Effects of bradykinin and endothelin-1 on the calcium homeostasis of mammalian cells

Bradykinin signaling counteracts cAMP-elicited aquaporin 2 translocation in renal cells

Bradykinin (BK) is one of the most important peptides regulating vascular tone, water, and ionic balance in the body, playing a key role in controlling BP. It is interesting that patients with essential hypertension excrete less BK than normotensive individuals. For elucidating the mechanism by which BK regulates renal water transport that contributes to its antihypertensive effect, aquaporin 2 (AQP2)-transfected collecting duct CD8 cells, expressing the BK type II receptor (BK2R), were used as an experimental model. In CD8 cells, BK pretreatment impaired forskolin-induced AQP2 translocation to the apical plasma membrane. For clarifying the signal transduction cascade associated with this effect, whether BK induced an increase in cytosolic calcium, via the G protein Gq, known to be coupled to BK2R, first was investigated. Spectrofluorometry using fura-2-AM revealed that 100 nM BK elicited a significant increase in Ca(i), which was abolished by the receptor antagonist HOE-140. BK acts through BK2R coupled to both Gq and Galpha13, a known upstream effector of Rho protein. In CD8 cells, BK causes an increase in Rho activity, likely as a result of Galpha13 activation. This results in stabilization of the cortical F-actin network, thus impairing AQP2 trafficking. These effects counteract physiologic vasopressin stimulation, which instead has an opposite effect on actin network organization through Rho inactivation.

Trypanosoma cruzi induces edematogenic responses in mice and invades cardiomyocytes and endothelial cells in vitro by activating distinct kinin receptor (B1/B2) subtypes

Trypanosoma cruzi, the protozoan that causes Chagas' heart disease, invades endothelial cells in vitro by activating the B2 kinin receptor (B2R). Here, we demonstrate that mice infected with trypomastigotes develop potent edema after treatment with the angiotensin-converting enzyme (ACE) (or kininase II) inhibitor captopril. Experiments performed with specific kinin receptor (B2R/B1R) antagonists and knockout mice revealed that the early-phase (3-h) edema is mediated by the constitutive B2R, whereas the late-phase (24-h) response depends on stimulation of the up-regulated B1R. Given previous evidence that parasite invasion of cells expressing B2R is potentiated by captopril, we investigated the prerequisites for in vitro infection of Chinese hamster ovary cells overexpressing either B1R or B2R, human umbilical vein endothelial cells activated by lipopolysaccharide, and neonatal rat cardiomyocytes. Our results indicate that captopril potentiates parasite invasion regardless of the kinin (B2/B1) activation pathways, whereas DL-2-mercaptomethyl-3-guanidino-ethylthiopropanoic acid (MGTA), an inhibitor of kininase I (carboxypeptidase M/N), selectively decreases parasite infectivity for B1R-expressing cells. These data suggest that formation of the B1R agonist, i.e., [des-Arg] kinins, critically depends on the processing action of kininase I, here proposed as a potential pathogenesis cofactor. Collectively, our data suggest that fluctuations in the levels of kininases may modulate parasite infectivity and pathological outcome in Chagas' disease.

Host cell invasion by Trypanosoma cruzi is potentiated by activation of bradykinin B(2) receptors

