(Received for publication, August 13, 1996, and in revised form, December 20, 1996)
From the Département de Pharmacologie, Faculté de Médecine, Université de Sherbrooke, Sherbrooke, Québec J1H 5N4, Canada
To identify binding domains between angiotensin II (AngII) and its type 2 receptor (AT2), two different radiolabeled photoreactive analogs were prepared by replacing either the first or the last amino acid in the peptide with p-benzoyl-L-phenylalanine (Bpa). Digestion of photolabeled receptors with kallikrein revealed that the two photoreactive analogs label the amino-terminal part of the receptor within the first 182 amino acids. Digestion of 125I-[Bpa1]AngII·AT2 receptor complex with endoproteinase Lys-C produced a glycoprotein of 80 kDa. Deglycosylation of this 80-kDa product decreased its apparent molecular mass to 4.6 kDa and further cleavage of this 4.6-kDa product with V8 protease decreased its molecular mass to 3.6 kDa, circumscribing the labeling site of 125I-[Bpa1]AngII within amino acids 3-30 of AT2 receptor. Treatment of 125I-[Bpa8]AngII·AT2 receptor complex with cyanogen bromide produced two major receptor fragments of 3.6 and 2.6 kDa. Cyanogen bromide hydrolysis of a mutant AT2 receptor produced two major fragments of 12.6 kDa and 2.6 kDa defining the labeling site of 125I-[Bpa8]AngII within residues 129-138 of AT2 receptor. Our results indicate that the amino-terminal tail of the AT2 receptor interacts with the amino-terminal end of AngII, whereas the inner half of the third transmembrane domain of AT2 receptor interacts with the carboxyl-terminal end of AngII.
The octapeptide angiotensin II (AngII)1 recognizes two distinct types of receptors on target cells: the type 1 receptor (AT1) and the type 2 receptor (AT2). The AT1 receptor mediates all the known physiological actions of AngII including regulation of blood pressure and water and electrolyte balance (1). The functional roles of the AT2 receptor are not well defined yet but recent studies suggest that it could act as a physiological antagonist of AT1-mediated pressor effect and also regulate central nervous system functions related to locomotion and exploratory behavior (2, 3). Other studies also suggest that the AT2 receptor inhibits cell proliferation and induces cell death (4, 5). AT1 and AT2 receptors have been cloned from several species. They are members of the G protein-coupled receptor superfamily, which is characterized by seven putative transmembrane helices. AT1 and AT2 receptors display a low degree (33%) of amino acid sequence similarity (6-9).
The elucidation of primary structures of numerous G protein-coupled
receptors has prompted investigators to look for and identify domains
in receptors directly involved in ligand binding. Most of this work was
done on members of the -adrenergic receptor family which bind
bioamines (<0.2 kDa) in the outer third of the plasma membrane between
transmembrane helices (10). Ligand-binding domains have also been
identified for larger agonist (>10 kDa) such as thyroid-stimulating
hormone and follicule-stimulating hormone which bind to the
extracellular amino-terminal region of their G protein-coupled
receptors (11). The ligand-binding domains of G protein-coupled
receptors for small bioactive peptides (0.5-5 kDa) have not been fully
characterized. Recent studies suggest, however, that certain proximal
loop regions as well as transmembrane regions may be important binding
determinants (12-16).
The localization of ligand-binding domains in the G protein-coupled receptor family has been mostly studied using approaches such as, site-directed mutagenesis, deletion analysis, and construction of chimeric receptors (17). Since these mutations may affect hormone binding indirectly by altering the conformation of a receptor or its expression at the plasma membrane, a more direct approach for the identification of the AT2 receptor ligand-binding domains should be envisaged. We previously reported the covalent labeling of the AT2 receptor with the photoreactive AngII analog [Bpa8]AngII (18-20). In the present study, another high affinity photoreactive analog was prepared by replacing the amino-terminal end of AngII with Bpa. These two photoreactive AngII analogs were used to label the AT2 receptor of PC-12 cells. The peptide-binding domains of the receptor were identified with each ligand after targetted enzymatic and chemical fragmentation.
