3D model for TM region of the AT-1 receptor in complex with angiotensin II independently validated by site-directed mutagenesis data

Structural determinants for binding, activation, and functional selectivity of the angiotensin AT1 receptor

The renin-angiotensin system (RAS) plays an important role in the pathophysiology of cardiovascular disorders. Pharmacologic interventions targeting the RAS cascade have led to the discovery of renin inhibitors, angiotensin-converting enzyme inhibitors, and AT(1) receptor blockers (ARBs) to treat hypertension and some cardiovascular and renal disorders. Mutagenesis and modeling studies have revealed that differential functional outcomes are the results of multiple active states conformed by the AT(1) receptor upon interaction with angiotensin II (Ang II). The binding of agonist is dependent on both extracellular and intramembrane regions of the receptor molecule, and as a consequence occupies more extensive area of the receptor than a non-peptide antagonist. Both agonist and antagonist bind to the same intramembrane regions to interfere with each other's binding to exhibit competitive, surmountable interaction. The nature of interactions with the amino acids in the receptor is different for each of the ARBs given the small differences in the molecular structure between drugs. AT(1) receptors attain different conformation states after binding various Ang II analogues, resulting in variable responses through activation of multiple signaling pathways. These include both classical and non-classical pathways mediated through growth factor receptor transactivations, and provide cross-communication between downstream signaling molecules. The structural requirements for AT(1) receptors to activate extracellular signal-regulated kinases 1 and 2 through G proteins, or G protein-independently through β-arrestin, are different. We review the structural and functional characteristics of Ang II and its analogs and antagonists, and their interaction with amino acid residues in the AT(1) receptor.

Angiotensin I converting enzyme inhibitory peptides obtained after in vitro hydrolysis of pea (Pisum sativum var. Bajka) globulins

Pea seeds represent a valuable source of active compounds that may positively influence health. In this study, the pea globulins were digested in vitro under gastrointestinal condition and potentially bioaccessible angiotensin I converting enzyme (ACE) inhibitory peptides were identified. The degree of hydrolysis after pepsin, 14.42%, and pancreatin, 30.65%, were noted. The peptides with the highest ACE inhibitory properties were separated using ion exchange chromatography on DEAE-cellulose. Thirteen peptides fractions were obtained but only four showed potential antihypertensive properties. The highest inhibitory activity was determined for the fraction F8 (IC50 = 0.0014 mg/mL). This fraction was separated on Sephadex G10 and two peptide fractions were obtained. The peptides fraction (B) with the highest ACE inhibitory activity (IC50 = 0.073 mg/mL) was identified by ESI-MS/MS. The sequences of ACE inhibitory peptides were GGSGNY, DLKLP, GSSDNR, MRDLK, and HNTPSR. Based on Lineweaver-Burk plots for the fraction B, the kinetic parameters as K m, Vmax, and K i and mode of inhibition were determined. This fraction belongs to uncompetitive inhibitor of ACE activity. The seeds of pea are the source of precursor protein, which releases the ACE inhibitory peptides as a result of enzymatic hydrolysis.

Differential mechanisms of activation of the Ang peptide receptors AT1, AT2, and MAS: using in silico techniques to differentiate the three receptors

The renin-angiotensin system is involved in multiple conditions ranging from cardiovascular disorders to cancer. Components of the pathway, including ACE, renin and angiotensin receptors are targets for disease treatment. This study addresses three receptors of the pathway: AT1, AT2, and MAS and how the receptors are similar and differ in activation by angiotensin peptides. Combining biochemical and amino acid variation data with multiple species sequence alignments, structural models, and docking site predictions allows for visualization of how angiotensin peptides may bind and activate the receptors; allowing identification of conserved and variant mechanisms in the receptors. MAS differs from AT1 favoring Ang-(1–7) and not Ang II binding, while AT2 recently has been suggested to preferentially bind Ang III. A new model of Ang peptide binding to AT1 and AT2 is proposed that correlates data from site directed mutagenesis and photolabled experiments that were previously considered conflicting. Ang II binds AT1 and AT2 through a conserved initial binding mode involving amino acids 111 (consensus 325) of AT1 (Asn) interacting with Tyr (4) of Ang II and 199 and 256 (consensus 512 and 621, a Lys and His respectively) interacting with Phe (8) of Ang II. In MAS these sites are not conserved, leading to differential binding and activation by Ang-(1–7). In both AT1 and AT2, the Ang II peptide may internalize through Phe (8) of Ang II propagating through the receptors’ conserved aromatic amino acids to the final photolabled positioning relative to either AT1 (amino acid 294, Asn, consensus 725) or AT2 (138, Leu, consensus 336). Understanding receptor activation provides valuable information for drug design and identification of other receptors that can potentially bind Ang peptides.