The parasitic protozoan Trypanosoma cruzi employs multiple molecular strategies to invade a broad range of nonphagocytic cells. Here we demonstrate that the invasion of human primary umbilical vein endothelial cells (HUVECs) or Chinese hamster ovary (CHO) cells overexpressing the B(2) type of bradykinin receptor (CHO-B(2)R) by tissue culture trypomastigotes is subtly modulated by the combined activities of kininogens, kininogenases, and kinin-degrading peptidases. The presence of captopril, an inhibitor of bradykinin degradation by kininase II, drastically potentiated parasitic invasion of HUVECs and CHO-B(2)R, but not of mock-transfected CHO cells, whereas the B(2)R antagonist HOE 140 or monoclonal antibody MBK3 to bradykinin blocked these effects. Invasion competence correlated with the parasites' ability to liberate the short-lived kinins from cell-bound kininogen and to elicit vigorous intracellular free calcium ([Ca(2+)](i)) transients through B(2)R. Invasion was impaired by membrane-permeable cysteine proteinase inhibitors such as Z-(SBz)Cys-Phe-CHN(2) but not by the hydrophilic inhibitor 1-trans-epoxysuccinyl-l-leucyl-amido-(4-guanidino) butane or cystatin C, suggesting that kinin release is confined to secluded spaces formed by juxtaposition of host cell and parasite plasma membranes. Analysis of trypomastigote transfectants expressing various cysteine proteinase isoforms showed that invasion competence is linked to the kinin releasing activity of cruzipain, herein proposed as a factor of virulence in Chagas' disease.

Involvement of the amino terminus of the B(2) receptor in agonist-induced receptor dimerization

The mechanisms and the functional importance of G-protein-coupled receptor dimerization are poorly understood. We therefore analyzed dimerization of the bradykinin B(2) receptor. The binding of the agonist bradykinin to the B(2) receptor endogenously expressed on PC-12 cells led to the formation of receptor dimers, whereas the B(2) antagonist HOE140 did not induce dimerization, suggesting that B(2) receptor dimerization was linked to receptor activation. Addition of a peptide corresponding to the amino terminus of the receptor reduced the amount of detected B(2) receptor dimers, whereas peptides derived from the extracellular loops had no effect. To further analyze the role of the amino terminus of the receptor in receptor dimerization, we created two different rat B(2) receptor variants with truncated amino termini, B(2)(53) and B(2)(65), starting at amino acids 53 and 65. In contrast to the wild-type B(2) receptor and to B(2)(53), bradykinin did not induce dimerization of the B(2)(65) receptor. Both receptor variants were similar to the wild-type B(2) receptor with respect to agonist binding and signal generation. However, B(2)(65) was not phosphorylated, did not desensitize, and was not downregulated upon bradykinin stimulation. Likewise, antibodies directed to the amino terminus of the receptor partially reduced internalization of [(3)H]bradykinin on PC-12 cells. These findings suggest that the amino terminus of the B(2) receptor is necessary for triggering agonist-induced B(2) receptor dimerization, and receptor dimers are involved in receptor-mediated signal attenuation.

Interaction of bradykinin with angiotensin, prostacyclin, and nitric oxide in myocardial preservation

This review focuses on the importance of bradykinin in myocardial preservation during ischemic arrest. Bradykinin is released from the heart spontaneously in response to ischemic stress, which may be viewed as a survival signal of the heart against ischemia. Bradykinin appears to function as a signaling molecule by controlling the release of other intracellular modulators, such as prostacyclins and nitric oxide, which also exert beneficial effects on the ischemic myocardium.

Calcium-binding properties and molecular organization of bradykinin A solution 1H-NMR study

The NMR features of bradykinin were investigated in dimethylsulfoxide containing 1% water. The temperature dependence of chemical shifts and ROESY maps were monitored for the major species where all X-Pro bonds are trans. The occurrence of a head-to-tail ionic interaction and intramolecular hydrogen bonds stabilizing a pseudo cyclic arrangement was inferred, a beta turn at the C-terminus being the main feature of the secondary structure. Calcium was shown to bind to the peptide with a dissociation constant Kd = 2.8 + 0.2 mm. 2Pro and 3Pro carbonyls, as well as the 9Arg carboxyl, were assigned as the metal-binding sites. A molecular model of the 1 : 1 metal-complex was obtained. In light of conformational changes experienced by the peptide upon interaction with calcium, a role for the metal was hypothesized in the process of conformational selection from the free to the receptor-bound state of bradykinin.

