A putative selectivity filter in the G-protein-coupled receptors for parathyroid hormone and secretion

Structural and functional insights into the juxtamembranous amino-terminal tail and extracellular loop regions of class B GPCRs

Class B guanine nucleotide-binding protein GPCRs share heptahelical topology and signalling via coupling with heterotrimeric G proteins typical of the entire superfamily of GPCRs. However, they also exhibit substantial structural differences from the more extensively studied class A GPCRs. Even their helical bundle region, most conserved across the superfamily, is predicted to differ from that of class A GPCRs. Much is now known about the conserved structure of the amino-terminal domain of class B GPCRs, coming from isolated NMR and crystal structures, but the orientation of that domain relative to the helical bundle is unknown, and even less is understood about the conformations of the juxtamembranous amino-terminal tail or of the extracellular loops linking the transmembrane segments. We now review what is known about the structure and function of these regions of class B GPCRs. This comes from indirect analysis of structure-function relationships elucidated by mutagenesis and/or ligand modification and from the more direct analysis of spatial approximation coming from photoaffinity labelling and cysteine trapping studies. Also reviewed are the limited studies of structure of some of these regions. No dominant theme was recognized for the structures or functional roles of distinct regions of these juxtamembranous portions of the class B GPCRs. Therefore, it is likely that a variety of molecular strategies can be engaged for docking of agonist ligands and for initiation of conformational changes in these receptors that would be expected to converge to a common molecular mechanism for activation of intracellular signalling cascades.

Identification of a contact site for residue 19 of parathyroid hormone (PTH) and PTH-related protein analogs in transmembrane domain two of the type 1 PTH receptor

Recent functional studies have suggested that position 19 in PTH interacts with the portion of the PTH-1 receptor (P1R) that contains the extracellular loops and seven transmembrance helices (TMs) (the J domain). We tested this hypothesis using the photoaffinity cross-linking approach. A PTHrP(1-36) analog and a conformationally constrained PTH(1-21) analog, each containing para-benzoyl-l-phenylalanine (Bpa) at position 19, each cross-linked efficiently to the P1R expressed in COS-7 cells, and digestive mapping analysis localized the cross-linked site to the interval (Leu232-Lys240) at the extracellular end of TM2. Point mutation analysis identified Ala234, Val235, and Lys240 as determinants of cross-linking efficiency, and the Lys240-->Ala mutation selectively impaired the binding of PTH(1-21) and PTH(1-19) analogs, relative to that of PTH(1-15) analogs. The findings support the hypothesis that residue 19 of the receptor-bound ligand contacts, or is close to, the P1R J domain-specifically, Lys240 at the extracellular end of TM2. The findings also support a molecular model in which the 1-21 region of PTH binds to the extracellular face of the P1R J domain as an alpha-helix.

Spatial approximation between two residues in the mid-region of secretin and the amino terminus of its receptor. Incorporation of seven sets of such constraints into a three-dimensional model of the agonist-bound secretin receptor

Photoaffinity labeling of receptors by bound agonists can provide important spatial constraints for molecular modeling of activated receptor complexes. Secretin is a 27-residue peptide hormone with a diffuse pharmacophoric domain that binds to the secretin receptor, a prototypic member of the Class B family of G protein-coupled receptors. In this work, we have developed, characterized, and applied two new photolabile probes for this receptor, with sites for covalent attachment in peptide positions 12 and 14, surrounding the previously most informative site of affinity labeling of this receptor. The [Tyr10,(BzBz)Lys12]rat secretin-27 probe covalently labeled receptor residue Val6, whereas the [Tyr10,(BzBz)Lys14]rat secretin-27 probe labeled receptor residue Pro38. When combined with previous photoaffinity labeling data, there are now seven independent sets of constraints distributed throughout the peptide and receptor amino-terminal domain that can be used together to generate a new molecular model of the ligand-occupied secretin receptor. The amino-terminal domain of this receptor presented a stable platform for peptide ligand interaction, with the amino terminus of the peptide hormone extended toward the transmembrane helix domain of the receptor. This provides clear insights into the molecular basis of natural ligand binding and supplies testable hypotheses regarding the molecular basis of activation of this receptor.

