Structural diagrams of the PDZ3 domain of PSD95, depicting interactions with a peptide ligand. (a) Ribbon diagram of the PDZ3 domain. The peptide (yellow) lies in the groove created by the βB strand and the αB helix, and the COOH-terminus of the peptide lies in the hydrophobic activity created by the βA–βB or carboxylate loop. (b) Chemical interactions involved in peptide binding. Stereoscopic images depicting hydrogen bonding (dashed white lines) between residues of the PDZ domains (blue) and the peptide ligand (orange). Oxygen atoms are shown in red and nitrogen atoms in blue. The green sphere is a well-ordered water molecule linking the carboxylate group to Arg³¹⁸. Reproduced, with permission, from ref. 4.