Histone 2A, a Heteromorphous Family of Eight Protein Species

The histone 2A faily of proteins is shown to consist of eight protein species. In addition to the previously described mammalian 2A variants H2A.1 and H2A.2, we describe two variants which are separable from each other and from variants 1 and 2 on both sodium dodecyl sulfate and acetic acid-urea gels. These two proteins H2A.X and H2A.Z are termed heteromorphous variants to distinguish them from the predominating form and its homeomorphous variants which require nonionic detergents for their resolution. The two heteromorphous variants are present in nucleosomal core particles isolated from mouse L1210 cells. In addition, these variants are found in normal mouse tissues, human HeLa cells, and chicken erythrocytes. On sodium dodecyl sulfate gels, one variant, H2A.X, has an apparent molecular weight approximately 1000 larger than H2A.1 and comprises approximately 11% of the total 2A in mouse L1210 cells. The second variant, H2A.Z, has an apparent molecular weight approximately 600 smaller than H2A.1 and comprises approximately 4% of the total 2A in mouse L1210 cells. The two heteromorphous variants have the same arginine/lysine ratio as H2A.1. In addition, a fraction of each of the four variants (approximately 11% in L1210 cells) is combined with ubiquitin. The molar sum of these eight H2A species approximately equals the number of moles of H4, H2B, or H3 in chromatin.