The extracellular domain of receptor activity-modifying protein 1 is sufficient for calcitonin receptor-like receptor function

A functional calcitonin gene-related peptide (CGRP) receptor requires dimerization of calcitonin receptor-like receptor (CRLR) with receptor activity-modifying protein 1 (RAMP 1). To determine the function of the three domains (extracellular, ECD; transmembrane, TM; and tail domains) of human RAMP 1, three mutants were constructed: RAMP 1 without the cytoplasmic tail, a chimera consisting of the ECD of RAMP 1 and the TM and tail of the platelet-derived growth factor receptor, and the ECD of RAMP 1 alone. These RAMP 1 mutants were examined for their ability to associate with CRLR to effect CGRP-stimulated cAMP accumulation, CGRP binding, CRLR trafficking, and cell surface expression. All RAMP 1 mutants were able to associate with CRLR with full efficacy for CGRP-stimulated cAMP accumulation. However, the RAMP 1/platelet-derived growth factor receptor chimera demonstrated a 10-fold decrease in potency for CGRP signaling and binding, and the RAMP 1-ECD mutant had a 4000-fold decrease in potency. In conclusion, the ECD of RAMP 1 is sufficient for normal CRLR association and efficacy. The presence of a TM domain and the specific sequence of the RAMP 1 TM domain contribute to CGRP affinity and potency. The C-terminal tail of RAMP 1 is unnecessary for CRLR function.

Different domains of the glucagon and glucagon-like peptide-1 receptors provide the critical determinants of ligand selectivity

(1) Glucagon and glucagon-like peptide-1 (GLP-1) are homologous peptide hormones with important functions in glucose metabolism. The receptors for glucagon and GLP-1 are homologous family B G-protein coupled receptors. The GLP-1 receptor amino-terminal extracellular domain is a major determinant of glucagon/GLP-1 selectivity of the GLP-1 receptor. However, the divergent residues in glucagon and GLP-1 that determine specificity for the GLP-1 receptor amino-terminal extracellular domain are not known. Less is known about how the glucagon receptor distinguishes between glucagon and GLP-1. (2) We analysed chimeric glucagon/GLP-1 peptides for their ability to bind and activate the glucagon receptor, the GLP-1 receptor and chimeric glucagon/GLP-1 receptors. The chimeric peptide GLP-1(7-20)/glucagon(15-29) was unable to bind and activate the glucagon receptor. Substituting the glucagon receptor core domain with the GLP-1 receptor core domain (chimera A) completely rescued the affinity and potency of GLP-1(7-20)/glucagon(15-29) without compromising the affinity and potency of glucagon. Substituting transmembrane segment 1 (TM1), TM6, TM7, the third extracellular loop and the intracellular carboxy-terminus of chimera A with the corresponding glucagon receptor segments re-established the ability to distinguish GLP-1(7-20)/glucagon(15-29) from glucagon. Corroborant results were obtained with the opposite chimeric peptide glucagon(1-14)/GLP-1(21-37). (3) The results suggest that the glucagon and GLP-1 receptor amino-terminal extracellular domains determine specificity for the divergent residues in the glucagon and GLP-1 carboxy-terminals respectively. The GLP-1 receptor core domain is not a critical determinant of glucagon/GLP-1 selectivity. Conversely, the glucagon receptor core domain contains two or more sub-segments which strongly determine specificity for divergent residues in the glucagon amino-terminus.

Human VPAC1 receptor selectivity filter. Identification of a critical domain for restricting secretin binding

The human VPAC1 receptor for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase activating peptide (PACAP) belongs to the class II family of G protein coupled receptors with seven transmembrane segments. It recognizes several VIP-related peptides and displays a very low affinity for secretin despite >70% homology between VIP and secretin. Conversely, the human secretin receptor has high affinity for secretin but low affinity for VIP. We took advantage of this reversed selectivity to identify a domain of the VPAC1 receptor responsible for selectivity toward secretin by constructing human VPAC1-secretin receptor chimeras. A first set of chimeras consisted of exchanging the entire N-terminal ectodomain or large parts of this domain. They were constructed by overlap PCR, transfected in COS-7 cells, and their ligand selectivity, expressed as the ratio of EC(50) for secretin/EC(50) for VIP (referred to as S/V), in stimulating cAMP production was measured. Two very informative chimeras respectively referred to as S144V and S123V were obtained by replacing the entire ectodomain or only the first 123 amino acids of the VPAC1 receptor by the corresponding sequences of the secretin receptor. Whereas S144V no longer discriminated between VIP and secretin (S/V = 1.2), S123V discriminated between the two peptides (S/V = 300) in the same manner as the wild-type VPAC1 receptor. The motif responsible for discrimination was determined by introducing small blocks or individual amino acids of secretin receptor in the 123-144 sequence of the S123V chimera. The data obtained from 14 new chimeras sustained that two nonadjacent pairs of amino acids, Gln(135) Thr(136) and Gly(140) Ser(141) in the C-terminal end of the N-terminal VPAC1 receptor ectodomain constitute a selective filter that strongly restricts access of secretin to the VPAC1 receptor.

