Modular PDZ domains, found in many cell junction-associated proteins, mediate the clustering of membrane ion channels by binding to their C- terminus. The X-ray crystallographic structures of the third PDZ domain from the synaptic protein PSD-95 in complex with and in the absence of its peptide ligand have been determined at 1.8 Å and 2.3 Å resolution, respectively. The structures reveal that a four-residue C-terminal stretch (X-Thr/Ser-X- Val-COO) engages the PDZ domain through antiparallel main chain interactions with a β sheet of the domain. Recognition of the terminal carboxylate group of the peptide is conferred by a cradle of main chain amides provided by a Gly-Leu-Gly-Phe loop as well as by an arginine side chain. Specific side chain interactions and a prominent hydrophobic pocket explain the selective recognition of the C-terminal consensus sequence.
Doyle, D. A., Lee, A., Lewis, J., Kim, E., Sheng, M., & MacKinnon, R. (1996). Crystal structures of a complexed and peptide-free membrane protein- binding domain: Molecular basis of peptide recognition by PDZ. Cell, 85(7), 1067–1076. https://doi.org/10.1016/S0092-8674(00)81307-0