The basic leucine zipper motif. This motif is often used to mediate protein-protein interactions in eukaryotic transcription factors. (a) The amino acid sequences of several basic leucine zipper (bZIP) proteins. Notice the Leu (L) residues at every seventh position in the zipper region and the number of basic residues—Lys (K) and Arg (R), and one invariant Asn (N)—in the DNA-binding region. A consensus sequence is shown at the bottom. (b) A basic leucine zipper from the yeast activator protein GCN4. Only the two “zippered” α helices, each from a different subunit of the dimeric protein, are shown. The helices wrap around each other in a coiled-coil. This molecular structure shows the interacting Leu residues (red), the basic residues in the DNA-binding region (yellow), and the invariant Asn residue (blue).