X-ray structure of the Engrailed protein homeodomain in complex with a 21-bp DNA containing its target sequence. The 60-residue protein is shown in ribbon form colored in rainbow order from its N-terminus (blue) to its C-terminus (red). The DNA is drawn in stick form colored according to atom type with the C atoms forming its TAAT subsite cyan, other C atoms green, N blue, O red, and P orange and with successive P atoms in the same chain linked by orange rods. Click the "Show DNA–Protein Interactions" button to view the interactions between the TAAT subsite and the protein's recognition helix (helix 3, residues 42–58; red and orange) and N-terminal segment (blue). Note that the protein's recognition helix is bound in the DNA's major groove, and that the N-terminal segment is inserted into the DNA's minor groove. A second homeodomain that binds to the lower end of the DNA in a nearly identical manner but does not contact the homeodomain shown has been omitted for clarity. [Based on an X-ray structure by Carl Pabo, MIT. PDBid 1HDD.]