The PDZ1 domain of the Na ؉
/H؉ exchanger regulatory factor (NHERF) binds with nanomolar affinity to the carboxyl-terminal sequence QDTRL of the cystic fibrosis transmembrane conductance regulator (CFTR) and plays a central role in the cellular localization and physiological regulation of this chloride channel. The crystal structure of human NHERF PDZ1 bound to the carboxyl-terminal peptide QDTRL has been determined at 1.7-Å resolution. The structure reveals the specificity and affinity determinants of the PDZ1-CFTR interaction and provides insights into carboxyl-terminal leucine recognition by class I PDZ domains. The peptide ligand inserts into the PDZ1 binding pocket forming an additional antiparallel -strand to the PDZ1 -sheet, and an extensive network of hydrogen bonds and hydrophobic interactions stabilize the complex. Remarkably, the guanido group of arginine at position ؊1 of the CFTR peptide forms two salt bridges and two hydrogen bonds with PDZ1 residues Glu 43 and Asn 22 , respectively, providing the structural basis for the contribution of the penultimate amino acid of the peptide ligand to the affinity of the interaction.
The Na؉ /H ؉ exchanger regulatory factor (NHERF) binds through its PDZ1 domain to the carboxyl-terminal sequences NDSLL and EDSFL of the  2 adrenergic receptor ( 2 AR) and platelet-derived growth factor receptor, respectively, and plays a critical role in the membrane localization and physiological regulation of these receptors. The crystal structures of the human NHERF PDZ1 domain bound to the sequences NDSLL and EDSFL have been determined at 1.9-and 2.2-Å resolution, respectively. The  2 AR and platelet-derived growth factor receptor ligands insert into the PDZ1 binding pocket by a -sheet augmentation process and are stabilized by largely similar networks of hydrogen bonds and hydrophobic contacts. In the PDZ1- 2 AR complex, the side chain of asparagine at position ؊4 in the  2 AR peptide forms two additional hydrogen bonds with Gly 30 of PDZ1, which contribute to the higher affinity of this interaction. Remarkably, both complexes are further stabilized by hydrophobic interactions involving the side chains of the penultimate amino acids of the peptide ligands, whereas the PDZ1 residues Asn 22 and Glu 43 undergo conformational changes to accommodate these side chains. These results provide structural insights into the mechanisms by which different side chains at the position ؊1 of peptide ligands interact with PDZ domains and contribute to the affinity of the PDZ-ligand interaction.
The Na(+)/H(+) exchanger regulatory factor (NHERF) contains two PDZ domains that mediate the assembly of transmembrane and cytosolic proteins into functional signal transduction complexes. The human NHERF PDZ1 domain, which spans residues 11-99, interacts specifically with carboxy-terminal residues of the beta2 adrenergic receptor and the cystic fibrosis transmembrane conductance regulator. The NHERF PDZ1 was expressed in Escherichia coli as a soluble protein, purified and crystallized in the unbound form using the vapor-diffusion method with 2 M ammonium sulfate as the precipitant. Diffraction data were collected to 1.5 A resolution using synchrotron radiation. The crystals belong to space group P3(1)21 or P3(2)21, with unit-cell parameters a = b = 51.6, c = 58.9 A, and one molecule in the asymmetric unit.
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