The histone core complex: an octamer assembled by two sets of protein-protein interactions.

A protein complex, extracted from calf thymus chromatin with 2 M NaCl, pH 7.5, containing approximately equal molar ratios of histones H2A, H2B, H3, and H4, has been characterized in this study. Gel filtration, sedimentation velocity, and sedimentation equilibrium experiments demonstrate that this complex, known as the core complex, has a molecular weight near that expected for a histone octamer (108 000 for a unit containing two each of the four inner histones) and far exceeding that of a histone tetramer (54 400). This finding suggests that the histone octamer, postulated to be the fundamental histone unit in chromatin, is stable in 2 M NaCl, pH 7.5, in the absence of DNA or chemical cross-linking reagents. In the second part of this study, we demonstrate that the bonds maintaining the octameric complex in 2 M NaCl are weak and distinctly different from the forces stabilizing the H2A-H2B dimer or H3-H4 tetramer. The octamer is dissociated into two H2A-H2B dimers and one H3-H4 tetramer by (i) increasing temperature; (ii) decreasing NaCl concentration; (iii) adding low concentrations of urea or guanidine hydrochloride; and (iv) lowering the pH below 7 or raising it above 10. These findings indicate that the octamer is assembled by two sets-of protein-protein interactions. The first set involves mostly hydrophobic interactions and yields the H2A-H2B dimer and the H3-H4 tetramer subunits. The second set involves the weak association of one H3-H4 tetramer with two H2A-H2B dimers to form an octamer. We suggest that these weak interactions might be derived predominantly from histidine-lysine or histidine-tyrosine hydrogen bonds between the dimer and tetramer subunits.