Histone variant macroH2A contains two distinct macrochromatin domains capable of directing macroH2A to the inactive X chromosome

Chromatin on the inactive X chromosome (Xi) of female mammals is enriched for the histone variant macroH2A that can be detected at interphase as a distinct nuclear structure referred to as a macro chromatin body (MCB). Green fluorescent protein-tagged and Myc epitope-tagged macroH2A readily form an MCB in the nuclei of transfected female, but not male, cells. Using targeted disruptions, we have identified two macrochromatin domains within macroH2A that are independently capable of MCB formation and association with the Xi. Complete removal of the non-histone C-terminal tail does not reduce the efficiency of association of the variant histone domain of macroH2A with the Xi, indicating that the histone portion alone can target the Xi. The non-histone domain by itself is incapable of MCB formation. However, when directed to the nucleosome by fusion to core histone H2A or H2B, the non-histone tail forms an MCB that appears identical to that of the endogenous protein. Mutagenesis of the non-histone portion of macroH2A localized the region required for MCB formation and targeting to the Xi to an ∼190 amino acid region.