MUTATIONAL ANALYSIS AND MOLECULAR MODELING OF THE NONAPEPTIDE HORMONE-BINDING DOMAINS OF THE [ARG(8)] VASOTOCIN RECEPTOR

To identify determinants that form nonapeptide hormone binding domains of the white sucker Catostomus commersoni [Arg(8)]vasotocin receptor, chimeric constructs encoding parts of the vasotocin receptor and part s of the isotocin receptor have been analyzed by [(3,5-H-3)Tyr(2), Arg (8)]vasotocin binding to membranes of human embryonic kidney cells pre viously transfected with the different cDNA constructs and by function al expression studies in Xenopus laevis oocytes injected with mutant c RNAs. The results indicate that the N terminus and a region spanning t he second extracellular loop and its flanking transmembrane segments, which contains a number of amino acid residues that are conserved thro ughout the nonapeptide receptor family, contribute to the affinity of the receptor for its ligand. Nonapeptide selectivity, however, is main ly defined by transmembrane region VI and the third extracellular loop . These results are complemented by a molecular model of the vasotocin receptor obtained by aligning its sequence with those of other G-prot ein coupled receptors as well as that of bacteriorhodopsin. The model indicates that amino acid residues of transmembrane regions II-VII tha t are located close to the extracellular surface also contribute to th e binding of vasotocin.