Investigation of the extracellular accessibility of the connecting loop between membrane domains I and II of the bradykinin B2 receptor

In analogy to the structure of rhodopsin, the seven hydrophobic segments of G-protein-coupled receptors are supposed to form seven membrane-spanning alpha-helices. To analyze the topology of the bradykinin B2 receptor, we raised site-directed antibodies to peptides corresponding to the loop regions and the amino and carboxyl terminus of this receptor. We found that a segment with predicted intracellular orientation according to the rhodopsin model, the connecting loop between membrane domains I and II of the bradykinin B2 receptor, was accessible to site-directed antibodies on intact fibroblasts, A431 cells, or COS cells expressing human B2 receptors. Extracellular orientation of this loop was further confirmed by the substituted cysteine accessibility method which showed that exchange of cysteine 94 for serine on this loop by point mutagenesis suppressed the effect of thiol modification by a membrane impermeant maleimide. In addition, this segment seemed to be involved in B2 receptor activation, since (i) thiol modification of cysteine 94 partially suppressed B2 receptor activation, and (ii) site-directed antibodies to the connecting loop between membrane domains I and II were agonists. The agonistic activity of the antibodies was suppressed by the B2 antagonist HOE140 confirming the B2 specificity of the antibody-generated signal. The extracellular orientation of the connecting loop between membrane domains I and II suggests a topology of the B2 receptor different from rhodopsin, consisting of five (instead of seven) transmembrane domains and two hydrophobic segments with both ends facing the extracellular side.

Correlations in palmitoylation and multiple phosphorylation of rat bradykinin B2 receptor in Chinese hamster ovary cells

Rat bradykinin B2 receptor from unstimulated Chinese hamster ovary cells transfected with the corresponding cDNA has been isolated, and subsequent mass spectrometric analysis of multiple phosphorylated species and of the palmitoylation attachment site is described. Bradykinin B2 receptor was isolated on oligo(dT)-cellulose using N-(epsilon-maleimidocaproyloxy)succinimide-Met-Lys-bradykinin coupled to a protected (dA)30-mer. This allowed a one-step isolation of the receptor on an oligo(dT)-cellulose column via variation solely of salt concentration. After enzymatic in-gel digestion, matrix-assisted laser desorption ionization and electrospray ion trap mass spectrometric analysis of the isolated rat bradykinin B2 receptor showed phosphorylation at Ser365, Ser371, Ser378, Ser380, and Thr374. Further phosphorylation at Tyr352 and Tyr161 was observed. Rat bradykinin receptor B2 receptor is also palmitoylated at Cys356. All of the phosphorylation sites except for Tyr161 cluster at the carboxyl-terminal domain of the receptor located on the cytoplasmic face of the cell membrane. Surprisingly, many of the post-translational modifications were shown by MSn mass spectroscopic analysis to be correlated pairwise, e.g. diphosphorylation at Ser365 and Ser371, at Ser378 and Ser380, and at Thr374 and Ser380 as well as mutually exclusive phosphorylation at Tyr352 and palmitoylation at Cys356. The last correlation may be involved in a receptor internalization motif. Pairwise correlations and mutual exclusion of phosphorylation and palmitoylation suggest critical roles of multiple post-translational modifications for the regulation of activity, coupling to intracelluar signaling pathways, and/or sequestration of the bradykinin receptor.

