Role of the third intracellular loop of the angiotensin II receptor subtype AT2 in ligand-receptor interaction

Predicting flexible loop regions that interact with ligands: the challenge of accurate scoring

Flexible loop regions play a critical role in the biological function of many proteins and have been shown to be involved in ligand binding. In the context of structure-based drug design, using or predicting an incorrect loop configuration can be detrimental to the study if the loop is capable of interacting with the ligand. Three protein systems, each with at least one flexible loop region in close proximity to the known binding site, were selected for loop prediction using the CorLps program; a six residue loop region from phosphoribosylglycinamide formyltransferase (GART), two nine residue loop regions from cytochrome P450 (CYP) 119, and an 11 residue loop region from enolase were selected for loop prediction. The results of this study indicate that the statistically based DFIRE scoring function implemented in the CorLps program did not accurately rank native-like predicted loop configurations in any protein system. In an attempt to improve the ranking of the native-like predicted loop configurations, the MM/GBSA and the optimized MM/GBSA-dsr scoring functions were used to re-rank the predicted loops with and without bound ligand. In general, single snapshot MM/GBSA scoring provided the best ranking of native-like loop configurations. Based on the scoring function analyses presented, the optimal ranking of native-like loop configurations is still a difficult challenge and the choice of the "best" scoring function appears to be system dependent.

New computational method for prediction of interacting protein loop regions

Flexible loop regions of proteins play a crucial role in many biological functions such as protein-ligand recognition, enzymatic catalysis, and protein-protein association. To date, most computational methods that predict the conformational states of loops only focus on individual loop regions. However, loop regions are often spatially in close proximity to one another and their mutual interactions stabilize their conformations. We have developed a new method, titled CorLps, capable of simultaneously predicting such interacting loop regions. First, an ensemble of individual loop conformations is generated for each loop region. The members of the individual ensembles are combined and are accepted or rejected based on a steric clash filter. After a subsequent side-chain optimization step, the resulting conformations of the interacting loops are ranked by the statistical scoring function DFIRE that originated from protein structure prediction. Our results show that predicting interacting loops with CorLps is superior to sequential prediction of the two interacting loop regions, and our method is comparable in accuracy to single loop predictions. Furthermore, improved predictive accuracy of the top-ranked solution is achieved for 12-residue length loop regions by diversifying the initial pool of individual loop conformations using a quality threshold clustering algorithm.

An ab initio study of AT2 antagonists

In this study we report ab initio molecular orbital calculations on the natural hormone angiotensin II, which induces activity at AT1/AT2 receptor subtypes leading to vasoconstriction and subsequent hypertension, and AT2 antagonists. Pharmacophoric features of AT2 antagonists have been studied. A model of AT2 receptor has been made, and angiotensin II as well as antagonists has been systematically docked and their interactions with the receptor analyzed. Calculated ligand-receptor interaction energies have been correlated with experimentally observed biological potency data. Our studies indicate that antagonists retain sufficient interactions to block the receptor but may not be adequate to induce activity at the receptor. A poor antagonist is, therefore, proposed as a close mimic of angiotensin II in terms of interacting with the receptor. These studies further explore the mechanistic aspects of this important class of drugs.

PMA- and ANG II-induced PKC regulation of the renal Na+-HCO3−cotransporter (hkNBCe1)

