A 3D MODEL OF THE DELTA-OPIOID RECEPTOR AND LIGAND-RECEPTOR COMPLEXES

A model for the 3D structure of the transmembrane domain of the delta opioid receptor was predicted from the sequence divergence analysis of 42 sequences of G-protein coupled peptide hormone receptors belonging to the opioid, somatostatin and angiotensin receptor families, No tem plate was used in the prediction steps, which include multiple sequenc e alignment, calculation of a variability profile of the aligned seque nces, use of the variability profile to identify the boundaries of tra nsmembrane regions, prediction of their secondary structure, optimizat ion of the packing shape in a helix bundle, prediction of side chain c onformations and structural refinement. The general shape of the model is similar to that of the low resolution rhodopsin structure in that the TM3 and TM7 helices are most buried in the bundle and the TM1 and TM4 helices are most exposed to the lipid phase, An initial assessment of this model was made by determining to what extent a binding site i dentified using four structurally disparate high affinity delta opioid ligands was consistent with known mutational studies, With the assump tion that the protonated amine nitrogen, a feature common to all delta opioid ligands, interacts with the highly conserved Asp127 in TM3, a pocket was found that satisfied the criteria of complementarity to the requirements for receptor recognition for these four diverse ligands, two delta selective antagonists (the fused ring naltrindole and the p eptide Tyr-Tic-Phe-Phe-NH2) and the two agonists lofentanil and BW373U 86 deduced from previous studies of the ligands alone, These ligands c ould be accommodated in a similar region of the receptor. The receptor binding site identified in the optimized complexes contained many res idues in positions known to affect ligand binding in G-protein coupled receptors, These results also allowed identification of key residues as candidates for point mutations for further assessment and refinemen t of this model as well as preliminary indications of the requirements for recognition of this receptor.