A role for amino acid residues in the third cytoplasmic loop in defining the ligand binding characteristics of the alpha2D-adrenergic receptor

Conserved structural, pharmacological and functional properties among the three human and five zebrafish alpha 2-adrenoceptors

1. Zebrafish has five distinct alpha(2)-adrenoceptors. Two of these, alpha(2Da) and alpha(2Db), represent a duplicated, fourth alpha(2)-adrenoceptor subtype, while the others are orthologue of the human alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptors. Here, we have compared the pharmacological properties of these receptors to infer structural determinants of ligand interactions. 2. The zebrafish alpha(2)-adrenoceptors were expressed in Chinese hamster ovary cells and tested in competitive ligand binding assays and in a functional assay (agonist-stimulated [(35)S]GTPgammaS binding). The affinity results were used to cluster the receptors and, separately, the ligands using both principal component analysis and binary trees. 3. The overall ligand binding characteristics, the order of potency and efficacy of the tested agonists and the G-protein coupling of the zebrafish and human alpha(2)-adrenoceptors, separated by approximately 350 million years of evolution, were found to be highly conserved. The binding affinities of the 20 tested ligands towards the zebrafish alpha(2)-adrenoceptors are generally comparable to those of their human counterparts, with a few compounds showing up to 40-fold affinity differences. 4. The alpha(2A) orthologues and the zebrafish alpha(2D) duplicates clustered as close pairs, but the relationships between the orthologues of alpha(2B) and alpha(2C) were not clearly defined. Applied to the ligands, our clustering methods segregated the ligands based on their chemical structures and functional properties. As the ligand binding pockets formed by the transmembrane helices show only minor differences among the alpha(2)-adrenoceptors, we suggest that the second extracellular loop--where significant sequence variability is located --might contribute significantly to the observed affinity differences.

Model structures of α-2 adrenoceptors in complex with automatically docked antagonist ligands raise the possibility of interactions dissimilar from agonist ligands

Antagonist binding to alpha-2 adrenoceptors (alpha2-ARs) is not well understood. Structural models were constructed for the three human alpha2-AR subtypes based on the bovine rhodopsin X-ray structure. Twelve antagonist ligands (including covalently binding phenoxybenzamine) were automatically docked to the models. A hallmark of agonist binding is the electrostatic interaction between a positive charge on the agonist and the negatively charged side chain of D3.32. For antagonist binding, ion-pair formation would require deviations of the models from the rhodopsin structural template, e.g., a rotation of TM3 to relocate D3.32 more centrally within the binding cavity, and/or creation of new space near TM2/TM7 such that antagonists would be shifted away from TM5. Thus, except for the quinazolines, antagonist ligands automatically docked to the model structures did not form ion-pairs with D3.32. This binding mode represents a valid alternative, whereby the positive charge on the antagonists is stabilized by cation-pi interactions with aromatic residues (e.g., F6.51) and antagonists interact with D3.32 via carboxylate-aromatic interactions. This binding mode is in good agreement with maps derived from a molecular interaction library that predicts favorable atomic contacts; similar interaction environments are seen for unrelated proteins in complex with ligands sharing similarities with the alpha2-AR antagonists.

The bovine alpha 2D-adrenergic receptor gene: structure, expression in retina, and pharmacological characterization of the encoded receptor

This report describes cloning of the bovine alpha 2D-adrenergic receptor (alpha 2D-AR) gene and determination of the transcription start site, unequivocal presence of the alpha 2D-AR transcript in the retina, and pharmacological characteristics of the encoded product. Furthermore, expression of the gene in selected bovine tissues has also been scrutinized. A genomic clone was isolated from lambda EMBL3 library and a 3 kb fragment was subcloned and sequenced. This fragment contained the putative TATA box and the coding region. The encoded receptor was transiently expressed in COS cells. The recombinant receptor expressed pharmacological characteristics almost identical to the wild-type bovine retinal receptor, which were typical of the alpha 2D-AR subtype. RNase protection analysis confirmed the expression of the gene in the retina. The bovine receptor was structurally close to its rat analogue which also encodes the alpha 2D-AR, but, the highest homology was observed with the porcine receptor expressing alpha 2A-AR pharmacological characteristics. Certain structural features of the bovine gene were unique to itself and not shared by any other alpha2-AR subtype. Among the tissues tested using reverse transcriptase-polymerase chain reaction (RT-PCR), the alpha 2D-AR message was the most abundant in retina, followed by the brain and olfactory lobe. Thus, the availability of the bovine receptor gene probe will become an important additional tool in the elucidation of molecular mechanisms behind the alpha 2D-AR physiology in neurosensory processes such as those occurring in the eye and the brain.