Requirement of Specific Intrahelical Interactions for Stabilizing the Inactive Conformation of Glycoprotein Hormone Receptors

Angela Schulz,Karsten Bruns,Peter Henklein,Gerd Krause,Mario Schubert,Thomas Gudermann,Victor Wray,Günter Schultz,Torsten Schöneberg

doi:10.1074/jbc.m006709200

Abstract

Systematic analysis of structural changes induced by activating mutations has been frequently utilized to study activation mechanisms of G-protein-coupled receptors (GPCRs). In the thyrotropin receptor and the lutropin receptor (LHR), a large number of naturally occurring mutations leading to constitutive receptor activation were identified. Saturating mutagenesis studies of a highly conserved Asp in the junction of the third intracellular loop and transmembrane domain 6 suggested a participation of this anionic residue in a salt bridge stabilizing the inactive receptor conformation. However, substitution of all conserved cationic residues at the cytoplasmic receptor surface did not support this hypothesis. Asp/Glu residues are a common motif at the N-terminal ends of alpha-helices terminating and stabilizing the helical structure (helix capping). Since Asp/Glu residues in the third intracellular loop/transmembrane domain 6 junction are not only preserved in glycoprotein hormone receptors but also in other GPCRs we speculated that this residue probably participates in an N-terminal helix-capping structure. Poly-Ala stretches are known to form and stabilize alpha-helices. Herein, we show that the function of the highly conserved Asp can be mimicked by poly-Ala substitutions in the LHR and thyrotropin receptor. CD and NMR studies of peptides derived from the juxtamembrane portion of the LHR confirmed the helix extension by the poly-Ala substitution and provided further evidence for an involvement of Asp in a helix-capping structure. Our data implicate that in addition to well established interhelical interactions the inactive conformation of GPCRs is also stabilized by specific intrahelical structures.

Highlights

G-protein-coupled receptors (GPCRs)1 comprise a large superfamily of integral membrane proteins that mediate transmembranous signal transduction of a remarkable diversity of
Because the replacement of Asp564 in the LHR by other amino acid residues except for Glu resulted in agonist-independent receptor activation, it was speculated that agonist- or mutation-induced disruption of a salt bridge involving this conserved Asp residue within the i3 loop contributes to lutropin/choriogonadotropin receptor; TFE, trifluoroethanol; TSHR, thyrotropin receptor; transmembrane domains (TMD), transmembrane domain; wt, wild type; PCR, polymerase chain reaction; IP, inositol phosphate
Because Asp/Glu residues in the i3 loop/TMD6 junction are conserved in glycoprotein hormone receptors and many other rhodopsin-like GPCRs, we speculate that the inactive receptor conformation depends on the stability of TMD6

Summary

EXPERIMENTAL PROCEDURES

Generation of Mutant LHR and TSHR Genes—All LHR mutations (see Figs. 1 and 2) were introduced into LHR-pcDps [19], a mammalian expression vector containing the entire coding sequence of the human LHR, using a PCR-based site-directed mutagenesis and restriction fragment replacement strategy [25]. For cAMP assay, transfected cells were washed once in serum-free Dulbecco’s modified Eagle’s medium containing 1 mM 3-isobutyl-1-methylxanthine (Sigma), followed by incubation in the presence of the indicated concentrations of human choriogonadotropin (hCG; from pregnancy urine, 3,000 units/ mg, Sigma) or bovine thyrotropin (Sigma); for 1 h at 37 °C. The peptides were dissolved at a concentration of 0.2 mg/ml in pure water or in aqueous TFE solutions containing 50% TFE by volume As these solutions adopted acidic pH value, further measurements were performed in 65 mM phosphate buffer or in phosphate-buffered TFE solution at neutral pH. The starting conformation of the i1 loop, the i2 loop, and the first portion of the C-terminal tail comprising the putative i4 loop of the LHR were adopted from the NMR structure of the rhodopsin cytosolic loop peptide complex [6] as described for the V2 vasopressin receptor [35]. Molecular dynamics simulations were performed at 300 K for 200 ps, where only the helix stability was maintained by restraints for hydrogen bonds of the TMD backbones

RESULTS AND DISCUSSION

TABLE I Functional characterization of LHR Ala mutants

Increase in cAMP levels

TABLE II

Adenosine receptors Somatostatin receptors