Human Spleen Histone H2A

Isolation and Four Variant Sequences

Abstract

The amino acid sequence of human spleen histone H2A was investigated as a study in histone evolution, following previous investigation of human spleen histone H2B [Ohe, Y., Hayashi, H., & Iwai, K. (1979) J. Biochem. 85, 615-624]. A mixture of H2A, H3, and H4, obtained from the whole histone by CM-cellulose chromatography, was fractionated into individual histones by Bio-Gel P-10 chromatography. The H2A fraction was further purified by Bio-Gel P-60 chromatography, and two subfractions, H2A (1) and H2A (2), were obtained in a ratio of about 2:1. Subfraction H2A (1) gave 21 tryptic peptides with reasonable recoveries, all of which, except for one, showed the compositions expected from the sequence of calf thymus H2A, including the partially phosphorylated N-terminal peptide. The one exception was due to a partial substitution of arginine for lysine at residue 99. Thus it was deduced that H2A (1) contains two variants; variant I accounts for 40% of the total and has the sequence of 129 amino acids identical with that of calf thymus H2A, and variant 2 accounts for 30% of the total and contains one substitution (Arg for Lys-99). Subfraction H2A (2) also gave the expected tryptic peptides, together with three unexpected peptides. From the sequence determination of such peptides and/or the position assignment of substituted residues, H2A (2) was deduced to contain two variants; each accounts for 15% of the total, variant 3 contains two substitutions (Ser for Thr-16 and Met for Leu-51), and variant 4 contains the same two substitutions and one deletion (His-123 or 124). The three substitutions and one deletion found in human H2A are compared with those in the known H2A sequences of other mammals and lower eukaryotes, and the implications are discussed.