identity and inheritance.

Figure 1

hMis18α, HJURP and CENP-A assemble at centromeres in a cell cycle-dependent manner. (A). Outline of the SNAP tagging experiments in (B) to establish temporal order of Mis18 recruitment to centromeres at mitotic exit and CENP-A loading. (B) Mis18 recruitment to centromeres precedes new CENP-A assembly at mitotic exit, as demonstrated with synchronized HeLa cells stably expressing CENP-A-SNAP. LAP-hMis18α is targeted to centromere in anaphase, prior to the late telophase/G1-CENP-A-SNAP assembly [adapted from (Silva and Jansen, 2009)]. (C) HJURP localization at centromere occurs after mitotic exit, as determined with indirect immunofluorescence for phospho-histone H3 and HJURP [adapted from (Dunleavy et al., 2009)].

Figure 2

Model of the multi-step pathway of CENP-A-containing centromeric chromatin replication utilizing sequential, temporally separated steps across the mammalian cell cycle. See full description in the main text.