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Published online 22 January 2001. doi:10.1083/jcb.152.2.375
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© The Rockefeller University Press, 0021-9525/2001/1/375/ $5.00
The Journal of Cell Biology, Volume 152, Number 2, January 22, 2001 375-384

Original Article

A Novel Chromatin Protein, Distantly Related to Histone H2A, Is Largely Excluded from the Inactive X Chromosome

Brian P. Chadwicka and Huntington F. Willarda
a Department of Genetics, Case Western Reserve University School of Medicine and Center for Human Genetics and Research Institute, University Hospitals of Cleveland, Cleveland, Ohio 44106-4955

Correspondence to: Huntington F. Willard, Department of Genetics, Case Western Reserve University School of Medicine, BRB 731, 2109 Adelbert Rd., Cleveland, OH 44106-4955. Tel:(216) 368-1617 Fax:(216) 368-3030 E-mail:hfw{at}po.cwru.edu.

Chromatin on the mammalian inactive X chromosome differs in a number of ways from that on the active X. One protein, macroH2A, whose amino terminus is closely related to histone H2A, is enriched on the heterochromatic inactive X chromosome in female cells. Here, we report the identification and localization of a novel and more distant histone variant, designated H2A-Bbd, that is only 48% identical to histone H2A. In both interphase and metaphase female cells, using either a myc epitope–tagged or green fluorescent protein–tagged H2A-Bbd construct, the inactive X chromosome is markedly deficient in H2A-Bbd staining, while the active X and the autosomes stain throughout. In double-labeling experiments, antibodies to acetylated histone H4 show a pattern of staining indistinguishable from H2A-Bbd in interphase nuclei and on metaphase chromosomes. Chromatin fractionation demonstrates association of H2A-Bbd with the histone proteins. Separation of micrococcal nuclease–digested chromatin by sucrose gradient ultracentrifugation shows cofractionation of H2A-Bbd with nucleosomes, supporting the idea that H2A-Bbd is incorporated into nucleosomes as a substitute for the core histone H2A. This finding, in combination with the overlap with acetylated forms of H4, raises the possibility that H2A-Bbd is enriched in nucleosomes associated with transcriptionally active regions of the genome. The distribution of H2A-Bbd thus distinguishes chromatin on the active and inactive X chromosomes.

Key Words: histones, X chromosome inactivation, euchromatin, histone H4 acetylation, macroH2A


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