High mobility group, HMG1/HMG2, subgroup <p>High mobility group (HMG or HMGB) proteins constitute a family of relatively low molecular weight non-histone components in chromatin. HMG1 and HMG2 are highly similar, and preferentially bind single-stranded DNA and unwind double-stranded DNA. Although they have no sequence specificity, they have a high affinity for bent or distorted DNA, and bend linear DNA. HMG1 and HMG2 contain two DNA-binding HMG-box domains (A and B) that show structural and functional differences, and have a long acidic C-terminal domain rich in aspartic and glutamic acid residues. The acidic tail modulates the affinity of the tandem HMG boxes in HMG1 and 2 for a variety of DNA targets. HMG1 and 2 appear to play important architectural roles in the assembly of nucleoprotein complexes in a variety of biological processes, for example V(D)J recombination, the initiation of transcription, and DNA repair [<cite idref="PUB00015133"/>].</p><p>The 3D structure of part of the sequence (57-136), termed box 2, has been determined using 3D NMR [<cite idref="PUB00004428"/>]. The protein exhibits an unusual all-alpha fold, which forms a V-shaped arrow-head, with helices along two edges and one rather flat face [<cite idref="PUB00004428"/>]. Such an architecture is not shown by any of the currently known DNA-binding motifs. The majority of conserved residues in the HMG box family are those involved in maintaining the 3D fold.</p>