The renal electrogenic Na+-HCO3−cotransporter (hkNBCe1) plays a major role in the bicarbonate reabsorption by the kidney. We examined how PMA- and ANG II-activated PKCs regulate hkNBCe1 expressed with or without the ANG II receptors AT1Bin Xenopus laevis oocytes. We found that 10 nM PMA halved the hkNBCe1 current detected in voltage-clamped oocytes. A PKC-specific inhibitor GF-109203X, and a specific inhibitor of Ca-dependent conventional PKCαβγ, GÖ-6976, significantly reduced PMA inhibition. PMA did not alter surface expression of the cotransporters, but it significantly increased hkNBCe1-PKCαβγ membrane association. We found that at 10−6M, ANG II halved the hkNBCe1 current detected in oocytes coexpressing cotransporters with AT1B. A PKC-specific inhibitor GF-109203X, and a PKCε translocation inhibitor εV1–2 peptide as well as BAPTA-AM (but not GÖ-6976), significantly reduced ANG II inhibition. At 10−6M, ANG II significantly decreased surface expression of the cotransporters and increased hkNBCe1-PKCε membrane association. Additionally, we found that at 10−11and 10−10M ANG II stimulated hkNBCe1 current. This effect was blocked by BAPTA-AM and partially reduced by GF-109203X. We also found that ANG II increased intracellular Ca2+in fluo 4-loaded oocytes. Our results suggest that 1) PMA inhibition of hkNBCe1 is mediated by Ca-dependent PKCαβγ and 10 nM PMA does not induce downregulation of cotransporter surface expression. 2) ANG II (10−6M) inhibition of hkNBCe1 is mediated by both Ca-independent PKCε and downregulation of cotransporter surface expression, possibly triggered by intracellular Ca2+mobilization. 3) Similar to proximal tubule, acute ANG II has a biphasic effect on hkNBCe1 coexpressed with AT1Bin X. laevis oocytes.

Roles of the intracellular regions of angiotensin II receptor AT2 in mediating reduction of intracellular cGMP levels

We have shown previously that the angiotensin II (Ang II) receptor AT2 reduces the intracellular levels of cGMP in Xenopus oocytes when activated by ligand binding, and the C-terminal cytoplasmic tail of the AT2 acts as a negative regulator of this function. Here we report the effects of mutations in the 2nd and 3rd intracellular loops of AT2 on AT2-mediated cGMP reduction. Mutating the highly conserved DRY motif (D141G-R142G-Y143A) of the 2nd ICL implicated in activating G(alpha) subunit of trimeric G-proteins did not affect AT2-mediated cGMP reduction. Moreover, anti-Gialpha antibody or phosphodiesterase inhibitor IBMX did not inhibit AT2-mediated cGMP reduction, suggesting that Gialpha activation and subsequent phosphodiesterase activation are not involved in this function. In contrast, mutations T250R-R251N and L255F-K256R located in the C-terminus of the 3rd ICL of AT2 retained ligand-binding properties of the wild-type AT2, and its ability to interact with the ErbB3 in yeast two-hybrid assay, but abolished AT2-mediated cGMP reduction. Similarities in the roles of ICLs of AT2 in AT2-mediated cGMP reduction in oocytes, and AT2-mediated SHP1 activation in COS-7 cells, (need of 3rd ICL for both functions and lack of involvement of DRY motif), suggest that the cascade of events in these two signaling mechanisms could be similar, and that an oocyte-specific SHP1-like protein may be involved in AT2-mediated cGMP reduction in these cells.

Role of Phe308 in the seventh transmembrane domain of the AT2 receptor in ligand binding and signaling

Studies on Angiotensin II (Ang II) receptor type AT1 have suggested that interaction between the two highly conserved residues, Tyr292 in the 7th transmembrane domain (TMD) and the Asp74 in the 2nd TMD, is critical for linking the Ang II binding and AT1 receptor-Gq protein coupling. In the Ang II receptor type AT2, the Asp is conserved (Asp90 in 2nd TMD), however, there is no Tyr residue in the 7th TMD and Phe308 occupies the analogous position to Tyr292 of the AT1. Replacing this Phe308 with Ala reduced receptor affinity to peptidic ligands (125)I-Ang II (K(d) = 0.37 nM) and (125)I-CGP42112A (K(d) = 0.56 nM), but retained the ability of the AT2 to reduce cGMP levels in Xenopus oocytes. Thus, the Phe308 of the AT2 does not mimic the role of Tyr292 of the AT1 in the receptor activation upon Ang II binding. We have also shown that the M8 mutant of the AT2 with the 7th TMD similar to that of wild type AT2 can couple to PLC like the AT1 and bind the AT2-specific ligands with high affinity. Since the Ang II is shown to bind to both the AT1 and the AT2 in an identical manner, we propose that the absence of Tyr in the 7th TMD of the AT2 does not prevent the receptor from coupling to Gq-protein, rather may contribute to the freedom of the AT2 to couple to trimeric G-proteins in both G- betagamma dependent and independent manners upon Ang II binding.