Directed evolution of angiotensin II-inhibiting peptides using a microbead display

Angiotensin II (ang II), an octapeptide (DRVYVHPF), can regulate blood pressure by binding specifically to its receptor, AT1. A peptide (VVIVIY) in the first transmembrane of AT1 has been found, via peptide array technology, to have an affinity for ang II. In this study, the peptide P2, which contained the VVIVIY sequence, was mutated and screened using microbead display technology that utilized emulsion PCR and cell-free protein synthesis. After one round of screening, the binding activities of collected mutants were estimated using flow cytometry and a peptide array. Two of these exhibited improved association rate constants to ang II, compared to the P2 peptide.

Preferential binding of allosteric modulators to active and inactive conformational states of metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) are G protein coupled receptors that play important roles in synaptic plasticity and other neuro-physiological and pathological processes. Allosteric mGluR ligands are particularly promising drug targets because of their modulatory effects--enhancing or suppressing the response of mGluRs to glutamate. The mechanism by which this modulation occurs is not known. Here, we propose the hypothesis that positive and negative modulators will differentially stabilize the active and inactive conformations of the receptors, respectively. To test this hypothesis, we have generated computational models of the transmembrane regions of different mGluR subtypes in two different conformations. The inactive conformation was modeled using the crystal structure of the inactive, dark state of rhodopsin as template and the active conformation was created based on a recent model of the light-activated state of rhodopsin. Ligands for which the nature of their allosteric effects on mGluRs is experimentally known were docked to the modeled mGluR structures using ArgusLab and Autodock softwares. We find that the allosteric ligand binding pockets of mGluRs are overlapping with the retinal binding pocket of rhodopsin, and that ligands have strong preferences for the active and inactive states depending on their modulatory nature. In 8 out of 14 cases (57%), the negative modulators bound the inactive conformations with significant preference using both docking programs, and 6 out of 9 cases (67%), the positive modulators bound the active conformations. Considering results by the individual programs only, even higher correlations were observed: 12/14 (86%) and 8/9 (89%) for ArgusLab and 10/14 (71%) and 7/9 (78%) for AutoDock. These findings strongly support the hypothesis that mGluR allosteric modulation occurs via stabilization of different conformations analogous to those identified in rhodopsin where they are induced by photochemical isomerization of the retinal ligand--despite the extensive differences in sequences between mGluRs and rhodopsin.

Modeling molecular mechanisms of binding of the anaphylatoxin C5a to the C5a receptor

This study presents the 3D model of the complex between the anaphylatoxin C5a and its specific receptor, C5aR. This is the first 3D model of a G-protein-coupled receptor (GPCR) complex with a peptide ligand deduced by a molecular modeling procedure analyzing various conformational possibilities of the extracellular loops and the N-terminal segment of the GPCR. The modeling results indicated two very different ways of interacting between C5a and C5aR at the two interaction sites suggested earlier based on the data of site-directed mutagenesis. Specifically, C5a and C5aR can be involved in "mutual-induced fit", where the interface between the molecules is determined by both the receptor and the ligand. The rigid core of the C5a ligand selects the proper conformations of the highly flexible N-terminal segment of C5aR (the first interaction site). At the same time, the binding conformation of the flexible C-terminal fragment of C5a is selected by well-defined interactions with the TM region of the C5aR receptor (the second interaction site). The proposed 3D model of C5a/C5aR complex was built without direct use of structural constraints derived from site-directed mutagenesis reserving those data for validation of the model. The available data of site-directed mutagenesis of C5a and C5aR were successfully rationalized with the help of the model. Also, the modeling results predicted that the full-length C5a and C5a-des74 metabolite would have different binding modes with C5aR. Modeling approaches employed in this study are readily applicable for studies of molecular mechanisms of binding of other polypeptide ligands to their specific GPCRs.