Bovine serum albumin, bacitracin, soybean trypsin inhibitor, and cyanogen bromide (CNBr) were from Sigma. L-158,809 and PD 123319 were generous gifts from Merck and Parke-Davis Warner-Lambert, respectively. Glycopeptidase-F (PNGase-F) (EC 3.5.1.52), V8 protease (EC 3.4.21.19), endroproteinase Lys-C (endo Lys-C) (EC 3.4.21.50), and tissue kallikrein (EC 3.4.21.35) were from Boehringer Mannheim. The cDNA clone of the rat AT2 receptor subcloned in the mammalian expression vector pcDNA1 was kindly provided by Dr. K. J. Catt (National Institutes of Health, Bethesda, MD). Lipofectamine and culture media were obtained from Life Technologies, Inc. [Bpa1]AngII and [Bpa8]AngII were synthesized in our laboratories by the solid phase method and purified by high performance liquid chromatography as described (21). 125I-AngII, 125I-[Bpa1]AngII, and 125I-[Bpa8]AngII (specific radioactivities ~1000 Ci/mmol) were prepared with IODO-GEN as described by Fraker and Speck (22). Briefly, 50 µl of the peptide solution (0.2 mM) was incubated with 5 µg of IODO-GEN (Pierce Chemical Co.), 1 mM Na125I (2200 Ci/mmol), 10 µl of acetic acid (2 M), and 30 µl of water for 30 min at room temperature. The labeled peptides were purified by high performance liquid chromatography on a C-18 column (10 µm) (Allteck Associates Inc.; number 29004) with a 20-40% acetonitrile gradient. The specific radioactivity of the labeled hormones was determined by self-displacement and saturation binding analysis.
Site-directed MutagenesispcDNA1 containing the rat AT2 receptor cDNA clone was digested with HindIII and XbaI endonucleases and cloned into M13mp18 also digested with HindIII and XbaI. The codon change in the rat AT2 cDNA was made by site-directed mutagenesis using an in vitro mutagenesis kit (Sculptor, Amersham). One oligonucleotide was constructed to induce a mutation at methionine 116. The mutagenic primer is listed (altered nucleotide is underlined): methionine 116 to leucine (rAT2M116L): 5-GGCTCTTTGGACCTGTGTGTGCAAAGTGT-3. After confirmation of site-directed mutation by DNA sequencing, the rAT2M116L gene was excised from the M13mp18RF form by digestion with HindIII and XbaI and subcloned into the multiple cloning site of pcDNA3 that had been digested by these same restriction enzymes.
Transfection of COS-7 CellsCOS-7 cells were grown in Dulbecco's modified Eagle's medium (DMEM) containing 2 mM L-glutamine and 10% (v/v) fetal bovine serum. Cells were seeded into 75-cm2 culture dishes at a density of 25,000 cells/cm2. When cells reached ~50% confluency, they were washed once with serum-free DMEM and transfected with 4 µg of plasmid DNA and 30 µl of Lipofectamine in 8 ml of serum-free DMEM. The cells were incubated for 5 h at 37 °C and the media were replaced with a complete DMEM medium containing 100 IU/ml penicillin and 100 µg/ml streptomycin. Transfected cells were grown for 48-72 h before photoaffinity labeling and binding assays.
Cell CulturePC-12 cells between the second and the fifteenth passages were used in all experiments. Cells were seeded into 75-cm2 culture dishes at a density of 25,000 cells/cm2 and grown in DMEM with 10% (v/v) fetal bovine serum, 50 IU/ml penicillin, 60 µg/ml streptomycin, and 2 mM L-glutamine. Cells were kept in culture at 37 °C, in a CO2 incubator, and the medium was changed daily or every other day depending on the state of confluency.
