The effect of salts on the stability of the H2A-H2B histone dimer.

The core nucleosome, which comprises an H3-H4 tetramer and two H2A-H2B dimers, is not a static DNA packaging structure. The nucleosome is a dynamic protein-DNA complex, and the modulation of its structure is an important component of transcriptional regulation. To begin to understand the molecular details of nucleosome dynamics, we have investigated the stability of the isolated H2A-H2B dimer. The urea-induced equilibrium responses of the heterodimer have been examined by far-UV circular dichroism and intrinsic tyrosine fluorescence. The two spectroscopic probes yielded coincident transitions, and global fitting of the reversible urea-induced unfolding further demonstrated that H2A-H2B unfolds by a two-state equilibrium response. At physiological ionic strengths, the free energy of unfolding in the absence of urea of H2A-H2B is 11.8 +/- 0.3 kcal mol(-)(1), moderate stability for a dimer of 26.4 kDa. The m value, or sensitivity of the unfolding to urea, is 2.9 +/- 0.1 kcal mol(-)(1) M(-)(1). This value is significantly larger than would be predicted for the unfolding of the dimerization motif alone ( approximately 2 kcal mol(-)(1) M(-)(1)), suggesting that the N-terminal tails may adopt a collapsed, solvent-excluding structure that undergoes an unfolding transition. The efficacies of several potassium salts and three chloride salts to stabilize the H2A-H2B dimer were determined. The salt-dependent stabilization of the H2A-H2B dimer shows that the Hofmeister effect is the predominant mode of stabilization. However, studies employing multiple salts suggest that there is a component of stabilization that must arise from screening of electrostatic repulsion in the highly basic heterodimer. The most highly charged regions of the dimer are the N-terminal tails, sites of posttranslational modifications such as acetylation and phosphorylation. These modifications, which alter the charge density of the tails, are involved in regulation of nucleosome dynamics.