Photoaffinity labeling and its application in structural biology

This review contains a brief consideration of some theoretical aspects of photoaffinity (photoreactive) labeling (PAL), and the most widely used photoreactive groups, such as arylazide, benzophenone, and 3-(trifluoromethyl)-3-phenyldiazirine, are characterized in comparison. Experimental methodology is described, including modern approaches of mass spectrometry for analysis of cross-linking products between the photoreactive probes and biomolecules. Examples of PAL application in diverse fields of structural biology during the last five-ten years are presented. Potential drug targets, transport processes, stereochemistry of interaction of G-protein-coupled receptors with ligands, as well as structural changes in nicotinic acetylcholine receptor are considered. Applications of photoaffinity ganglioside and phospholipid probes for studying biological membranes and of nucleotide probes in investigations of replicative and transcriptional complexes, as well as photoaffinity glycoconjugates for detecting carbohydrate-binding proteins are covered. In combination with modern techniques of instrumental analysis and computer-aided modeling, PAL remains the most important approach in studies on the organization of biological systems.

Interactions between Conserved Residues in Transmembrane Helices 2 and 7 during Angiotensin AT1Receptor Activation

Site-directed mutagenesis studies and independent molecular modeling studies were combined to investigate the network of inter-residue interactions within the transmembrane region of the angiotensin AT(1a) receptor. Site-directed mutagenesis was focused on residues Tyr292, Asn294, Asn295, and Asn298 in transmembrane helix 7, and the conserved Asp74 in helix 2 and other polar residues. Functional interactions between pairs of residues were evaluated by determining the effects of single and double-reciprocal mutations on agonist-induced AT(1a) receptor activation. Replacement of Tyr292 by aspartate in helix 7 abolished radioligand binding to both Y292D and D74Y/Y292D mutant receptors. Reciprocal mutations of Asp74/Asn294, Ser115/Asn294, Ser252/Asn294, and Asn298/Sen115 caused additive impairment of function, suggesting that these pairs of residues make independent contributions to AT(1a) receptor activation. In contrast, mutations of the Asp74/Tyr298 pair revealed that the D74N/N298D reciprocal mutation substantially increased the impaired inositol phosphate responses of the D74N and N298D receptors. Extensive molecular modeling yielded 3D models of the TM region of the AT(1) receptor and the mutants as well as of their complexes with angiotensin II, which were used to rationalize the possible reasons of impairing of function of some mutants. These data indicate that Asp74 and Asn298 are not optimally positioned for direct strong interaction in the resting conformation of the AT(1a) receptor. Balance of interactions between residues in helix 2 (as D74) and helix 7 (as N294, N295 and N298) in the AT(1) receptors, however, has a crucial role both in determining their functional activity and levels of their expression.

Insight into the binding mode for cyclopentapeptide antagonists of the CXCR4 receptor

The finding that the chemokine receptor CXCR4 is involved in T-cell HIV entry has encouraged the development of antiretroviral drugs targeting this receptor. Additional evidence that CXCR4 plays a crucial role in both angiogenesis and metastasis provides further motivation for the development of a CXCR4 inhibitor for therapeutic applications in oncology. To facilitate the design of such ligands, we have investigated the possible binding modes for cyclopentapeptide CXCR4 antagonists by docking 11 high/medium affinity cyclopentapeptides to a developed three-dimensional model of the CXCR4 G-protein-coupled receptor's transmembrane region. These ligands, expected to bind in the same mode to the receptor, were docked in the previously deduced receptor-bound conformation [Våbenøet al., in press; doi 10.1002/bip.20508]. Ligand-receptor complexes were generated using an automated docking procedure that allowed ligand flexibility. By comparing the resulting ligand poses, only two binding modes common for all 11 compounds were identified. Inspection of these two ligand-receptor complexes identified several CXCR4 contact residues shown by mutation to be interaction sites for ligands and important for HIV gp120 binding. Thus, the results provide further insight into the mechanism by which these cyclopentapeptides block HIV entry as well as a basis for rational design of CXCR4 mutants to map potential contacts with small peptide ligands.

A role for transmembrane domains V and VI in ligand binding and maturation of the angiotensin II AT1 receptor

Several studies have proposed that angiotensin II (Ang II) binds to its receptor AT1 through interactions with residues in helices V and VI, suggesting that the distance between these helices is crucial for ligand binding. Based on a 3D model of AT1 in which the C-terminus of Ang II is docked, we identified the hydrophobic residues of TM V and VI pointing towards the external face of the helices, which may play a role in the structure of the binding pocket and in the structural integrity of the receptor. We performed a systematic mutagenesis study of these residues and examined the binding, localization, maturation, and dimerization of the mutated receptors. We found that mutations of hydrophobic residues to alanine in helix V do not alter binding, whereas mutations to glutamate lead to loss of binding without a loss in cell surface expression, suggesting that the external face of helix V may not directly participate in binding, but may rather contribute to the structure of the binding pocket. In contrast, mutations of hydrophobic residues to glutamate in helix VI lead to a loss in cell surface expression, suggesting that the external surface of helix VI plays a structural role and ensures correct folding of the receptor.