Binding AssaysCell membrane preparation and binding assays were performed as described previously (19, 20).
Photoaffinity LabelingTransfected COS-7 cells and PC-12
cells (~1 × 107) were incubated in 4 ml of binding
medium containing the photoreactive radioligands (6 nM), in
the presence of L-158,809 (1 µM) (an AT1
selective non peptide analog). After 45 min at room temperature, cells
were washed with 20 ml of ice-cold binding medium (without bovine serum albumin) and irradiated for 60 min at 0 °C under filtered UV light (365 nm) (mercury vapor lamp serial number JC-Par-38 from Westinghouse and Raymaster black light filters number 5873 from Gates and Co. Inc.,
Long Island, NY). Cells were then gently scraped with a rubber
policeman and centrifuged (200 × g) for 10 min at
4 °C. The pellet was solubilized in a buffer containing 100 mM Na2HPO4, pH 8.5, 25 mM EDTA, 0.1 mg/ml soybean trypsin inhibitor, and 1% (v/v)
Nonidet P-40. After centrifugation (13,000 × g for 10 min at 4 °C), the supernatants were kept at 80 °C until further
analysis.
The solubilized
photolabeled receptors were diluted with an equal volume of 2 × loading buffer (120 mM Tris-HCl, pH 6.8, 20% (v/v)
glycerol, 4% (w/v) SDS, 200 mM dithiothreitol, and 0.05% (w/v) bromphenol blue) and incubated for 2 h at 37 °C.
SDS-polyacrylamide gel electrophoresis (PAGE) was performed as
described by Laemmli (23) using 1.5-mm 8% gels. The gel was then dried
and exposed to x-ray film (Kodak XAR-5) with an intensifying screen.
The labeled proteins were isolated from the preparative gel using a
passive elution protocol similar to that described by Blanton and Cohen (24). After autoradiography, radioactive bands were excised from dried
gels and rehydrated with appropriate digestion buffer. The gel slices
were macerated and eluted with 2 ml of buffer for 3-4 days at 4 °C
under gentle agitation. Under these conditions we repeatedly recovered
at least 80% of the initial radioactivity. The eluted proteins were
filtered (Acrodisc 0.22 µm; Gelman Sciences) and the gel slices were
washed with 10 ml of digestion buffer. The whole eluate (12-15 ml) was
concentrated approximately 150 times using Centriprep-10 and
Centricon-10 (Amicon) and the partially purified proteins were
aliquoted in fractions of 2-4 × 105 cpm,
lyophilized, and kept at 80 °C.
Partially purified photolabeled receptors were resuspended in digestion buffers containing 0.2% (v/v) Nonidet P-40. PNGase-F (33-100 units/ml) was added and samples were incubated for different periods of time as indicated.
Protease DigestionThe partially purified photolabeled receptors (10,000-300,000 cpm) were resuspended in 25 µl of digestion buffer containing 100 mM NH4HCO3, pH 8.0, and 0.1% (w/v) SDS. Under these conditions V8 protease is known to cleave at the carboxyl-terminal side of glutamate residues. Samples were incubated for 3-4 days at room temperature with the indicated amounts of V8 protease. Partially purified photolabeled receptors (10,000-300,000 cpm) were also digested for 18-24 h at 37 °C with indicated amounts of endo Lys-C in 25 µl of digestion buffer containing 25 mM Tris-HCl, pH 8.5, 1 mM EDTA, and 0.1% (w/v) SDS. For kallikrein digestions, photolabeled receptors (~20-50 µg of proteins) were resuspended in 25 µl-50 µl of digestion buffer containing 100 mM Na2HPO4, pH 8.5, 25 mM EDTA, 1 mg/ml soybean trypsin inhibitor, 0.1 mg/ml L-1-chloro-3-(4-tosylamido)-4-phenyl-2-butanon and 0.2% (v/v) Nonidet P-40. Indicated amounts of kallikrein were added and samples were incubated for 2 h at 37 °C. All digestions were terminated by adding an equal volume of 2 × loading buffer (previously described) and boiling the samples for 3 min. When subsequent digestions were needed, products of the first digestion were identified by SDS-PAGE and autoradiography, and recovered from non-fixed gels by passive elution for 12-18 h at 37 °C with 200-400 µl of digestion buffer. Extracted proteins were lyophilized and submitted to chemical or enzymic digestion as described.
