From the Department of Pharmacology, University of Iowa College of Medicine, Iowa City, Iowa 52242-1109
Received for publication, November 20, 2000
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ABSTRACT |
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Alanine scanning mutagenesis of the second
extracellular loop of the human lutropin receptor (hLHR) showed that
mutation of most of the residues present in this region either enhance
or impair the internalization of agonist. A more complete
analysis of four mutants, two that enhanced internalization (F515A and T521A) and two that impaired internalization (S512A and V519A), showed
that the two mutants that impaired internalization also show a decrease
in the sensitivity for agonist-induced cAMP accumulation, whereas the
two mutants that enhanced internalization show an increase in the
sensitivity for agonist-induced cAMP accumulation. None of these
mutants had an effect on the agonist-induced phosphorylation of the
hLHR, however. We conclude that, in contrast to the prevailing view of
the relative importance of receptor phosphorylation in the
internalization of G protein-coupled receptors, the phosphorylation of
the hLHR is less important than the agonist-induced activation of the
hLHR in the process of internalization.
Internalization of G protein-coupled receptors
(GPCRs)1 is a ubiquitous
response that follows agonist activation (reviewed in Refs. 1-3).
Although several GPCR internalization pathways can now be recognized
(3), the most common and best understood pathway is dependent on the G
protein-coupled receptor kinase (GRK)-catalyzed phosphorylation of
GPCRs and the subsequent formation of a complex between the
agonist-activated and phosphorylated GPCRs and a family of proteins
known as the nonvisual arrestins or The follitropin, lutropin and thyrotropin receptors (FSHR, LHR, and
TSHR, respectively) are members of the
rhodopsin/ Like many other GPCRs, the binding of agonist to the LHR triggers the
internalization of the agonist-receptor complex via clathrin-coated
pits by a pathway that is dependent on receptor activation and
phosphorylation and requires the participation of the nonvisual
arrestins and dynamin (13-17). Whereas the model derived from the
large number of studies on the The studies presented herein describe a novel set of mutations of the
LHR (all located in the second extracellular loop) that enhance or
impair receptor activation but do not affect agonist-induced phosphorylation. These mutations affect the rate of internalization of
agonist in a manner that parallels their effects on receptor activation
and thus highlight the importance of receptor activation, as opposed to
phosphorylation, in the agonist-induced internalization of the LHR.
Plasmids and Cells--
Full-length cDNAs encoding for the
hLHR and rLHR (20, 21) were modified with the Myc epitope at the N
terminus (17, 22) and subcloned into pcDNAI/Neo (rLHR) or pcDNA
3.1(hLHR) for expression. Site-directed mutagenesis was performed using conventional PCR strategies. The identity of each mutant was verified by automated DNA sequencing of the mutated region (performed by the DNA
core of the Diabetes and Endocrinology Research Center of the
University of Iowa).
Expression vectors for arrestin-3 and arrestin-3-(284-489) (23)
were generously provided by Dr. Jeff Benovic (Thomas Jefferson University, Philadelphia, PA). An expression vector for dynamin-K44A (24) was donated by Dr. Sandra Schmid (Scripps Research Institute, La
Jolla, CA). The expression of these constructs has been documented previously (25, 26).
Human embryonic kidney (293) cells were obtained from the American Type
Culture Collection (CRL 1573) and maintained in Dulbecco's modified
Eagle's medium containing 10 mM Hepes, 10% newborn calf serum, and 50 µg/ml gentamicin, pH 7.4. Cells were plated in 100-mm dishes or 35-mm wells that had been coated with gelatin and transfected with 0.5-1 µg of plasmid/35-mm well or 10 µg of plasmid/100-mm dish when 70-80% confluent using the calcium phosphate method of Chen
and Okayama (27). After an overnight incubation, the cells were washed
and incubated for an additional 24 h prior to use.
