Partial Agonism through a Zinc-Ion Switch Constructed between Transmembrane Domains III and VII in the Tachykinin NK₁ Receptor

Abstract

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.