Equilibrium folding of the core histones: the H3-H4 tetramer is less stable than the H2A-H2B dimer

To compare the stability of structurally related dimers and to aid in understanding the thermodynamics of nucleosome assembly, the equilibrium stabilities of the recombinant wild-type H3-H4 tetramer and H2A-H2B dimer have been determined by guanidinium-induced denaturation, using fluorescence and circular dichroism spectroscopies. The unfolding of the tetramer and dimer are highly reversible. The unfolding of the H2A-H2B dimer is a two-state process, with no detected equilibrium intermediates. The H3-H4 tetramer is unstable at moderate ionic strengths (mu approximately 0.2 M). TMAO (trimethylamine-N-oxide) was used to stabilize the tetramer; the stability of the H2A-H2B dimer was determined under the same solvent conditions. The equilibrium unfolding of H3-H4 was best described by a three-state mechanism, with well-folded H3-H4 dimers as a populated intermediate. When compared to H2A-H2B, the H3-H3 tetramer interface and the H3-H4 histone fold are strikingly less stable. The free energy of unfolding, in the absence of denaturant, for the H3-H4 and H2A-H2B dimers are 12.4 and 21.0 kcal mol(-)(1), respectively, in 1 M TMAO. It is postulated that the difference in stability between the histone dimers, which contain the same fold, is the result of unfavorable tertiary interactions, most likely the partial to complete burial of three salt bridges and burial of a charged hydrogen bond. Given the conservation of these buried interactions in histones from yeast to mammals, it is speculated that the H3-H4 tetramer has evolved to be unstable, and this instability may relate to its role in nucleosome dynamics.