Binding, Internalization, and cAMP Assays--
The methods used
to measure the internalization of 125I-hCG have been
described (18, 28). Single point assays were done using a 30-min
incubation for cells expressing the rLHR or a 10-min incubation for
cells expressing the hLHR. The results of these experiments are
expressed as an internalization index that is defined as the ratio of
internalized/surface-bound hormone. The length of the incubations was
different because the half-time of internalization of
125I-hCG mediated by the rLHR (~120 min) is much slower
than that mediated by the hLHR (~20 min) and the internalization
index is linear as a function of time for ~60 min for the rLHR and
for ~30 min for the hLHR (18, 28).
Determinations of the rates of internalization were done using at least
five different data points collected at 3-10-min intervals after the
addition of 125I-hCG (depending on the construct
transfected). The endocytotic rate constant (ke)
was calculated from the slope of the line obtained by plotting the
internalized radioactivity against the integral of the surface-bound
radioactivity (18, 29-32). The half-time of internalization (t
1/2) is defined as
0.693/ke.
The hCG binding properties of the different receptor constructs were
ascertained using intact cells cotransfected with dynamin-K44A to
prevent internalization (see "Results"). Binding was measured in
the cotransfected cells that had been incubated with seven different
concentrations of 125I-hCG for 1 h at room
temperature. All binding assays were corrected for nonspecific binding,
which was measured in the presence of 50 IU/ml partially purified hCG
(3,000 IU/mg). The binding data were fitted to a sigmoidal equation
(33) using DeltaGraph® software (Delta Point, Monterey,
CA) to calculate the maximal amount of cell-associated hormone and the
concentration of hCG required to attain half of this value (apparent
Kd).
Hormonal responsiveness was assessed by measuring cAMP accumulation in
intact transfected cells plated in gelatin-coated 35-mm wells (see
above). Total cAMP was measured at the end of a 2-h incubation
(37 °C) with increasing concentrations of hCG as shown in Fig. 3 or
with a single concentration of cholera toxin (0.6 nM) known
to be maximally effective (17). The parameters that describe these dose
responses (i.e. EC50 and maximal response) were
calculated by fitting the data obtained to a sigmoidal equation (33) as
described above.
Phosphorylation Assays--
Cells were plated in 100-mm dishes
that had been coated with gelatin and were transfected as described
above. After an overnight incubation, the cells were washed,
trypsinized, and plated in gelatin-coated 100-mm dishes and in
gelatin-coated 35-mm wells and incubated overnight in Dulbecco's
modified Eagle's medium containing 10 mM Hepes, 1% bovine
serum albumin, and 50 µg/ml gentamicin, pH 7.4. The cells plated in
35-mm wells were used to assess cell surface receptor expression by
measuring the binding of a single, saturating concentration of
125I-hCG during a 1-h incubation at room temperature as
described above. The cells plated in the 100-mm dishes were lysed, and
the crude lysates were immunoprecipitated and analyzed as described recently (17). The immunoprecipitates were resolved on SDS gels, the
gels were visualized and quantitated using a PhosphorImager and the
images were captured in a digital format for presentation.
Hormones and Supplies--
Purified hCG (CR-127, ~13,000
IU/mg) was kindly provided by Dr. Al Parlow and the National Hormone
and Pituitary Agency of the NIDDK, National Institutes of Health
(Bethesda, MD). 125I-hCG was prepared as described
elsewhere (34). Partially purified hCG (~3,000 IU/mg) was purchased
from Sigma, and it was used only for the determination of nonspecific
binding (see above). 125I-cAMP and cell culture medium were
obtained from the Iodination Core and the Media and Cell Production
Core, respectively, of the Diabetes and Endocrinology Research Center
of the University of Iowa. Other cell culture supplies and reagents
were obtained from Corning and Life Technologies, Inc., respectively.
All other chemicals were obtained from commonly used suppliers.
