NMR STRUCTURE OF HMFB FROM THE HYPERTHERMOPHILE, METHANOTHERMUS-FERVIDUS, CONFIRMS THAT THIS ARCHAEAL PROTEIN IS A HISTONE

The three-dimensional structure of the recombinant histone rHMfB from Methanothermus fervidus, an archaeon that grows optimally at 83 degree s C, has been determined by nuclear magnetic resonance methods. This i s only the third structure of a protein from a hyperthermophilic organ ism (optimal growth at temperatures above 80 degrees C). Signal assign ments were made using a combination of homonuclear-correlated, N-15-do uble resonance and N-15, C-13 triple resonance NMR experiments. Long r ange dipolar interactions for the symmetric homodimer were identified from two-dimensional C-13-double half-filtered and three-dimensional C -13-filtered NMR data obtained for a heterolabeled-dimer. A family of 33 structures was calculated using DSPACE(TM) with a total of 609 NOE- derived interproton distance restraints, including 22 intraresidue, 19 2 sequential, 300 medium-range (two to five residues), 86 long-range i ntramolecular (more than five residues) and 112 intermolecular distanc e restraints. The monomer subunits consist of three alpha-helices, ext ending from residues Pro4 to Ala15 (helix I), Ser21 to Ala50 (helix II ) and Lys56 to Lys68 (helix III), as well as two short segments of bet a-strand comprised of residues Arg19 to Ser21 and Thr54 to Ile55. Heli ces I, II and III contain N-terminal capping boxes, and helices I and II contain C-terminal caps. The structure of the (rHMfB)(2) dimer appe ars very similar to the dimer subunits within the histone core octamer of the chicken nucleosome. The presence of a canonical ''histone fold '' motif in rHMfB is consistent with the HMf family of archaeal histon es and the eukaryal nucleosome core histones having evolved from a com mon ancestor. The (rHMf8)2 dimer contains several structural features that may impart thermal stability (or non-lability), including two nov el hydrophobic ''proline Ncaps'', four interhelical hydrogen bonds and short N- and C-terminal disordered tails. (C) 1996 Academic Press Lim ited