Proteolytic digestion studies of chromatin core-histone structure. Identification of a limit peptide of histone H2A

Tryptic digestion of chicken erythrocyte nuclei, to a level at which no intact histone remained, resulted in a set of resistant peptides. These were partially separated by exclusion chromatography. One of the peptides was shown to represent the central sequence 12--118 of histone H2A. This was established by amino acid analysis and by Edman degradations. Comparison of the sequence of histone H2A from a wide range of cell types shows that the tryptic cleavage points correspond closely to the limits of the highly conserved central sequence and not to the limits of the strongly basic regions. It is proposed that the 11 N-terminal and 10 C-terminal residues cleaved by trypsin are exposed in chromatin and play a structural and functional role different from the central 107 residues. The exposed position of the 118--119 bond accords with the known linkage point of ubiquitin to residue 119 of histone H2A in the semi-histone A24.