The bZIP domain consists of two structural features located on a contiguous alpha-helix: first, a basic region of ~ 16 amino acid residues containing a nuclear localization signal followed by an invariant N-x7-R/K motif that contacts the DNA; and, second, a heptad repeat of leucines or other bulky hydrophobic amino acids positioned exactly nine amino acids towards the C-terminus, creating an amphipathic helix. To bind DNA, two subunits adhere via interactions between the hydrophobic sides of their helices, which creates a superimposing coiled-coil structure. The ability to form homo- and heterodimers is influenced by the electrostatic attraction and repulsion of polar residues flanking the hydrophobic interaction surface of the helices.
Plant bZIP proteins preferentially bind to DNA sequences with an ACGT core. Binding specificity is regulated by flanking nucleotides. Plant bZIPs preferentially bind to the A-box (TACGTA), C-box (GACGTC) and G-box (CACGTG), but there are also examples of nonpalindromic binding sites.