Structural determinants of agonist-induced signaling and regulation of the angiotensin AT1 receptor

Angiotensin II (Ang II) regulates aldosterone secretion by stimulating inositol phosphate production and Ca(2+) signaling in adrenal glomerulosa cells via the G(q)-coupled AT(1) receptor, which is rapidly internalized upon agonist binding. Ang II also binds to the heptahelical AT(2) receptor, which neither activates inositol phosphate signaling nor undergoes receptor internalization. The differential behaviors of the AT(1) and AT(2) receptors were analyzed in chimeric angiotensin receptors created by swapping the second (IL2), the third (IL3) intracellular loops and/or the cytoplasmic tail (CT) between these receptors. When transiently expressed in COS-7 cells, the chimeric receptors showed only minor alterations in their ligand binding properties. Measurements of the internalization kinetics and inositol phosphate responses of chimeric AT(1A) receptors indicated that the CT is required for normal receptor internalization, and IL2 is a determinant of G protein activation. In addition, the amino-terminal portion of IL3 is required for both receptor functions. However, only substitution of IL2 impaired Ang II-induced ERK activation, suggesting that alternative mechanisms are responsible for ERK activation in signaling-deficient mutant AT(1) receptors. Substitution of IL2, IL3, or CT of the AT(1A) receptor into the AT(2) receptor sequence did not endow the latter with the ability to internalize or to mediate inositol phosphate signaling responses. These data suggest that the lack of receptor internalization and inositol phosphate signal generation by the AT(2) receptor is a consequence of its different activation mechanism, rather than the inability of its cytoplasmic domains to couple to intracellular effectors.

Molecular cloning, characterization, and distribution of the gerbil angiotensin II AT2 receptor

We isolated a cDNA clone encoding the gerbil AT2 receptor (gAT2) gene from a gerbil adrenal gland cDNA library. The full-length cDNA contains a 1,089-bp open reading frame encoding 363 amino acid residues with 90.9, 96.1, and 95.6% identity with the human (hAT2), rat (rAT2), and mouse AT2 (mAT2) receptors, respectively. There are at least seven nonconserved amino acids in the NH2-terminal domain and in positions Val196, Val217, and Met293, important for angiotensin (ANG) II but not for CGP-42112 binding. Displacement studies in adrenal sections revealed that affinity of the gAT2 receptor was 10-20 times lower for ANG II, ANG III, and PD-123319 than was affinity of the rAT2 receptor. The affinity of each receptor remained the same for CGP-42112. When transfected into COS-7 cells, the gAT2 receptor shows affinity for ANG II that is three times lower than that shown by the hAT2 receptor, whereas affinities for ANG III and the AT2 receptor ligands CGP-42112 and PD-123319 were similar. Autoradiography in sections of the gerbil head showed higher binding in muscles, retina, skin, and molars at embryonic day 19 than at 1 wk of age. In situ hybridization and emulsion autoradiography revealed that at embryonic day 19 the gAT2 receptor mRNA was highly localized to the base of the dental papilla of maxillary and mandibular molars. Our results suggest selective growth-related functions in late gestation and early postnatal periods for the gAT2 receptor and provide an essential basis for future mutagenesis studies to further define structural requirements for agonist binding.

