Crystal structure of a PDZ domain.

PDZ domains (also known as DHR domains or GLGF repeats) are ~90-residue repeats found in a number of proteins implicated in ion-channel and receptor clustering, and the linking of receptors to effector enzymes1. PDZ domains are protein-recognition modules; some recognize proteins containing the consensus carboxy-terminal tripeptide motif S/TXV with high specificity2–4. Other PDZ domains form homotypic dimers: the PDZ domain of the neuronal enzyme nitric oxide synthase binds to the PDZ domain of PSD-95, an interaction that has been implicated in its synaptic association5. Here we report the crystal structure of the third PDZ domain of the human homologue of the Drosophila discs-large tumour-suppressor gene product, DlgA. It consists of a five-stranded antiparallel β-barrel flanked by three α-helices. A groove runs over the surface of the domain, ending in a conserved hydrophobic pocket and a buried arginine; we suggest that this is the binding site for the C-terminal peptide.