Model of the whole rat AT1 receptor and the ligand-binding site

We present a three-dimensional model of the rat type 1 receptor (AT1) for the hormone angiotensin II (Ang II). Ang II and the AT1 receptor play a critical role in the cell-signaling process responsible for the actions of renin-angiotensin system in the regulation of blood pressure, water-electrolyte homeostasis and cell growth. Development of improved therapeutics would be significantly enhanced with the availability of a 3D-structure model for the AT1 receptor and of the binding site for agonists and antagonists. This model was constructed using a combination of computation and homology-modeling techniques starting with the experimentally determined three-dimensional structure of bovine rhodopsin (PDB#1F88) as a template. All 359 residues and two disulfide bonds in the rat AT1 receptor have been accounted for in this model. Ramachandran-map analysis and a 1 nanosecond molecular dynamics simulation of the solvated receptor with and without the bound ligand, Ang II, lend credence to the validity of the model. Docking calculations were performed with the agonist, Ang II and the antihypertensive antagonist, losartan. [Figure: see text].

Molecular mechanisms of constitutive activity: mutations at position 111 of the angiotensin AT1 receptor

A possible molecular mechanism for the constitutive activity of mutants of the angiotensin type 1 receptor (AT1) at position 111 was suggested by molecular modeling. This involves a cascade of conformational changes in spatial positions of side chains along transmembrane helix (TM3) from L112 to Y113 to F117, which in turn, results in conformational changes in TM4 (residues I152 and M155) leading to the movement of TM4 as a whole. The mechanism is consistent with the available data of site-directed mutagenesis, as well as with correct predictions of constitutive activity of mutants L112F and L112C. It was also predicted that the double mutant N111G/L112A might possess basal constitutive activity comparable with that of the N111G mutant, whereas the double mutants N111G/Y113A, N111G/F117A, and N111G/I152A would have lower levels of basal activity. Experimental studies of the above double mutants showed significant constitutive activity of N111G/L112A and N111G/F117A. The basal activity of N111G/I152A was higher than expected, and that of N111G/Y113A was not determined due to poor expression of the mutant. The proposed mechanism of constitutive activity of the AT(1) receptor reveals a novel nonsimplistic view on the general problem of constitutive activity, and clearly demonstrates the inherent complexity of the process of G protein-coupled receptor (GPCR) activation.

Molecular Dynamics Simulations of Angiotensin II in Aqueous and Dimethyl Sulfoxide Environments

Angiotensin II (Ang II) is an octapeptidic hormone, which plays an important role in the mechanisms of blood pressure control. In this work, extensive molecular dynamics (MD) simulations have been carried out on this peptide, both in aqueous and in dimethyl sulfoxide (DMSO) environments. Experimentally proposed models for the structure of angiotensin II in both environments are not consensual and the results obtained have provided some further insight about the structural properties of this hormone. In these simulations, the N-terminus of Ang II in the aqueous environment has been associated with a considerable larger flexibility than the correspondent C-terminus, but this was not found in the case of the DMSO environment. This is consistent with the assumption that the biological activity of Ang II is associated with its C-terminal residues embedded in a hydrophobic environment of its AT1 receptor. Other features detected in DMSO environment were an H(His6 imidazole)-O(Phe8 carboxylate) hydrogen bond and a salt-bridge structure involving the Asp1 and Arg2 side chains. An additional important conformational feature is the spatial proximity between Tyr4 and His6 in both water and DMSO environments. This molecular feature may trigger the interest for the synthetic chemists to apply rational design for the synthesis of novel AT1 antagonists.