Chemical DigestionFor CNBr hydrolysis, partially purified photolabeled receptors (60,000-450,000 cpm) were resuspended in 50 µl of 70% (v/v) trifluoroacetic acid and CNBr (50 µl) was added to a final concentration of 100 mg/ml. Samples were incubated at room temperature, in the dark, for 24-36 h. Reactions were terminated by adding 500 µl of water. Samples were lyophilized, resuspended in denaturating buffer, and analyzed by SDS-PAGE.
Analysis of Products of Proteolysis and Chemical CleavageThe products of proteolysis and chemical cleavage were analyzed by SDS-PAGE using 16.5% acrylamide Tris-Tricine gels (Bio-Rad) followed by autoradiography on x-ray films (Kodak XAR-5). 14C-Labeled low molecular protein standards (Life Technologies, Inc.) were used to determine apparent molecular masses. Running conditions, fixation, and coloration of gels were performed according to the manufacturer's instructions.
Fig. 1 shows the primary
structures of AngII and photoreactive AngII analogs used in this study.
Asp1 and Phe8 were, respectively, replaced by
Bpa to give [Bpa1]AngII and [Bpa8]AngII. In
competitive binding assays, [Bpa1]AngII and
[Bpa8]AngII exhibited high affinities for AT2
receptor of PC-12 cells with respective IC50 values of
1.07 ± 0.38 and 0.37 ± 0.21 nM, comparable to
that of AngII (0.40 ± 0.10 nM) (mean ± S.D. of
three experiments). In photoaffinity labeling experiments (Fig.
2), 125I-[Bpa1]AngII
(lane 1) and 125I-[Bpa8]AngII
(lane 4) specifically labeled the AT2 receptor
which migrated as a glycoprotein of 140 kDa as described previously
(19, 20). The labeling of the AT2 receptor by the two
photoreactive analogs was completely abolished by PD123319 (10 µM) (an AT2 receptor selective ligand)
(lanes 2 and 5) and by AngII (1 µM)
(lanes 3 and 6) thereby confirming the
specificity and the selectivity of the labeling. Although both
photoreactive analogs successfully labeled the AT2
receptor, determination of covalent incorporation yields (calculated
from the ratio of total radioactivity found in isolated bands to total
specific binding observed before photolysis) revealed that
125I-[Bpa1]AngII, with a ~10% yield of
covalent incorporation, was approximately six times less effective than
125I-[Bpa8]AngII (~60% yield of covalent
incorporation). These differences are well illustrated in Fig. 2 where
the intensity of 125I-[Bpa1]AngII labeling
(lane 1) is clearly weaker than that of
125I-[Bpa8]AngII labeling (lane
4). It indicates that 125I-[Bpa1]AngII
and 125I-[Bpa8]AngII interact distinctly with
the AT2 receptor.
125I-[Bpa1]AngII and 125I-[Bpa8]AngII Label the Amino-terminal Portion of the AT2 Receptor
Tissue kallikrein is a
serine protease that cleaves after phenylalanine-arginine or
leucine-arginine amino acid combinations. The AT2 receptor
contains only four of these amino acid combinations, including one
located precisely halfway in the receptor molecule at arginine 182. The
three other combinations are located in the carboxyl-terminal end of
the molecule at arginines 330, 334, and 356 (Fig. 3).
Kallikrein-treated
125I-[Bpa8]AngII·AT2 complex
migrated with a molecular mass of 70 kDa (Fig. 4a,
lane 1). This receptor fragment was the final digestion product since prolonged incubation with kallikrein (Fig. 4a, lanes 3 and 5) did not reveal any lower molecular weight fragment.