Bradykinin-evoked Ca2+ mobilization in Madin Darby canine kidney cells

We studied the mechanisms underlying the bradykinin-evoked changes in intracellular calcium concentration ([Ca2+]i) in Madin Darby canine kidney (MDCK) cells. Bradykinin evoked a [Ca2+]i transient in a dose-dependent manner, measured by fura-2 fluorimetry and digital video imaging. The transient consisted of a rise and a decay and [Ca2+]i returned to baseline without oscillations. External Ca2+ influx occurred, as demonstrated by Mn2+ quench and external Ca2+ removal measurements. Bradykinin acted by stimulating bradykinin B2 receptors as evidenced by blockade by D-arginyl-L-arginlyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl -3-(2-thienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolineca rbonyl-L-(2alpha,3beta,7alphabeta)-octahydro-1 H-indole-2-carbonyl-L-arginine (HOE 140) but not by D-arginyl-L-arginlyl-L-prolyl-trans-4-hydroxy-L-proylglycyl- 3-(2-thienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolinecar bonyl-L-(2alpha,3beta,7alphabeta)-octahydro-1 H-indole-2-carbonyl ([Des-Arg]HOE 140). The [Ca2+]i signal was abolished by 1-(6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1 H-pyrrole-2,5-dione (U73122) and partially inhibited by neomycin, implying mediation by phospholipase C. The transient was initiated by a release of Ca2+ from internal stores since it was abolished by pretreatment with thapsigargin or cyclopiazonic acid. The mobilization of the internal Ca2+ store subsequently triggered a 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1 H-imidazole hydrochloride (SKF 96365)-insensitive Ca2+ entry. Pretreatment with carbonylcyanide m-chlorophynylhydrozone and gly-phe-beta-naphthylamide did not alter the transient, thus excluding the participation of mitochondria and lysosomes. Efflux via Ca2+ pumps contributed to the decay of the transient. Efflux via Na+/Ca2+ exchange or sequestration by mitochondria and lysosomes was insignificant. The transient was blunted by the protein kinase C activator phorbol 12-myristate 13-acetate, and was enhanced by the protein kinase C inhibitors sphingosine and chelerythrine, the protein kinase A inhibitor 2,5-di-(t-butyl)-1,4-hydroquinone, N-[2-(p-bromocinnamylamino)ethyl]5-isoquinolinesulfonamide (H-89), the agent 8-(diethylamino)octyl 3,4,5-trimethoxybenzoate (TMB-8), and agents that elevated levels of 3',5'-cyclic guanosine monophosphate. The transient did not heterologously desensitize with that evoked by ATP, ADP or UTP.

Negative cooperativity in the human bradykinin B2 receptor

A human kidney bradykinin (BK) B2 receptor cDNA was transfected in CHO-K1 cells to establish cell lines that express stably and at high density a receptor exhibiting B2 receptor properties in terms of coupling to cell signaling effectors, desensitization, and internalization. A cell line with a density of 1.3 x 10(6) receptors/cell allowed us to carry out a detailed study of BK-receptor interaction over a wide range of BK concentrations. A model assuming that BK binds to two receptor affinity states (depending on guanine nucleotide-sensitive coupling) was not sufficient to account for the kinetics of BK binding. Equilibrium kinetic analysis and studies of the effects of receptor occupancy by agonists or antagonists on the kinetics of BK-receptor complex dissociation revealed features typical of negative cooperative binding. The negative cooperativity phenomenon was also observed in isolated membranes in both the presence and absence of guanine nucleotide. Thus, following the interaction with BK, B2 receptor molecules likely interact with each other, resulting in an acceleration of bound ligand dissociation and a decrease in the apparent affinity of the receptor for BK. This phenomenon can participate in the desensitization process.

Ligand-induced phosphorylation/dephosphorylation of the endogenous bradykinin B2 receptor from human fibroblasts

We have studied the ligand-induced phosphorylation/dephosphorylation of the bradykinin B2 receptor endogenously expressed in human HF-15 fibroblasts. An antiserum (AS346) to a synthetic peptide (CRS36), derived from the extreme carboxyl terminus of the human B2 receptor, precipitated the receptor from solubilized membranes of HF-15 cells that had been labeled with [32P]orthophosphate. A low basal level of B2 receptor phosphorylation was found in the absence of a ligand. Stimulation of the cells with the B2 receptor agonists bradykinin, [Lys0,Hyp3]bradykinin, kallidin, and T-kinin resulted in a rapid and efficient phosphorylation of the receptor. The B2 receptor antagonist HOE140 and the B1 receptor agonist des-Arg9-bradykinin failed to induce significant phosphorylation of the B2 receptor. Phosphoamino acid analysis revealed that the B2 receptor is phosphorylated on serine and threonine, but not on tyrosine residues. The ligand-induced phosphorylation of the receptor was concentration-dependent, with an apparent EC50 of 33 nM, and peaked at 1 min after challenge. The kinin-stimulated phosphorylation of the B2 receptor was rapid and transient and paralleled the kinetics of desensitization/resensitization of the receptor as followed by [Ca2+]i release and radioligand binding assay, respectively. The ligand-induced phosphorylation of the B2 receptor was independent of the protein kinase C pathway. In vitro experiments suggest betaARK1 (beta-adrenergic receptor kinase) as a candidate kinase that could mediate the homologous B2 receptor phosphorylation. Inhibitors of protein phosphatases 1 and 2A effectively blocked the dephosphorylation, but did not affect the internalization of the B2 receptor, whereas inhibitors of receptor internalization delayed its dephosphorylation. These finding point to a role of ligand-induced phosphorylation in the desensitization and redistribution of the bradykinin receptor in human fibroblasts.

