Abstract
MacroH2A (mH2A) is a histone variant that is enriched in the inactivated X-chromosomes of mammalian females. To characterize the role of this protein in other nuclear processes we isolated chromatin particles from chicken liver, a vertebrate system that does not undergo X-inactivation. Chromatin digestion and fractionation studies determined that mH2A is evenly distributed at several levels of chromatin structure and stabilizes the nucleosome core particle in solution. However, at the level of the chromatosome, selective salt precipitation showed the existence of a mutually exclusive relationship between mH2A and H1, which may reveal functional redundancy between these proteins. Two-dimensional gel electrophoresis demonstrated the presence of one major population of mH2A containing nucleosomes, which may become ADP-ribosylated. This report provides new clues into how mH2A distribution and a previously unidentified post-translational modification may help regulate the repression of autosomal chromatin.
Highlights
The incorporation of histone variants into nucleosomes has been causally linked to such events as gene expression, DNA repair, and meiosis
The direct interplay between histone variants and downstream effector proteins is proving to be important for nuclear metabolism [11,12,13,14,15]
Its name was derived from the C-terminal nonhistone region (NHR) that comprises two-thirds of its molecular mass
Summary
The incorporation of histone variants into nucleosomes has been causally linked to such events as gene expression, DNA repair, and meiosis (see Ref. 1 for a recent review). The core histone H2A has the largest family of described heteromorphous variants that specialize nucleosomes for defined functions This family of proteins displays the greatest chemical variability at their C terminus, which has implications for nucleosome stability, the binding of H1, and the formation of higher order chromatin structures (4 – 6). The crystallographic structure of a protein with homology to the NHR was resolved, which has heightened interest in this histone variant [17] Bioinfomatical analysis of this region suggests that it may function in directly regulating the ADP-ribosylation of histones [17, 18], suggesting for the first time that a histone variant may have inherent enzymatic potential [19]. Nucleosome complexes purified from chicken hepatocytes provide an interesting look into the relationship between mH2A and autosomal chromatin
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