NMR Structure and Comparison of the Archaeal Histone HFoB from the Mesophile Methanobacterium formicicum with HMfB from the Hyperthermophile Methanothermus fervidus

The solution-state structure of the recombinant archaeal histone rHFoB, from the mesophile Methanobacterium formicicum, has been determined by two- and three-dimensional (3D) proton homonuclear correlated nuclear magnetic resonance (NMR) methods. On the basis of 951 nuclear Overhauser effect (NOE)-derived distance restraints, rHFoB monomers form the histone fold and assemble into symmetric (rHFoB)2 dimers that have a structure consistent with assembly into archaeal nucleosomes. rHFoB exhibits approximately 78% sequence homology with rHMfB from the hyperthermophile Methanothermus fervidus, and the results obtained demonstrate that these two proteins have very similar 3D structures, with a root-mean-square deviation for backbone atoms of 0.65 +/- 0.13 A2. (rHFoB)2 dimers however unfold at lower temperatures and require a higher salt environment for stability than (rHMfB)2 dimers, and comparing the structures, we predict that these differences result from unfavorable surface-located ionic interactions and a larger, more solvent-accessible cavity adjacent to residue G36 in the hydrophobic core of (rHFoB)2.