Partial Agonism through a Zinc-Ion Switch Constructed between Transmembrane Domains III and VII in the Tachykinin NK1Receptor

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 β₂-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 Zn²⁺ affinity transferred to the tachykinin NK₁receptor. In contrast to the similarly mutated β₂-adrenergic receptor, signal transduction—i.e., inositol phosphate turnover—could be stimulated by both Zn²⁺ and by the natural agonist, Substance P in the mutated NK₁ 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 β₂-adrenergic receptor increased both the potency and the agonistic efficacy of Zn²⁺ or Cu²⁺ in complex with the chelator, also bound to the metal-ion site-engineered NK₁ 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 Zn²⁺ 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.