N. Bhowmick et al., Identification of ionizable amino acid residues on the extracellular domain of the lutropin receptor involved in ligand binding, ENDOCRINOL, 140(10), 1999, pp. 4558-4563
The LH receptor (LHR) is a G protein-coupled receptor characterized by a re
latively large N-terminal extracellular domain responsible for high affinit
y ligand binding. Based on a model proposed for a major portion of the extr
acellular domain that contains a number of leucine-rich repeats, nine ioniz
able amino acid residues (Glu(57), Glu(80), Lys(158) Glu(181), Lys(183), Gl
u(184), Glu(188), Lys(190), and Asp(206)) were selected for charge reversal
mutagenesis based on their locations in the proposed model and their poten
tial to serve as ligand contact sites. Mutant LHR complementary DNAs were t
ransiently transfected into COS-7 cells, and the expressed receptors were c
haracterized by Western blot analysis, competitive ligand (hCG) binding, an
d ligand-mediated cAMP production. The most interesting mutants were K158E,
K183E, E184K and D206K, which were present on the plasma membrane fraction
, as judged by Western blots, but were incapable of binding hCG and, of cou
rse, were deficient in hCG-mediated cAMP production. Other replacements at
these positions, K158R,Q,G; K183R, Q,G; E184N; and D206E,Q, led to cell sur
face binding and signaling. The mutants E57K, E189K and K190E behaved simil
arly to wild-type LHR; E80K was trapped intracellularly, but bound ligand i
n solubilized cells; and E181K was not expressed or was rapidly degraded. T
hese results, based on 18 point mutants of LHR, indicate that Lys(158) Lys(
183), Glu(184), and Asp(206) are involved, either directly or indirectly, i
n gonadotropin binding and support the general nature of the proposed model
.