Pharmacophore, Drug Metabolism, and Pharmacokinetics Models on Non-Peptide AT1, AT2, and AT1/AT2Angiotensin II Receptor Antagonists

About 20 non-peptide angiotensin II receptor antagonists are in various stages of clinical development. Different modeling approaches were used to predict the pharmacophoric requirements for AT(1) (angiotensin II receptor subtype 1) affinity. However, to our knowledge, none was used to predict both the selectivity toward AT(1) and AT(2) (angiotensin II receptor subtype 2) receptor subtypes. In this paper, partial least squares discriminant analysis is applied to derive the chemical features guiding AT(1) and AT(2) selectivity or mixed AT(1)/AT(2) receptor binding. The method can be used to modulate AT(1) versus AT(2) selectivity. Concerns that unopposed stimulation of the AT(2) receptor might produce adverse effects initiated a search for new balanced antagonists. Moreover, it can serve as a fast filtering procedure in database searches. Finally, some relevant pharmacokinetics and metabolic properties of the database of 53 compounds are calculated using the VolSurf and MetaSite software to allow the simultaneous characterization of pharmacodynamic and pharmacokinetics properties of the chemical space of angiotensin II receptor antagonists.

Mutagenesis of the AT1 receptor reveals different binding modes of angiotensin II and [Sar1]-angiotensin II

Homology modeling of the structure of the AT1 receptor, based on the high resolution rhodopsin crystal structure, indicated that it is unlikely that the binding of AngII to AT1 involves simultaneously all the receptor's residues reported in the literature to participate in this process. Site-directed mutagenesis using Ala substitution of charged residues Lys20, Arg23, Glu91 and Arg93 was performed to evaluate the participation of their side-chains in ligand binding and in triggering the cell's response. A comparative analysis by competition binding and functional assays using angiotensin II and the analog [Sar1]-angiotensin II suggests an important role for Arg23 of AT1 receptor in binding of the natural agonist. It is discussed whether some receptor's residues participate directly in the binding with AngII or whether they are part of a regulatory site.

Loss-of-function polymorphic variants of the human angiotensin II type 1 receptor

The angiotensin II type 1 (AT1) receptor is the primary effector for angiotensin II (Ang II), a key peptide regulator of blood pressure and fluid homeostasis. AT1 receptors are involved in the pathogenesis of several cardiovascular diseases, including hypertension, cardiac hypertrophy, and congestive heart failure, which are characterized by significant interindividual variation in disease risk, progression, and response to pharmacotherapy. Such variation could arise from genomic polymorphisms in the AT1 receptor. To pursue this notion, we have pharmacologically characterized seven known and putative nonsynonymous AT1 receptor variants. Functional analysis using the cell-based assay receptor selection and amplification technology (R-SAT) revealed that three variants (AT1-G45R, AT1-F204S, and AT1-C289W) displayed altered responses to Ang II and other AT1 receptor agonists and antagonists. Agonist responses to Ang II were absent for AT1-G45R and significantly reduced in potency for AT1-C289W (11-fold) and AT1-F204S (57-fold) compared with the wild-type (WT) receptor. AT1-F204S also displayed reduced relative efficacy (57%). Quantitatively similar results were obtained in two additional functional assays, phosphatidyl inositol hydrolysis and extracellular signal-regulated kinase activation. Radioligand binding studies revealed that AT1-G45R failed to bind Ang II, whereas cell surface staining clearly showed that it trafficked to the cell surface. AT1-C289W and AT1-F204S displayed reduced binding affinities of 3- and 5-fold and reduced cell surface expression of 43 and 60% of that observed for the WT receptor, respectively. These data demonstrate that polymorphic variation in the human AT1 receptor induces loss of functional phenotypes, which may constitute the molecular basis of variability of AT1 receptor-mediated physiological responses.

Angiotensin II inhibitory peptide found in the receptor sequence using peptide array

Peptide array consisting of hundreds of peptides spatially addressed and synthesized on a cellulose membrane support was used to screen ligand-inhibitory peptides. As a model, angiotensin II (Ang II), a significant peptide related to the treatment of cardiovascular diseases, was chosen as the target ligand. Peptide arrays covering the Ang II receptor type 1 sequence were prepared, and peptide domains with high affinity to the Ang II fluorescein conjugate were investigated. The peptide (VVIVIY) within the first transmembrane region exhibited the highest affinity to Ang II. The synthesized soluble VVIVIY peptide had an 84% inhibitory effect on Ang II-induced aorta contraction. These results indicate that our screening strategy utilizing peptide array is an effective approach for the peptide drug development.

A 3D-QSAR of Angiotensin II (AT1) Receptor Antagonists Based on Receptor Surface Analysis

A hypothetical receptor surface model has been constructed for a set of 38 AT1 antagonists using activity data of each molecule as a weight in the building of the receptor surface. The best model was derived by optimizing various parameters such as atomic partial charges, surface fit, and the manner of representation of electrostatics on the surface. Descriptors such as van der Waals energy, electrostatic energy, and total nonbonded energy were used individually or in combination to derive a family of quantitative structure-activity relationship equations using G/PLS as the statistical method.

