THE REGULATION OF THE BINDING-AFFINITY OF THE LUTEINIZING-HORMONE CHORIOGONADOTROPIN RECEPTOR BY SODIUM-IONS IS MEDIATED BY A HIGHLY CONSERVED ASPARTATE LOCATED IN THE 2ND TRANSMEMBRANE DOMAIN OF G-PROTEIN-COUPLED RECEPTORS
J. Quintana et al., THE REGULATION OF THE BINDING-AFFINITY OF THE LUTEINIZING-HORMONE CHORIOGONADOTROPIN RECEPTOR BY SODIUM-IONS IS MEDIATED BY A HIGHLY CONSERVED ASPARTATE LOCATED IN THE 2ND TRANSMEMBRANE DOMAIN OF G-PROTEIN-COUPLED RECEPTORS, Molecular endocrinology, 7(6), 1993, pp. 767-775
Sequence alignment shows that there is a highly conserved aspartate in
the second transmembrane helix of virtually all G protein-coupled rec
eptors. A previous study on the alpha2-adrenergic receptor demonstrate
d that substitution of this acidic residue for the corresponding amide
slightly decreases the affinity of the receptor for agonists and comp
letely abolishes the effect of Na+ on the affinity for agonists. Since
we have previously shown that Na+ modulates the binding affinity of t
he LH/CG receptor for ovine LH (oLH) [but not for human CG (hCG)], the
experiments described here were designed to determine if the correspo
nding residue (D383) of the rat LH/CG receptor also mediates this Naeffect. We used site-directed mutagenesis to create an LH/CG receptor
mutant in which D383 was substituted by N. The wild type and mutant re
ceptor [designated rLHR(D383N)] were expressed in human embryonic kidn
ey 293 cells, and the transfected cells were tested for their ability
to bind hCG and oLH in medium containing Na+ or an isoosmolar concentr
ation of an appropriate sodium substitute. The results presented here
show that this single point mutation of the LH/CG receptor leads to a
slight reduction in affinity for hCG and oLH but completely abolishes
the effects of Na+ removal on the affinity for oLH. Thus, regardless o
f the presence or absence of Na+, cells expressing rLHR(D383N) bind oL
H with a low affinity comparable to that of the wild type receptor ass
ayed in the presence of Na+. We also measured the ability of hCG and o
LH to increase cAMP accumulation in cells expressing the wild type and
mutant receptors. Since the EC50s for both hormones are one order of
magnitude higher in cells expressing rLHR(D383N) than in cells express
ing the wild type receptor, our data show that D383 is also important
for receptor activation.