Histone heterodimers fold in MD simulations and NMR experiments, not monomers.

(A) Q values as a function of the annealing temperature are plotted for simulated histone monomers, with the mean and standard deviation displayed as circles and error bars. Aligned final snapshots of H2A by α2 helix (orange) show that no stable tetiary structure formed. (B-C) Q analysis shows that histone monomers (blue, green) cooperatively fold and bind into a dimer (magenta). Two example systems are shown, human H2A/H2B (B) and archaeal histones (HMfA)2 (C). The final conformations (magenta) are well aligned with their native states (gray). (D-E) NMR and CD studies of H2A and H2B upon their complex formation. (D) 1H-15N NMR spectra of 15N-labeled H2A alone (i) and in the presence of unlabeled H2B at a 1:1 molar ratio (iii). Heteronuclear steady-state 15N{1H} NOE spectra with amide proton presaturation recorded for 15N-labeled H2A alone (ii) and in the presence of unlabeled H2B at an equimolar ratio (iv). In these spectra, contours with positive intensities are colored black while negative intensities are blue. (E) CD spectra of H2A (blue) and H2B (red) alone and in an equimolar mixture (black). The total concentrations of the proteins are the same in all three cases. Also shown (dashed magenta) is the expected CD spectrum of the equimolar mixture if no structural changes in either protein. The table shows the secondary structure composition of the proteins estimated from these experimental CD data using K2D algorithm.