The N-terminal amino group of [Tyr8]bradykinin is bound adjacent to analogous amino acids of the human and rat B2 receptor

To obtain data of the bradykinin B2 receptor's agonist binding site, we used a combined approach of affinity labeling and "immunoidentification" of receptor fragments generated by cyanogen bromide cleavage. Domain-specific antibodies to the various extracellular receptor domains were applied to detect receptor fragments with covalently attached [125I-Tyr8]bradykinin. As a cross-linker we used the homobifunctional reagent disuccinimidyl tartarate (DST), which reacts preferentially with primary amines. With this technique a [125I-Tyr8]bradykinin-labeled receptor fragment derived from the third extracellular domain was identified. The epsilon-amino group of lysine (Lys172) of the human B2 receptor provides the only primary amino group within this receptor fragment. This strongly suggests that DST attached the N-terminal amino group of [Tyr8]bradykinin to Lys172 of the human B2 receptor. Next we asked whether DST attaches [Tyr8]bradykinin to the analogous residue, Lys174 of the rat B2 receptor, which is 81% identical to the human B2 receptor, and we attempted to label the wild-type rat B2 receptor and a rat B2 receptor mutant where Lys174 had been exchanged for alanine. Affinity labeling of the wild-type rat B2 receptor worked efficiently, whereas DST did not attach detectable amounts of [125I-Tyr8]bradykinin to the K174A rat B2 receptor mutant. Taken together these observations indicate that the N-terminal amino group of [Tyr8]bradykinin is bound to analogous positions of the rat and of the human B2 receptor, i.e. [Tyr8]bradykinin's N terminus is bound adjacent to Lys172 of the human and Lys174 of the rat B2 receptor.

Two distinct Ca2+ influx pathways activated by the bradykinin B2 receptor

The hormone-induced depletion of cellular Ca stores provides a signal for the Ca2+ influx into electrically non-excitable cells; however, the underlying molecular mechanisms remain elusive. Therefore, we analyzed bradykinin-activated Ca2+ influx into human foreskin fibroblast cells, HF-15, by fura-2 and 45Ca labeling to discriminate between Ca2+ influx into the fura-sensitive compartment and Ca uptake into fura-insensitive Ca stores. Bradykinin-activated Ca2+ influx into the fura-sensitive compartment was blocked by inhibitors of NO synthases. These inhibitors also suppressed bradykinin-activated increases in cGMP, indicating that the NO-dependent increase in cGMP is involved in the activation of the Ca2+ influx into the fura-sensitive compartment. The cGMP-dependent kinase inhibitors KT5823 and Rp-8-(parachlorophenylthio)-cGMP (Rp-8-pCPT-cGMPS) blocked bradykinin-activated Ca2+ influx into the fura-sensitive compartment, suggesting that a cGMP-dependent kinase step participates in the activation of this Ca2+ influx pathway. In addition to the NO/cGMP-mediated Ca2+ influx into the fura-sensitive compartment, bradykinin enhanced 45Ca uptake into Ca stores that were not accessible to fura-2. This enhanced 45Ca uptake was insensitive to blockers of the NO/cGMP pathway, indicating that the 45Ca uptake pathway is distinct from the NO-dependent Ca2+ influx into the fura-sensitive compartment. Furthermore, bradykinin enhanced 45Ca uptake into proliferating but not into quiescent HF-15 fibroblasts. Hence, bradykinin stimulates two distinct Ca2+ influx pathways in HF-15 cells, (a) Ca2+ influx into the fura-sensitive compartment which is NO/cGMP-dependent and (b) Ca uptake into Ca stores which bypasses the cytoplasm, which is NO insensitive and which is linked to cell proliferation.