Our interest on a potential role of the second extracellular loop
of the LHR in the rate of internalization of hCG arose from two sets of
studies. First, a recent analysis (35) of a truncation of the rLHR (at
residue 653 in the C-terminal tail) that was initially reported to
impair the internalization of hCG revealed that the decrease in
internalization was due to an additional mutation (V497A in the second
extracellular loop) that was inadvertently introduced (and went
unnoticed) during the construction of the original C-terminally
truncated mutant. Second, other investigators have documented the
potential involvement of this loop in modulation the hCG binding
affinity and signaling properties of the rLHR (36). Alanine scanning
mutagenesis of each of the 20 residues present in the second
extracellular loop of the rLHR confirmed that the V497A mutation
decreases the internalization of hCG in the context of the full-length
receptor and showed that the mutation of many other residues present in
this loop also enhance or decrease internalization (Fig. 1,
right panel). Four mutants of
this receptor region (Y486A, I491A, C492A, and P494A) could not be
analyzed for internalization because the binding of
125I-hCG to cells transfected with these plasmids was very
low or undetectable (these are marked with asterisks in Fig.
1). The reduced or undetectable expression of the I491A, C492A, and
P494A mutants is in agreement with the previous study cited above (36) involving alanine scanning mutagenesis of this region. We disagree on
the expression of the S484A mutant, however. Ryu and co-workers (36)
reported that cells expressing the S484A mutant had no detectable
125I-hCG binding. In our experiments, however, the
expression of this mutant was normal.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
-arrestins. The nonvisual
arrestins (arrestin-2 and -3) target the activated and phosphorylated
GPCRs to clathrin-coated pits by virtue of their ability to bind with
high affinity to clathrin and to adaptor protein-2 (4, 5). Once
localized to clathrin-coated pits, the GPCRs are internalized by a
process that requires the participation of dynamin, a GTPase involved
in the fission of clathrin-coated pits (6).
2-adrenergic-like subfamily of GPCRs (7, 8).
They form a small subfamily of GPCRs, collectively known as the
glycoprotein hormone receptors, that is characterized by the presence
of relatively large extracellular domains composed of leucine-rich
repeats (9-11). Additional leucine-rich repeat-containing G
protein-coupled receptors that are homologous to the glycoprotein
hormone receptors have been recently identified in mammals and other
organisms, but their ligands and functions are not yet known (reviewed
in Ref. 12).
2-adrenergic receptor
emphasizes the importance of GPCR phosphorylation on the process of
internalization (1-3), recent mutagenesis studies suggest that the
agonist-induced activation and phosphorylation of the human (h) LHR may
play redundant roles in the agonist-induced internalization of this
receptor (17). Additional studies on the LHR, FSHR, and TSHR have also
uncovered an unusual, but as yet unexplained, role for their
extracellular domains on the rate of internalization and/or the fate of
the internalized receptors (18, 19).
MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
View larger version (60K):
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Fig. 1.
Effect of mutations of the second
intracellular loop of the rLHR and the hLHR on the internalization of
125I-hCG. Each of the 20 residues of the second
extracellular loop of the rLHR (right panel) or
the hLHR (left panel) was substituted with
alanine as indicated. The resulting plasmids were transiently
transfected into 293 cells (0.5 µg/35-mm well) and the
internalization of 125I-hCG was measured as described under
"Experimental Procedures." Each bar represents the mean ± S.E. of at least three independent transfections. The absence of an
error bar indicates that the S.E. is too small to be shown. The
internalization of hCG mediated by the mutants marked with an
asterisk could not be analyzed because of their poor
expression. The vertical line emphasizes the
internalization index of the LHR-wt, and the arrows mark the
four mutants that were chosen for further analysis.
Since the transient expression of the human (h) LHR is much more robust than that of the rLHR (16, 17) and the rate of internalization of hCG mediated by the hLHR is faster than that mediated by the rLHR (16), we reasoned that using the hLHR to examine the potential effect of mutations of the second extracellular loop may facilitate detection of changes in the rate of internalization and it would be a more amenable model system to examine these effects in more detail. The results of alanine-scanning mutagenesis of the second extracellular loop residues of the hLHR are shown in Fig. 1, left panel.2 Only one mutant of the hLHR (C514A, marked with an asterisk in Fig. 1 was not properly expressed and could not be analyzed for internalization. Alanine scanning of the remaining 19 residues, showed that only one residue (Glu520) had no effect on internalization. Of the remaining 18 residues, the mutation of 12 residues resulted in an increase in internalization and the mutation of 6 residues resulted in a decrease in internalization. Based on these results we chose two mutants that enhanced internalization and two that decreased internalization of the hLHR (see arrows in the left panel of Fig. 1) for more detailed studies.