Purification and characterization of a receptor for human parathyroid hormone and parathyroid hormone-related peptide

The human parathyroid hormone (PTH) receptor (hPTH1R), containing a 9-amino acid sequence of rhodopsin at its C terminus, was transiently expressed in COS-7 cells and solubilized with 0.25% n-dodecyl maltoside. Approximately 18 microg of hPTH1R were purified to homogeneity per mg of crude membranes by single-step affinity chromatography using 1D4, a monoclonal antibody to a rhodopsin epitope. The N terminus of the hPTH1R is Tyr(23), consistent with removal of the 22-amino acid signal peptide. Comparisons of hPTH1R by quantitative immunoblotting and Scatchard analysis revealed that 75% of the receptors in membrane preparations were functional; there was little, if any, loss of functional receptors during purification. The binding affinity of the purified hPTH1R was slightly lower than membrane-embedded hPTH1R (K(d) = 16.5 +/- 1.3 versus 11.9 +/- 1.9 nm), and the purified receptors bound rat [Nle(8,21),Tyr(34)]PTH-(1-34)-NH(2) (PTH-(1-34)), and rat [Ile(5),Trp(23),Tyr(36)]PTHrP-(5-36)-NH(2) with indistinguishable affinity. Maximal displacement of (125)I-PTH-(1-34) binding by rat [alpha-aminoisobutyric acid (Aib)(1,3),Nle(8),Gln(10),Har(11),Ala(12),Trp(14),Arg(19),Tyr(21)]PTH-(1-21)-NH(2) and rat [Aib(1,3),Gln(10),Har(11),Ala(12),Trp(14)]PTH-(1-14)-NH(2) of 80 and 10%, respectively, indicates that both N-terminal and juxtamembrane ligand binding determinants are functional in the purified hPTH1R. Finally, PTH stimulated [(35)S]GTP gamma S incorporation into G alpha(s) in a time- and dose-dependent manner, when recombinant hPTH1R, G alpha(s)-, and beta gamma-subunits were reconstituted in phospholipid vesicles. The methods described will enable structural studies of the hPTH1R, and they provide an efficient and general technique to purify proteins, particularly those of the class II G protein-coupled receptor family.

Analysis of parathyroid hormone (PTH)/secretin receptor chimeras differentiates the role of functional domains in the pth/ pth-related peptide (PTHrP) receptor on hormone binding and receptor activation

The type 1 parathyroid hormore receptor (PTH1r) belongs to the class II family of G protein-coupled receptors. To delineate the sites in the PTH1r's N-terminal region, and the carboxy-core domain (transmembrane segments + extracellular loops) involved in PTH binding, we have evaluated the functional properties of 27 PTH1-secretin chimeras receptors stably expressed in HEK-293 cells. The wild type and chimeric receptors were analyzed for cell surface expression, binding for PTH and secretin, and functional responsiveness (cAMP induction) toward secretin and PTH. The expression levels of the chimeric receptors were comparable to that of the PTH1r (60-100%). The N-terminal region of PTH1r was divided into three segments that were replaced either singly or in various combinations with the homologous region of the secretin receptor (SECr). Substitution of the carboxy-terminal half (residues 105-186) of the N-terminal region of PTH1r for a SECr homologous segment did not reduced affinity for PTH but abolished signaling in response to PTH. This data indicate that receptor activation is dissociable from high affinity hormone binding in the PTH1r, and that the N-terminal region might play a critical role in the activation process. Further segment replacements in the N-termini focus on residues 105-186 and particularly residues 146-186 of PTH1r as providing critical segments for receptor activation. The data obtained suggest the existence of two distinct PTH binding sites in the PTH1r's N-terminal region: one site in the amino-terminal half (residues 1-62) (site 1) that participates in high-affinity PTH binding; and a second site of lower affinity constituted by amino acid residues scattered throughout the carboxy-terminal half (residues 105-186) (site 2). In the absence of PTH binding to site 1, higher concentrations of hormone are required to promote receptor activation. In addition, elimination of the interaction of PTH with site 2 results in a loss of signal transduction without loss of high-affinity PTH binding. Divers substitutions of the extracellular loops of the PTH1r highlight the differential role of the first- and third extracellular loop in the process of PTH1r activation after hormone binding. A chimera containing the entire extracellular domains of the PTH1r and the transmembrane + cytoplasmic domains of SECr had very low PTH binding affinity and did not signal in response to PTH. Further substitution of helix 5 of PTH1r in this chimera increased affinity for PTH that is close to the PTH affinity for the wild-type PTH1r but surprisingly, did not mediate signaling response. Additional substitutions of PTH1r's helices in various combinations emphasize the fundamental role of helix 3 and helix 6 on the activation process of the PTH1r. Overall, our studies demonstrated that several PTH1r domains contribute differentially to PTH binding affinity and signal transduction mechanism and highlight the role of the N-terminal domain and helix 3 and helix 6 on receptor activation.