Na+ ions binding to the bradykinin B2 receptor suppress agonist-independent receptor activation

Control of the balance between receptor activation and inactivation is a prerequisite for seven transmembrane domain (7TM) receptor function. We asked for a mechanism to stabilize the inactive receptor conformation which prevents agonist-independent receptor activation. Na+ ions have reciprocal effects on agonist versus antagonist interaction with various 7TM receptors. To investigate the Na+ dependence of receptor activation we chose the bradykinin B2 receptor as a prototypic 7TM receptor. Decrease of the intracellular Na+ content from 40 mM to 10 mM of COS-1 cells transiently expressing rat B2 receptors activated the B2 receptor in the absence of agonist as shown by a 3-fold increase in the basal release of inositolphosphates and increased the intrinsic activity of bradykinin to 1.2. In contrast, under increased intracellular Na+ (148 mM) the intrinsic activity of bradykinin decreased to 0.72. When the interaction of Na+ with the B2 receptor was prevented by exchanging a conserved aspartate in transmembrane domain II for asparagine the B2 receptor was also constitutively-activated in the absence of agonist. Agonist-independence B2 receptor activation under decreased intracellular Na+ was similarly observed with primary human fibroblasts endogenously expressing human B2 receptors by a 2.5-fold increase in basal inositolphosphates. Activation of human B2 receptors in the absence of agonist under decreased intracellular Na+ was further evident by an increased basal phosphorylation of the B2 receptor protein. Thus our data suggest that the interaction of Na+ ions with the B2 receptor stabilizes or induces an inactive receptor conformation thereby providing a mechanism to suppress agonist-independent receptor activation in vivo.

Palmitoylation of endothelin receptor A. Differential modulation of signal transduction activity by post-translational modification

Post-translational modifications such as phosphorylation and palmitoylation play important roles for the function and regulation of receptors coupled to heterotrimeric guanyl nucleotide-binding proteins. Here we demonstrate that the human endothelin receptor A (ETA) incorporates [3H]palmitate. Mutation of a cluster of five cysteine residues present in the cytoplasmic tail of ETA into serine or alanine residues completely prevented palmitoylation of the receptor. The ligand binding affinity of the non-palmitoylated ETA mutants was essentially unchanged as compared to the palmitoylated wild type ETA suggesting that the replacement of the cysteine residues did not alter the overall structure of the receptor. Furthermore, the ligand-induced stimulation of adenylyl cyclase by the mutant ETA was unaffected by the mutation. In contrast, the mutated non-palmitoylated receptors but not the wild type receptor failed to stimulate phosphatidylinositol hydrolysis by phospholipase C activation upon challenge by endothelin-1. Furthermore, the mutant receptors failed to stimulate the ligand-induced transient increase in the cytoplasmic calcium seen with the wild type ETA. Endothelin-1 induced mitogenic stimuli via the wild type receptors but not through the mutated receptors suggesting an important role for phospholipase C in this signal transduction pathway. The differential regulation of distinct signal transduction pathways by post-translational modification suggests that palmitoylation of the ETA provides a novel mechanism of modulating ETA receptor activity.

