Recognition by Max of its cognate DNA through a dimeric b/HLH/Z domain

The three-dimensional structure of the basic/helix-loop-helix/leucine zipper domain of the transcription factor Max complexed with DNA has been determined by X-ray crystallography at 2.9 Å resolution. Max binds as a dimer to its recognition sequence CACGTG by direct contacts between the α-helical basic region and the major groove. This symmetric homodimer, a new protein fold, is a parallel, left-handed, four-helix bundle, with each monomer containing two α-helical segments separated by a loop. The two α-helical segments are composed of the basic region plus helix 1 and helix 2 plus the leucine repeat, respectively. As in GCN4, the leucine repeat forms a parallel coiled coil.