Functional role of transmembrane helix 7 in the activation of the heptahelical lutropin receptor.

Abstract

A member of the G protein-coupled receptor superfamily, the LH receptor (LHR), and the two other glycoprotein hormone receptors are distinguished from the other members by the presence of a relatively large N-terminal extracellular domain that is responsible for high-affinity ligand binding. Transmembrane helix (TMH) 7 of LHR is amphipathic, with an extended face containing only hydrophobic side chains and another containing both hydrophobic and polar side chains with potential hydrogen bond donor and acceptor functions. Since several reports have shown the importance of this helix in ligand-mediated signaling, we have used Ala scanning mutagenesis to study eight amino acid residues of rat LHR that are invariant in the three glycoprotein hormone receptors, Leu586, Val587, Asn593, Ser594, Cys595, Asn597, Phe604, and Thr605. The wild type (WT) and mutant cDNAs were transiently transfected into COS-7 cells for characterization by human CG (hCG) binding and cAMP production. No differences were detected in dissociation constants (K(d)S) or basal cAMP production relative to WT LHR, but three categories of LHR mutants were distinguished from WT LHR based upon their expression levels and responsiveness to hCG: 1) comparable or higher expression but reduced ligand responsiveness (N593A and C595A), 2) reduced expression and ligand responsiveness (N597A and T605A), and 3) comparable expression and responsiveness (L586A, V587A, S594A, and F604A). Three other mutants, C595M, F604Y, and T605Y, were comparable to WT LHR in ligand responsiveness. To provide more information on Asn593 and Asn597, a total of 12 replacements were investigated. Of considerable interest and potential significance was the finding that many of the replacements in LHR resulted in either loss of function (N593A, Q, S; N597R) or gain of function (N593R and N597Q), this being the first evidence of a position in LHR that, depending upon the nature of the amino acid residue, can result in constitutive activation and/or diminished responsiveness to ligand. The results of molecular modeling and energy minimization of TMHs 6 and 7, based on a postulated interaction between Asp556 (TMH 6) and Asn593/Asn597 (TMH 7), indicated that, while there is not a correlation between function and predicted energies of WT LHR and the mutants, reorientation of one or both helices is responsible for the functional changes observed. Possible interactions of TMHs 3 and 4 and of 5 and 6 were suggested by molecular modeling. Ten mutants were prepared of two amino acid residues that are invariant in the glycoprotein hormone receptors and have side chain hydrogen bond donor and acceptor function, Glu429 in TMH 3 and Asn513 in TMH 5. Expression levels and hCG-mediated signaling were reduced in most of the LHR mutants, but none of these exhibited constitutive receptor activation. We conclude that Glu429 is not critical for receptor function, while Asn513 appears to be particularly important in receptor folding and/or trafficking. The results reported herein indicate an important role for TMH 7, and particularly Asn593 and Asn597, in the process of receptor activation. Moreover, these two asparagines, although in close proximity to each other in TMH 7, are quite distinct in function as evidenced by certain replacements that can lead to loss of function in one and gain of function in the other.