STRUCTURE OF THE A-DOMAIN OF HMG1 AND ITS INTERACTION WITH DNA AS STUDIED BY HETERONUCLEAR 3-DIMENSIONAL AND 4-DIMENSIONAL NMR-SPECTROSCOPY

HMG1 has two homologous, folded DNA-binding domains (''HMG boxes''), A and B, linked by a short basic region to an acidic C-terminal domain. Like the whole protein, which may perform an architectural role in ch romatin, the individual boxes bind to DNA without sequence specificity , have a preference for distorted or prebent DNA, and are able to bend DNA and constrain negative superhelical turns. They show qualitativel y similar properties with quantitative differences. We have previously determined the structure of the HMG box from the central B-domain (77 residues) by two-dimensional NMR spectroscopy, which showed that it c ontains a novel fold [Weir et al. (1993) EMBO J. 12, 1311-1319]. We ha ve now determined the structure of the A-domain (as a Cys --> Ser muta nt at position 22 to avoid oxidation, without effect on its DNA-bindin g properties or structure) using heteronuclear three- and four-dimensi onal NMR spectroscopy. The A-domain has a very similar global fold to the B-domain and the Drosophila protein HMG-D [Jones et al. (1994) Str ucture 2, 609-627]. There are small differences between A and B, in pa rticular in the orientation of helix I, where the B-domain is more sim ilar to HMG-D than it is to the A-domain; these differences may turn o ut to be related to the subtle differences in functional properties be tween the two domains [Tee et al. (1995) Eur. J. Biochem. 230, 943-950 ] and will be the subject of further investigation. NMR studies of the interaction of the A-domain of HMG1 with a short double-stranded olig onucleotide support the notion that the protein binds via the concave face of the L-shaped structure; extensive contacts with the DNA are ma de by the N-terminal extended strand, the N-terminus of helix I, and t he C-terminus of helix II. These contacts are very similar to those se en in the LEF-1 and SRY-DNA complexes [Love et al. (1995) Nature 376, 791-795; Werner et al. (1995) Cell 81, 705-714].