Streptococcal cysteine proteinase releases kinins: a virulence mechanism

Previous work has indicated a crucial role for the extracellular cysteine proteinase of Streptococcus pyogenes in the pathogenicity and virulence of this important human pathogen. Here we find that the purified streptococcal cysteine proteinase releases biologically active kinins from their purified precursor protein, H-kininogen, in vitro, and from kininogens present in the human plasma, ex vivo. Kinin liberation in the plasma is due to the direct action of the streptococcal proteinase on the kininogens, and does not involve the previous activation of plasma prekallikrein, the physiological plasma kininogenase. Judged from the amount of released plasma kinins the bacterial proteinase is highly efficient in its action. This is also the case in vivo. Injection of the purified cysteine proteinase into the peritoneal cavity of mice resulted in a progressive cleavage of plasma kininogens and the concomitant release of kinins over a period of 5 h. No kininogen degradation was seen in mice when the cysteine proteinase was inactivated by the specific inhibitor, Z-Leu-Val-Gly-CHN2, before administration. Intraperitoneal administration into mice of living S. pyogenes bacteria producing the cysteine proteinase induced a rapid breakdown of endogenous plasma kininogens and release of kinins. Kinins are hypotensive, they increase vascular permeability, contract smooth muscle, and induce fever and pain. The release of kinins by the cysteine proteinase of S. pyogenes could therefore represent an important and previously unknown virulence mechanism in S. pyogenes infections.

Structure of the bradykinin B2 receptors' amino terminus

The peptide hormone bradykinin exerts important biological functions by binding to and activating bradykinin B2 receptors. B2 receptors belong to the seven transmembrane domain (7TM) receptor family. Cloning of the cDNA sequences for the rat, human, and mouse bradykinin B2 receptor revealed several in-frame AUG triplets as potential initiation sites for translation. Due to "Kozak-like" consensus nucleotides, the AUG codon closest to transmembrane domain 1 was assumed the preferred initiation site for translation, but the real amino terminus of the B2 receptor protein was unknown. The amino terminus of several 7TM receptors has been shown to be essentially involved in receptor activation and/or ligand binding. Therefore we determined the amino terminus of the human and of the rat B2 receptor using domain-specific antipeptide antibodies, amino acid sequence analysis, and in vitro transcription/translation. We report that the human and rat B2 receptor protein start with the methionine which is translated from the first in-frame AUG. This start site extends the known amino terminus of the human and rat B2 receptors by 27 or 30 amino acid residues, respectively. Antibodies raised against a peptide of the initial 27 amino acids of the human B2 receptor stained B2 receptors on intact cells. This finding excludes the existence of a signal sequence for this receptor.

Extracellular domains of the bradykinin B2 receptor involved in ligand binding and agonist sensing defined by anti-peptide antibodies

Many of the physiological functions of bradykinin are mediated via the B2 receptor. Little is known about binding sites for bradykinin on the receptor. Therefore, antisera against peptides derived from the putative extracellular domains of the B2 receptor were raised. The antibodies strongly reacted with their corresponding antigens and cross-reacted both with the denatured and the native B2 receptor. Affinity-purified antibodies to the various extracellular domains were used to probe the contact sites between the receptor and its agonist, bradykinin or its antagonist HOE140. Antibodies to extracellular domain 3 (second loop) efficiently interfered, in a concentration-dependent manner, with agonist and antagonist binding and vice versa. Antibodies to extracellular domain 4 (third loop) blocked binding of the agonist but not of the antagonist, whereas antibodies to extracellular domains 1 and 2 or to intracellular domains failed to block ligand binding. Antibodies to ectodomain 3 competed with agonistic anti-idiotypic antibodies for B2 receptor binding. Further, affinity-purified antibodies to the amino-terminal portion of extracellular domain 3 transiently increased intracellular free Ca2+ concentration and thus are agonists. The Ca2+ signal was specifically blocked by the B2 antagonist HOE140. By contrast, antibodies to the carboxyl-terminal segment of extracellular domain 4 failed to trigger Ca2+ release. The specific effects of antibodies to the amino-terminal portion of extracellular domain 3 suggest that this portion of the B2 receptor may be involved in ligand binding and in agonist function.