Figure 24-40

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X-ray structure of the Max(22-113) dimer in complex with a 22-bp DNA containing the protein's palindromic 6-bp target sequence. The complex is viewed with its 2-fold axis vertical. The DNA is drawn in stick form with C magenta, N blue, O red, and P orange and with successive P atoms connected by orange rods. The homodimeric protein is shown in ribbon form. Its N-terminal basic region (green) forms an α helix that engages its target sequence in the DNA's major groove and then merges smoothly with the H1 helix (yellow) of the helix–loop–helix (HLH) motif. Following the loop (red), the protein's two H2 helices (purple) of the HLH motif form a parallel left-handed four-helix bundle with the two H1 helices. Each H2 helix then merges smoothly with the leucine zipper (Z) motif (cyan) to form a parallel coiled coil. The protein's C-terminal ends are pink. Click the "Show DNA Binding Region" button for a close-up of the DNA binding region in which the basic region's side chains are drawn in stick form in blue. Note that the two polypeptides engage the DNA half a helical turn apart. [Based on an X-ray structure by Edward Ziff and Stephen Burley, The Rockefeller University. PDBid 1AN2.]