Identification of the region of AT2 receptor needed for inhibition of the AT1 receptor-mediated inositol 1,4,5-triphosphate generation

Increase in the intracellular inositol triphosphate (IP3) levels in Xenopus oocytes in response to expression and activation of rat angiotensin II (Ang II) receptor AT1 was inhibited by co-expression of rat AT2 receptor. To identify which region of the AT2 was involved in this inhibition, ability of three AT2 mutants to abolish this inhibition was analyzed. Deletion of the C-terminus of the AT2 did not abolish this inhibition. Replacing Ile249 in the third intracellular loop (3rd ICL) of the AT2 with proline, corresponding amino acid in the AT1, in the mutant M6, resulted in slightly reduced affinity to [125I]Ang II (K(d)=0.259 nM), however, did not abolish the inhibition. In contrast, replacing eight more amino acids in the 3rd ICL of the AT2 (at positions 241-244, 250-251 and 255-256) with that of the AT1 in the mutant M8, not only increased the affinity of the AT2 receptor to [125I]Ang II (K(d)=0.038 nM) but also abolished AT2-mediated inhibition. Interestingly, activation of the M8 by Ang II binding also resulted in increase in the intracellular IP(3) levels in oocytes. These results imply that the region of the 3rd ICL of AT2 spanning amino acids 241-256 is sufficient for the AT2-mediated inhibition of AT1-stimulated IP3 generation. Moreover, these nine mutations are also sufficient to render the AT2 with the ability to activate phospholipase C.

The angiotensin II type 2 receptor: an enigma with multiple variations

Since it was discovered ten years ago, the angiotensin II (ANG II) type 2 (AT(2)) receptor has been an enigma. This receptor binds ANG II with a high affinity but is not responsible for mediating any of the classical physiological actions of this peptide, all of which involve the ANG II type 1 (AT(1)) receptor. Furthermore, the AT(2) receptor exhibits dramatic differences in biochemical and functional properties and in patterns of expression compared with the AT(1) receptor. During the past decade, much information has been gathered about the AT(2) receptor, and the steadily increasing number of publications indicates a growing interest in this new and independent area of research. A number of studies suggest a role of AT(2) receptors in brain, renal, and cardiovascular functions and in the processes of apoptosis and tissue regeneration. Despite these advances, nothing stands out as the major singular function of these receptors. The study of AT(2) receptors has reached a crossroads, and innovative approaches must be considered so that unifying mechanisms as to the function of these unique receptors can be put forward. In this review we will discuss the advances that have been made in understanding the biology of the AT(2) receptor. Furthermore, we will consider how these discoveries, along with newer experimental approaches, may eventually lead to the elusive physiological and pathophysiological functions of these receptors.

Identification of an interaction between the angiotensin II receptor sub-type AT2 and the ErbB3 receptor, a member of the epidermal growth factor receptor family

To identify the proteins that interact and mediate angiotensin II receptor AT2-specific signaling, a random peptide library was screened by yeast-based Two-Hybrid protein-protein interaction assay technique. A peptide that shared significant homology with the amino acids located between the residues Gly-Xaa-Gly-Xaa-Xaa-Gly721 and Lys742, the residues predicted to be important for ATP binding of the ErbB3 and ErbB2 receptors, was identified to be interacting with the AT2 receptor. The interaction between the human ErbB3 receptor and the AT2 receptor was further confirmed using the cytoplasmic domain (amino acids 671-782) of the human ErbB3 receptor. Moreover, an AT2 receptor peptide that spans the amino acids 226-363, (spans the third ICL and carboxy terminal domain) could also interact with the AT2 receptor in a yeast Two-Hybrid protein-protein interaction assay. Studies using mutated and chimeric AT2 receptors showed that replacing the third intracellular loop (ICL) of the AT2 receptor with that of the AT1 abolishes the interaction between the ErbB3 and the AT2 in yeast Two-Hybrid protein-protein interaction assay. Thus the interaction between the AT2 receptor and the ErbB3 receptor seems to require the region spanning the third ICL and carboxy terminus of the AT2 receptor. Since the third ICL of the AT2 receptor is essential for exerting its inhibitory effects on cell growth, possible involvement of this region in the interaction with the cytoplasmic domain of the ErbB3 receptor suggests a novel signaling mechanism for the AT2 receptor mediated inhibition of cell growth. Furthermore, since both the AT2 and the ErbB3 receptors are expressed during fetal development, we propose that the existence of direct interaction between these two receptors may play a role in the regulation of growth during the initial stages of development.