As shown in Fig. 3, the AT2 receptor is
N-glycosylated exclusively in its amino-terminal
extracellular tail (6, 7). The relatively high molecular mass of the
70-kDa digestion product and its glycoprotein-like migration behavior
(broad band) suggest that it corresponds to the labeled 1-182 fragment
of the AT2 receptor. To confirm the location of the 70-kDa
receptor fragment, kallikrein-treated
125I-[Bpa8]AngII·AT2 complex
was deglycosylated with PNGase-F. Under these conditions a labeled
fragment of 18 kDa was produced (Fig. 4b, lane 5). This
deglycosylation fragment exhibited about half the size of the
nonkallikrein-treated deglycosylated AT2 receptor (molecular mass of 35 kDa) (lane 3). Identical results were
obtained with AT2 receptor labeled with
125I-[Bpa1]AngII. Together, these results
show that 125I-[Bpa1]AngII and
125I-[Bpa8]AngII are labeling sites within
the first 182 residues of the AT2 receptor.
Mapping the 125I-[Bpa1]AngII Binding Domain
After photolabeling with
125I-[Bpa1]AngII, AT2 receptor
was partially purified and digested with endo Lys-C which cleaves on
the carboxyl-terminal side of lysine residues. The digestion product migrated as a broad band of ~80 kDa (Fig. 5a,
lane 2). Again the high Mr and the
broadness of the band suggested a glycoprotein nature. After extraction
of the endo Lys-C digestion product, treatment with PNGase-F resulted
in a 4.6-kDa fragment which migrated as a sharp band suggesting that it
was completely deglycosylated (Fig. 5c, lane 2).
Interestingly, similar results were obtained with a simplified protocol
where the photolabeled AT2 receptor was simultaneously
digested with endo Lys-C and PNGase-F (Fig. 5b, lane 2).
Since as previously mentioned, the AT2 receptor is N-glycosylated exclusively in its amino-terminal ectodomain,
the cleavage probably occurred at one of two lysine residues, located at positions 38 and 42 (Fig. 3). Cleavage at these residues should produce either a 5.2-kDa fragment (including the photolabel of 1.3 kDa)
that corresponds to the labeled 3-38 peptide or a 5.6-kDa fragment
that corresponds to the labeled 3-42 peptide.
To further define the
125I-[Bpa1]AngII-binding domain, the 4.6-kDa
fragment obtained after co-digestion with endo Lys-C and PNGase-F was
submitted to digestion with V8 protease. Fig. 6,
lane 3, shows that, under these conditions, the 4.6-kDa
fragment was converted to a 3.6-kDa fragment. The only site of cleavage
for V8 protease within amino acids 3-42 is after glutamate 30 (Fig.
3). Cleavage of the 4.6-kDa fragment at this site should produce either
a labeled 2.6-kDa fragment (31-42 peptide + photolabel), a labeled
2.2-kDa fragment (31-38 peptide + photolabel), or a labeled 4.3-kDa
fragment (3-30 peptide + photolabel) (Fig. 3). Based on the relatively high molecular mass of the digestion product (3.6 kDa), the 3-30 peptide is clearly a better candidate than the 31-42 or 31-38 peptides for the binding domain of
125I-[Bpa1]AngII. This conclusion is further
strengthened by experiments in which photolabeled-AT2
receptor was co-digested with PNGase-F and V8 protease. Under these
conditions, a 4.1-kDa fragment was obtained, locating the binding
domain within the first 30 amino acids of the AT2 receptor
(results not shown). Together these results show that
125I-[Bpa1]AngII is labeling a site within
residues 3-30 of the extracellular amino-terminal tail of the
AT2 receptor (Fig. 10).
