Dynamic Equilibrium in Histone Assembly: Self-assembly of Single Histones and Histone Pairs

The assembly of acid-extracted, purified F2a1, F3, F2a2, and F2b histones and their six possible pairwise combination into organized structures has been studied by: (1) sedimentation velocity, (2) sedimentation equilibrium, (3) electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate after cross-linking the protein solution with dimethyl suberimidate, and (4) electron microscopy. Each of the purified histone fractions can renature and assemble into high molecular weight organized structures. This assembly is dependent on the ionic strength, protein concentration, and temperature of the solutions. The four histones studied assemble into structures of similar dimensions and shape. In each case the first structure observed is a bent rod with a diameter of 22 A. Conditions which favor assembly lead to formation of fibers with diameters of about 44 A. The conditions which lead to assembly into organized structures are similar for the arginine-rich histones, F2a1 and F3. Higher ionic strength is required for the assembly of the lysine-rich histones, F2a2 and F2b. Certain pairs of histones interact. Strong interactions among pairs of histones interfere with the self-assembly of single histones into large structures. Howver, increase in protein concentration or ionic stregth leads to formation of large molecular structures even in solutions of pairs of strongly interacting histones. These structures are similar to those obtained with single histones. The results suggest that aggregation and complexing of histones represent a reversible, ordered process of assembly. The various assembled forms are in a dynamic equilibrium. The final assembled form, which is similar in all cases, is dependent on the environmental conditions to which the histones are exposed. It is suggested that each of the assembled histone structures, regardless whether it is composed of a single histone or a pair of histones, can serve as a core around which the DNA can be wrapped.