Site-directed mutagenesis of the rat beta1-adrenoceptor. Involvement of Tyr356 (7.43) in (+/-)cyanopindolol but not (+/-)[125Iodo]cyanopindolol binding.

To determine the role played by Tyr(356 (7.43)) in the rat beta(1)-adrenoceptor in binding the antagonists (+/-)cyanopindolol (4-[3-(t-butylamino]-3-(2'-cyano-indoloxy)-2-propanolol) and its iodinated analogue (+/-)[(125)Iodo]cyanopindolol (1-(t-butylamino]-3-(2'-cyano-3'-iodo-indoloxy)-2-propanolol), Tyr(356 (7.43)) was mutated to either Phe or Ala and binding affinities determined for wild type and mutant rat beta(1)-adrenoceptors. Our results indicate that Tyr(356 (7.43)) is important for (+/-)cyanopindolol, but not (+/-)[(125)Iodo]cyanopindolol, binding and that (+/-)cyanopindolol adopts a "reverse" binding orientation whereas (+/-)[(125)Iodo]cyanopindolol cannot be accommodated in this binding mode. We define a "reverse" antagonist binding mode as one where the aryloxy moiety interacts with residues on transmembrane helices 1, 2, 3 and 7. The beta(1)-adrenoceptor site-directed mutagenesis results are the first to support a "reverse" antagonist binding orientation and the involvement of Tyr(356 (7.43)) in this binding mode.