Mapping the 125I-[Bpa8]AngII-binding Domain
125I-[Bpa8]AngII-photolabeled
AT2 receptor was partially purified and digested with V8
protease. The patterns of fragmentation were clearly distinct from
those obtained with
125I-[Bpa1]AngII·AT2 complex.
Digestion with V8 protease produced 28- and 15.2-kDa fragments (Fig.
7, lane 2). Deglycosylation had no effect on
the mobility of the 15.2-kDa fragment indicating that it was not a
glycosylated fragment (result not shown). Knowing that the binding
domain of 125I-[Bpa8]AngII is located within
amino acids 1-182 (Fig. 4) and that the 15.2-kDa digestion product is
not glycosylated, the fragment located between alanine 46 and glutamate
188 (estimated molecular mass of the peptide + photolabel: 17.8 kDa) is
the best candidate for the binding domain of
125I-[Bpa8]AngII (Fig. 3). Prolonged
incubations in the presence of PNGase-F reduced the proportion of the
28-kDa fragment and increased the proportion of a fragment migrating
close to the 15.2-kDa fragment, suggesting that the 28-kDa fragment is
the glycosylated 31-188 peptide of AT2 receptor,
containing a putative site of glycosylation at asparagine 34 (Fig.
3).
To further define the
125I-[Bpa8]AngII-binding domain, photolabeled
AT2 receptor was submitted to hydrolysis with CNBr which
cleaves specifically at the carboxyl-terminal side of methionine
residues. Fig. 8, lane 2, shows that, under
these conditions, two major digestion products of 3.6 and 2.6 kDa were
obtained. These results suggest that the photolabel binds to 117-138
peptide (estimated molecular mass of 3.7 kDa, including the photolabel)
in the third transmembrane domain of AT2. Hydrolysis at
methionine 128 of this peptide would produce two fragments of
comparable apparent sizes (estimated size of 2.6 and 2.4 kDa for
117-128 peptide and 129-138 peptide, respectively). To confirm this
hypothesis and to identify the 2.6-kDa fragment, we generated a mutant
of the rat AT2 receptor in which methionine 116 was
replaced by a leucine, therefore abolishing that putative CNBr cleavage
site. In competitive binding assays, AngII exhibited a similar affinity
for wild-type (IC50 of 0.34 ± 0.03 nM;
mean ± S.D. of three experiments) and mutant
rAT2M116L (0.37 ± 0.17 nM) receptors
expressed in COS-7 cells. [Bpa8]AngII displayed also
comparable binding affinities for the wild-type (0.31 ± 0.26 nM) and the mutant (0.23 ± 0.15 nM)
receptors. The wild-type and the mutant rAT2M116L receptors
were photolabeled with 125I-[Bpa8] AngII,
partially purified, and submitted to hydrolysis by CNBr. Fig.
9, lane 1, shows that CNBr hydrolysis of the
wild-type AT2 receptor produced the previously described
3.6- and 2.6-kDa fragments. CNBr hydrolysis of the
rAT2M116L receptor still produced the 2.6-kDa fragment and
a longer 12.6-kDa fragment (Fig. 9, lane 3). Under these
conditions the only possibility is that the 2.6-kDa fragment is the
129-138 peptide located in the inner half of the third transmembrane
domain of mutant AT2 receptor. Indeed, if labeling had
occurred between leucine 116 and methionine 128 in the mutant receptor,
exclusively higher molecular mass labeled receptor fragments (13-15
kDa) would have been produced. The 12.6-kDa fragment produced by CNBr
hydrolysis of rAT2M116L most probably corresponds to the mutant receptor 54-138 peptide (estimated molecular mass of the peptide + photolabel: 10.9 kDa). The incomplete CNBr hydrolysis of both
native and mutant receptors suggest that cleavage at methionine 128 is
impaired. This may result from reduced solubility of the protein in
strong dissociating agents (like trifluoroacetic acid), sterical
masking of the methionine residue by surrounding amino acids, or
oxidation of the methionine residue occurring during protein
manipulations and/or acid hydrolysis (25). Together these results show
that 125I-[Bpa8]AngII is labeling a site
within residues 129-138 of AT2 receptor (Fig.