We first considered the possibility that the mutants in question were internalized by pathways that do not require the participation of the visual arrestins and/or dynamin. This was tested by measuring the rates of internalization of hCG in cells cotransfected with arrestin-3-(284-409), a dominant-negative mutant of the nonvisual arrestins (23), or with dynamin-K44A, a dominant-negative mutant of dynamin (24). These constructs have been previously shown to slow the t1/2 of internalization of hCG mediated by the hLHR-wt (16, 17). As shown in Table I, dynamin-K44A was an effective inhibitor of the internalization of hCG mediated by the hLHR-wt and the four mutants tested. In contrast, arrestin-3-(284-409) inhibited the internalization of the hLHR-wt and the two mutants with a fast t1/2 of internalization (F515A and T521A) but had little or no effect on the two mutants with a slow t1/2 of internalization (S512A and V519A). Thus, it appears that the slow mutants internalize hCG by a pathway that is still dependent on dynamin but independent of the nonvisual arrestins. The data presented in Table I also show, however, that the slow rate of internalization of hCG mediated by these mutants can be rescued by overexpression of arrestin-3.
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Since the agonist-induced phosphorylation of the hLHR is also important
to the process of internalization (17) we measured the phosphorylation
of the four receptor mutants in cells incubated with or without hCG.
These results are presented in Fig. 2 and show that the four mutations in question had little or no effect on
basal or agonist-induced phosphorylation.
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The ability of the four extracellular loop mutants to bind hCG and to transduce the signal into cAMP accumulation were next measured. The results of the binding assays are presented in Table II and show that none of the mutants affected receptor expression at the cell surface. The apparent Kd for hCG binding to cells expressing one of the mutants, S512A, was higher than that of cells expressing hLHR-wt, however. These data are somewhat different than those reported for mutations of the rLHR (36). In the rLHR, mutations equivalent to S512A and T521A were reported to reduce the Kd for hCG binding whereas mutations equivalent to F515A and V519A were reported to have no effect on hCG binding affinity.
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A representative experiment documenting the ability of cells expressing
each of these four mutants to respond to hCG with increases in cAMP
accumulation is shown in Fig. 3 and a
summary of several experiments is presented in Table
III. As pointed out before (17), there is
some inherent variability associated with measuring the cAMP responses
of transiently transfected cells and we attempted to correct for it by
normalizing the basal and hCG-induced cAMP responses mediated by the
different mutants to the cholera toxin-induced cAMP response measured
in the same cells (see columns labeled basal/cholera toxin and
hCG/cholera toxin in Table III). Thus, instead of using the absolute
levels of cAMP, the responsiveness of cells expressing the different
mutants shown in Table III is best compared by using these ratios.
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The results presented in Table III show that the basal levels of cAMP
were normal in cells transfected with any of the four mutants selected
(see basal/cholera toxin response ratio in Table III). These results
also show that the two mutants that lengthen the
t1/2 of internalization (S512A and V519A,
cf. Table I) display a rightward shift in the
EC50 whereas the two mutants that shorten the
t1/2 of internalization (F515A and T521A) display a
leftward shift in the EC50. The maximal cAMP response to
hCG was also reduced in cells expressing the F515A or T521A mutants
(see hCG/cholera toxin response ratio in Table III). These results are
also somewhat different than those reported for mutations of the rLHR
(36). The four equivalent mutations of the rLHR mutations were reported to increase the EC50 and to have little or no effect on the
maximal cAMP response.