Interaction of angiotensin II with the C-terminal 300-320 fragment of the rat angiotensin II receptor AT1a monitored by NMR

Interaction between angiotensin II (Ang II) and the fragment peptide 300-320 (fCT300-320) of the rat angiotensin II receptor AT1a was demonstrated by relaxation measurements, NOE effects, chemical shift variations, and CD measurements. The correlation times modulating dipolar interactions for the bound and free forms of Ang II were estimated by the ratio of the nonselective and single-selective longitudinal relaxation rates. The intermolecular NOEs observed in NOESY spectra between HN protons of 9Lys(fCT) and 6His(ang), 10Phe(fCT) and 8Phe(ang), HN proton of 3Tyr(fCT) and Halpha of 4Tyr(ang), 5Phe(fCT)Hdelta and Halpha of 4Tyr(ang) indicated that Ang II aromatic residues are directly involved in the interaction, as also verified by relaxation data. Some fCT300-320 backbone features were inferred by the CSI method and CD experiments revealing that the presence of Ang II enhances the existential probability of helical conformations in the fCT fragment. Restrained molecular dynamics using the simulated annealing protocol was performed with intermolecular NOEs as constraints, imposing an alpha-helix backbone structure to fCT300-320 fragment. In the built model, one strongly preferred interaction was found that allows intermolecular stacking between aromatic rings and forces the peptide to wrap around the 6Leu side chain of the receptor fragment.

Conformational features of a synthetic model of the first extracellular loop of the angiotensin II AT1A receptor

The angiotensin II AT1A receptor belongs to the G-protein coupled receptors (GPCRs). Like other membrane proteins, GPCRs are not easily amenable to direct structure determination by the currently available methods. The peptide encompassing the putative first extracellular loop of AT1A (residues Thr88-Leu100, el1) has been synthesized along with a cyclic model where the linear peptide has been covalently linked to a template designed to keep the distance between the peptide termini as expected in the receptor. The conformational features of the two molecules have been studied using circular dichroism and NMR techniques. The region W94PFG97 forms a type-II beta-turn and undergoes a Trp-Pro peptide bond cis-trans isomerization in both peptides confirming that these characteristics are intrinsic to el1. In addition, the presence of the spacer seems to modulate the flexibility of the peptide.

The crystallographic model of rhodopsin and its use in studies of other G protein-coupled receptors

G protein-coupled receptors (GPCRs) are integral membrane proteins that respond to environmental signals and initiate signal transduction pathways activating cellular processes. Rhodopsin is a GPCR found in rod cells in retina where it functions as a photopigment. Its molecular structure is known from cryo-electron microscopic and X-ray crystallographic studies, and this has reshaped many structure/function questions important in vision science. In addition, this first GPCR structure has provided a structural template for studies of other GPCRs, including many known drug targets. After presenting an overview of the major structural elements of rhodopsin, recent literature covering the use of the rhodopsin structure in analyzing other GPCRs will be summarized. Use of the rhodopsin structural model to understand the structure and function of other GPCRs provides strong evidence validating the structural model.

Angiotensin II is bound to both receptors AT1 and AT2, parallel to the transmembrane domains and in an extended form

We have applied photoaffinity labelling methods combined with site-directed mutagenesis towards the two principal angiotensin II (AnglI) receptors AT1 and AT2 in order to determine contact points between AngII and the two receptors. We have first identified the receptor contact points between an N- and a C-terminal residue of the AngII molecule and the AT1 receptor and constructed with this stereochemical restriction a molecular model of AT1. A similar approach with a modified procedure of photoaffinity labelling has allowed us now to determine contact points also in the AT2 receptor. Molecular modelling of AT2 on the rhodopsin scaffold and energy minimisation of AngII binding into this AT2 model produced a model strikingly similar to the AT11 structure. Superposition of the experimentally obtained contact points of AngII with AT2 upon this model revealed excellent congruence between the experimental and modelling results.

CONCLUSIONS

(i) athough AT1 and AT2 have quite low sequence homology, they both bind AngII with similar affinity and in an almost identical fashion, as if the ligand dictates the way it has to be bound, and (ii) in its bound form, AngII adopts an extended conformation in both AT1 and AT2, contrary to all previous predictions.