10).
Our results indicate that, upon binding, the amino-terminal tail of the AT2 receptor interacts with the amino-terminal end of AngII whereas the inner half of the third transmembrane domain of AT2 receptor interacts with the carboxyl-terminal end of AngII. To our knowledge, this is the first study providing data on the binding domains of the AT2 receptor. Our results and those of other groups suggest that there may be a common scheme for the binding domains of small peptide hormones in the G-protein-coupled receptor superfamily. For example, it has been shown that charged residues in the second and third extracellular loops of the AT1 receptor are major docking points for the amino-terminal end (Asp1 and Arg2 residues, respectively) of AngII (13, 26). On the other hand, the hydrophobic nature of the phenyl group at position 8 in the AngII molecule (the carboxyl-terminal end of the peptide) suggests that its site of interaction with the AT1 receptor is within the membrane. Actually, recent studies suggest that upon binding, the carboxyl-terminal end of the AngII molecule occupies a space between helices III, V, VI, and VII of the AT1 receptor (26, 27). Similarly, binding domains for the undecapeptide substance-P (SP) were identified in the first and second extracellular loops, the amino-terminal ectodomain and the outer half of the transmembrane helices II and VII of the NK-1 receptor (14, 15, 28). In an elegant study using photoreactive analogs of SP, Li et al. (15) have shown that the amino-terminal side (the fourth position) of the 125I-[Tyr1,Bpa4]SP peptide interacts with the extracellular amino-terminal tail of the NK-1 receptor while the COOH-terminal side (the ninth position) of the 125I-[Tyr1,Bpa9]SP peptide interacts with the second extracellular loop. The authors proposed a model in which the carboxyl-terminal end of the peptide positions itself between helices in the outer part of the plasma membrane whereas the amino-terminal portion of the peptide is stabilized by ectodomains of NK-1 receptor. Based on our results, it is tempting to propose that the AngII molecule binds the AT2 receptor in a similar fashion, with the carboxyl-terminal end sitting deep within the transmembrane domains and the amino-terminal end interacting with the ectodomains of the receptor. The location of the carboxyl-terminal portion of AngII deep in the plasma membrane fits well with known pharmacological properties of AngII. The agonistic nature of AngII is conferred by its carboxyl-terminal phenylalanine residue (29) and intrahelical amino acids are known to play a major role in G-protein coupled receptor activation upon direct interaction with ligands (10). Our results also suggest that the carboxyl-terminal end of AngII interacts with the third transmembrane domain of the AT2 receptor. Similarly, it has been shown that residues in the third transmembrane domain of the AT1 receptor are required for high affinity binding (16, 30). Our results also raise another interesting point. If the AngII molecule interacts at the same time with residues in the amino-terminal extracellular tail as well as residues deeply located in the third transmembrane domain, one could speculate that the ectodomain must lie near the outer membrane surface. The AngII molecule has an estimated length of about ~30 Å (probably less in solution) which is shorter than a transmembrane helix (~40 Å). To account for such a requirement, the putative disulfide bridge located between the amino-terminal tail and the third extracellular loop of the AT2 receptor may play an important conformational role in bringing the amino-terminal tail in close proximity to the plasma membrane surface (Fig. 10).
In conclusion, we have identified two peptide-binding regions in the AT2 receptor with the use of highly potent and specific AngII photoreactive analogs. This approach also allowed the determination of the AngII molecule's orientation in its binding pocket. Based on these results, we conclude that AngII interacts with an extracellular segment and a transmembrane helix of AT2 receptor. This interaction pattern, also found in other G-protein coupled receptors for small bioactive peptides, may correspond to a highly conserved feature among this very large family or receptors. By recovering large amounts of labeled receptor, it will be possible to sequence the fragments and pinpoint the precise interaction sites.