STRUCTURE OF THE HMG BOX MOTIF IN THE B-DOMAIN OF HMG1

The conserved, abundant chromosomal protein HMG1 consists of two highly homologous, folded, basic DNA-binding domains, each of approximately 80 amino acid residues, and an acidic C-terminal tail. Each folded domain represents an 'HMG box', a sequence motif recently recognized in certain sequence-specific DNA-binding proteins and which also occurs in abundant HMG1-like proteins that bind to DNA without sequence specificity. The HMG box is defined by a set of highly conserved residues (most distinctively aromatic and basic) and appears to define a novel DNA-binding structural motif. We have expressed the HMG box region of the B-domain of rat HMG1 (residues 88 - 164 of the intact protein) in Escherichia coli and we describe here the determination of its structure by 2D H-1-NMR spectroscopy. There are three alpha-helices (residues 13 - 29, 34 - 48 and 50 - 74), which together account for approximately 75% of the total residues and contain many of the conserved basic and aromatic residues. Strikingly, the molecule is L-shaped, the angle of approximately 80-degrees between the two arms being defined by a cluster of conserved, predominantly aromatic, residues. The distinctive shape of the HMG box motif, which is distinct from hitherto characterized DNA-binding motifs, may be significant in relation to its recognition of four-way DNA junctions.