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DISCUSSION |
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Studies performed with rhodopsin as well as the adrenergic and
muscarinic receptors have shown that the GRK-catalyzed phosphorylation of these GPCRs is dependent on receptor activation (1-3) and that
arrestin binding is in turn exquisitely dependent on the GRK-catalyzed
phosphorylation of these GPCRs (37-39). One would thus predict that
the agonist-induced internalization of GPCRs that are internalized by a
nonvisual arrestin-dependent pathway would also be
dependent on receptor activation and phosphorylation. Such dependence
can be readily demonstrated with the 2-adrenergic (
2AR) and the µ-opioid receptors. Thus, mutation of
the phosphorylation sites of the
2AR inhibits
agonist-induced internalization 2-3-fold and an activation impaired
mutant (Y326A in the transmembrane helix-7) inhibits agonist-induced
internalization 3-6-fold (40, 41). In transfected cells, the
agonist-induced phosphorylation and internalization of
2AR-Y326A are decreased by ~90% but they can both be
rescued by cotransfection with GRK2, provided that the phosphorylation
sites are intact (41). These results suggest that the agonist-provoked
phosphorylation of the
2AR is more important than
receptor activation in the process of internalization. More recent
studies (42, 43) have also nicely documented the importance of
phosphorylation on the agonist-induced internalization of the
µ-opioid receptor. When expressed in transfected cells, the
µ-opioid receptor becomes phosphorylated and internalized upon
exposure to etorphine, but not upon exposure to morphine even though
both compounds are full agonists that can activate the µ-opioid
receptor to the same extent. In this case, again, the phosphorylation
and internalization of the morphine-activated µ-opioid receptor can
be induced by cotransfection with GRK2.
Unlike the studies summarized above, however, our recent work with the hLHR suggests that the agonist-induced activation and the phosphorylation of this GPCR play a redundant role in internalization (17). This conclusion was based on the following findings. First, an activation-competent but phosphorylation-impaired mutant of the hLHR (constructed by mutation of the phosphorylation sites) lengthens the t1/2 of internalization of the hLHR less than 2-fold. Second, three different activation-impaired mutants of the hLHR with intact phosphorylation sites are resistant to agonist-induced phosphorylation and lengthen the t1/2 of internalization 5-7-fold. Third, the already long t1/2 of internalization of these activation-impaired mutants cannot be further lengthened by mutation of their phosphorylation sites. Fourth, the phosphorylation of these activation-impaired mutants and the long t1/2 of internalization can be rescued by overexpression of GRK2 but only if the phosphorylation sites are intact. It appears then that, although the agonist-induced internalization of the hLHR is readily impaired by mutations that impair both activation and phosphorylation, the agonist-induced internalization of the LHR is minimally impaired when activation is intact and phosphorylation is impaired or when activation is impaired and phosphorylation is intact.
The studies presented herein characterize a novel class of mutations of
second extracellular loop residues of the hLHR that continue to
emphasize a remarkable correlation between the state of activation of
the LHR and the rate of agonist internalization (17, 44-46) but
de-emphasize the importance of receptor phosphorylation in this
process. We show that two mutations of the second extracellular loop
that impair receptor activation (S512A and the V519A) also impair
internalization (Table I and III). In contrast to all previously described signaling-impaired mutations that have been studied in detail
(D405N, Y546F, and I625K, all of which are in the transmembrane helices; see Ref. 17), the S512A and V519A mutants reported here are
the first signaling-impairing mutations of the hLHR that impair
internalization without affecting receptor phosphorylation (Fig. 2).
The differential effect of these two classes of mutations on receptor
phosphorylation may be related to the extent of impairment in
signaling. The second extracellular loop mutants result in only a
2-3-fold rightward shift in the EC50 for cAMP accumulation (see Table III), whereas the previously examined transmembrane mutants
result in a 10-50-fold rightward shift in this EC50 (17, 47). Despite the milder effects of the extracellular loop mutants on
activation and the lack of effect on phosphorylation, the extent of
impairment in internalization induced by them is similar to that
detected with the transmembrane mutants that induce a more drastic
change in activation and block phosphorylation (see Table I and Ref.
