-μ opiate receptor: Charged transmembrane domain amino acids are critical for agonist recognition and intrinsic activity

The μ opiate receptor is a principal brain site for activities of morphine, other opiate drugs, and opioid peptides in modulating pain and altering mood. Recent cloning of cDNAs encoding rat and human μ receptors reveals charged amino acid residues within putative transmembrane domains (TMs) II, III, and VI, a substantial N-terminal extracellular domain, and a C-terminal intracellular domain. Deletion of 64 N-terminal amino acids produced little effect on receptor function (Wang, J. B., Imai, Y., Eppler, C. M., Gregor, P., Spivak, C. E., and Uhl, G. R. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 10230-10234). Further deletion of 33 C-terminal amino acids yielded a receptor at which morphine, but not the substituted enkephalin DAMGO ([D-Ala²,MePhe⁴,Glyol⁵]enkephalin), inhibited adenylate cyclase. Alanine substitution for each charged TM residue in the N-terminally deleted receptor reduced affinities for morphine, DAMGO, and the opiate antagonist naloxone. Replacement of TM II Asp¹¹⁴ with asparagine or glutamic acid increased μ receptor affinity for naloxone. TM II and TM III glutamic acid substitutions for Asp¹¹⁴ and Asp¹⁴⁷ reduced agonist binding affinities but allowed full inhibition of adenylate cyclase at high agonist concentrations. TM VI histidine substitution with alanine yielded a receptor that produced almost twice the cyclase inhibition displayed by the wild type receptor in parallel transient expression assays. These findings underscore the importance of charged residues in TM II, III, and VI for different receptor functions and the modest involvement of extensive portions of N- and C-terminal receptor domains in these processes.