Partly due to lack of detailed knowledge of the molecular recognition of ligands the structural basis for partial versus full agonism is not known. In the beta(2)-adrenergic receptor the agonist binding site has previously been structurally and functionally exchanged with an activating metal-ion site located between AspIII:08-or a His residue introduced at this position in transmembrane domain (TM)-III-and a Cys residue substituted for AsnVII:06 in TM-VII. Here, this interhelical, bidentate metal-ion site is without loss of Zn(2+) affinity transferred to the tachykinin NK(1) receptor. In contrast to the similarly mutated beta(2)-adrenergic receptor, signal transduction-i.e., inositol phosphate turnover-could be stimulated by both Zn(2+) and by the natural agonist, Substance P in the mutated NK(1) receptor. The metal-ion acted as a 25% partial agonist through binding to the bidentate zinc switch located exactly one helical turn below the two previously identified interaction points for Substance P in, respectively, TM-III and -VII. The metal-ion chelator, phenantroline, which in the beta(2)-adrenergic receptor increased both the potency and the agonistic efficacy of Zn(2+) or Cu(2+) in complex with the chelator, also bound to the metal-ion site-engineered NK(1) receptor, but here the metal-ion chelator complex instead acted as a pure antagonist. It is concluded that signaling of even distantly related rhodopsin-like 7TM receptors can be activated through Zn(2+) coordination between metal-ion binding residues located at positions III:08 and VII:06. It is suggested that only partial agonism is obtained through this simple well defined metal-ion coordination due to lack of proper interactions with residues also in TM-VI.
Keywords: Amino Acid Sequence; Animals; Animals, Genetically Modified; COS Cells; Humans; Membrane Proteins; Molecular Sequence Data; Protein Conformation; Protein Engineering; Radioligand Assay; Receptors, Neurokinin-1; Signal Transduction; Transfection; Zinc