17). The slow rate of internalization of both sets of mutants also
becomes insensitive to inhibition by a dominant-negative mutant of the
nonvisual arrestins but can be rescued by overexpression of arrestin-3
(Table I and Ref. 17). Previous studies have also shown that several
distinct mutations of the rLHR or the hLHR (L435R, D578G, D578Y, and
D578H, all of which are located in the transmembrane helices) that
induce constitutive activity also enhance the internalization of hCG in
transfected cells (17, 46, 48). The results presented here show, for
the first time, that two mutations of the second extracellular loop
that have no effect on basal cAMP levels but enhance receptor
activation by hCG (F515A and T521A) also enhance internalization
(Tables I and III). As with the signaling-impairing mutations, the
increased internalization of hCG mediated by the F515A and T521A
mutants occurs independently of changes in receptor phosphorylation
(Fig. 2). Based on these results, we must now conclude that, in
contrast to the model derived from the study of the 2-AR
and the µ-opioid receptors (see above), for the hLHR the
agonist-induced activation is more important than phosphorylation in
the process of internalization of this receptor. A similar conclusion
may be drawn from recent studies on the FSHR and the parathyroid
hormone receptor. In the case of the FSHR, it has been shown that the
agonist-induced phosphorylation of a signaling-impaired mutation can be
rescued by cotransfection with GRK2 but such treatment does not rescue
the rate of internalization of agonist (49). In the case of the
parathyroid hormone receptor, it has been shown that the
agonist-induced internalization of this receptor is not affected by
mutation of its phosphorylation sites (50).
The emphasis that these results have placed on receptor activation (as
opposed to phosphorylation) on the agonist-induced internalization of
the hLHR agrees well with recent studies showing that the mutation (17)
or removal of the phosphorylation sites of the hLHR have a relatively
small effect on internalization (16). They also complement the finding
that the rate of internalization of hCG mediated by the hLHR can be
drastically reduced by mutation of a few intracellular residues that do
not become phosphorylated upon agonist stimulation (16). Finally,
whereas we have shown previously that extracellular residues can have a
dramatic effect on the rate of internalization of the LHR without
affecting receptor activation (18), the studies presented here
highlight the importance of a discrete extracellular region of the LHR,
the second extracellular loop, in modulating the agonist-induced
internalization of the hLHR. As documented here the involvement of this
extracellular region in the process of internalization seems to be
indirectly mediated by changes in receptor activation. The importance
of this region is underscored by the finding that the mutation of just
about every residue in this loop either enhances or impairs internalization (Fig. 1B).
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ACKNOWLEDGEMENTS |
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We thank Dr. Deborah L. Segaloff for critically reading this manuscript, Dr. Jeff Benovic for providing us with the expression vectors for arrestin-3 and arrestin-3-(284-409), and Dr. Sandra Schmid for providing us with the expression vector for dynamin-K44A. We also thank Ares Serono for providing us with a plasmid coding for the hLHR. We gratefully acknowledge the services and facilities provided by the Diabetes and Endocrinology Research Center of the University of Iowa (supported by National Institutes Health Grant DK-25295).
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FOOTNOTES |
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* This work was supported by National Institutes of Health Grant CA-40629 (to M. A.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
To whom correspondence should be addressed: Dept. of Pharmacology,
University of Iowa, 2-319B BSB, 51 Newton Rd., Iowa City, IA
52242-1109. Tel.: 319-335-9907; Fax: 319-335-8930; E-mail: mario-ascoli@uiowa.edu.
Published, JBC Papers in Press, December 15, 2000, DOI 10.1074/jbc.M010482200
2 Note that the amino acid sequences of the second extracellular loops of the rLHR and the hLHR are identical except for two residues (Leu493 and Ser499 in the rLHR are replaced by Phe515 and Thr521 in the hLHR). Additionally, note that the different numbers assigned to equivalent residues is artificially caused by differences in the numbering of amino acids. Since the N terminus of the mature hLHR is not known, residue number 1 is taken to the be the methionine present at the N terminus of the signal peptide. In contrast, since the N terminus of the mature rLHR is known, this residue (which corresponds to residue 23 of the hLHR) is taken to be residue number 1.
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ABBREVIATIONS |
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The abbreviations used are:
GPCR, G
protein-coupled receptor;
GRK, G protein-coupled receptor kinase;
FSHR, follitropin receptor;
LHR, lutropin receptor;
TSHR, thyrotropin
receptor;
CG, chorionic gonadotropin;
wt, wild type;
2AR,
2-adrenergic receptor.
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