Receptor activity modifying proteins

Our understanding of G protein-coupled receptor (GPCR) function has recently expanded to encompass novel protein interactions that underlie both cell-surface receptor expression and the exhibited phenotype. The most notable examples are those involving receptor activity modifying proteins (RAMPs). RAMP association with the calcitonin (CT) receptor-like receptor (CRLR) traffics this receptor to the cell surface where individual RAMPs dictate the expression of unique phenotypes. A similar function has been ascribed to RAMP interaction with the CT receptor (CTR) gene product. This review examines our current state of knowledge of the mechanisms underlying RAMP function.

Structural insights into the amino-terminus of the secretin receptor: I. Status of cysteine and cystine residues

The secretin receptor is prototypic of the class II family of G protein-coupled receptors, with a long extracellular amino-terminal domain containing six highly conserved Cys residues and one Cys residue (Cys(11)) that is present only in the most closely related family members. This domain is critical for function, with some component Cys residues believed to be involved in key disulfide bonds, although these have never been directly demonstrated. Here, we examine the functional importance of each of these residues and determine their involvement in disulfide bonds. Secretin binding was markedly diminished after treating cells with cell-impermeant reducing reagents, supporting the presence of important extracellular disulfide bonds. To determine whether the amino-terminal domain was covalently attached to the receptor body by disulfide linkage, a strategy was implemented that involved introduction of an acid-labile Asp-Pro sequence to enable specific cleavage at the boundary of these domains. Under nonreducing conditions, the amino terminus was released from the receptor body, supporting the absence of covalent association between these domains. Quantitative [(14)C]iodoacetamide incorporation into the isolated amino-terminal domain of the receptor in the absence and presence of chemical reduction established the ratio of free to total Cys residues as 1:7, consistent with three disulfide bonds. Mutagenesis of each of the amino-terminal Cys residues to Ala was tolerated only for Cys(11), suggesting that these bonds linked the conserved Cys residues. This was further supported by treatment of intact cells expressing wild-type or C11A mutant secretin receptor with a cell-impermeant sulfhydryl-reactive reagent. Thus, the functionally important amino terminus of the secretin receptor represents a structurally independent, highly folded, and disulfide-bonded domain, with a pattern that is likely critical and conserved throughout this receptor family.

Identification of two pairs of spatially approximated residues within the carboxyl terminus of secretin and its receptor

The carboxyl-terminal domains of secretin family peptides have been shown to contain key determinants for high affinity binding to their receptors. In this work, we have examined the interaction between carboxyl-terminal residues within secretin and the prototypic secretin receptor. We previously utilized photoaffinity labeling to demonstrate spatial approximation between secretin residue 22 and the receptor domain that includes the first 30 residues of the amino terminus (Dong, M., Wang, Y., Pinon, D. I., Hadac, E. M., and Miller, L. J. (1999) J. Biol. Chem. 274, 903-909). Here, we further refined the site of labeling with the p-benzoyl-phenylalanine (Bpa(22)) probe to receptor residue Leu(17) using progressive cleavage of wild type and mutant secretin receptors (V13M and V16M) and sequence analysis. We also developed a new probe incorporating a photolabile Bpa at position 26 of secretin, closer to its carboxyl terminus. This analogue was also a potent agonist (EC(50) = 72 +/- 6 pm) and bound to the secretin receptor specifically and with high affinity (K(i) = 10.3 +/- 2.4 nm). It covalently labeled the secretin receptor at a single site saturably and specifically. This was localized to the segment between residues Gly(34) and Ala(41) using chemical and enzymatic cleavage of labeled wild type and A41M mutant receptor constructs and immunoprecipitation of epitope-tagged receptor fragments. Radiochemical sequencing identified the site of covalent attachment as residue Leu(36). These new insights, along with our recent report of contact between residue 6 within the amino-terminal half of secretin and this same amino-terminal region of this receptor (Dong, M., Wang, Y., Hadac, E. M., Pinon, D. I., Holicky, E. L., and Miller, L. J. (1999) J. Biol. Chem. 274, 19161-19167), support a key role for this region, making the molecular details of this interaction of major interest.

Role of second extracellular loop in the function of human vasoactive intestinal polypeptide/pituitary adenylate cyclase activating polypeptide receptor 1 (hVPAC1R)

To elucidate the functional role of the second extracellular loop of human vasoactive intestinal polypeptide/pituitary adenylate cyclase-activating polypeptide (VIP/PACAP) receptor (hVPAC1R), surface expression, ligand binding, and receptor activation were analyzed. Amino acids in the entire second extracellular loop were individually substituted by alanine by site-directed mutagenesis. The mutant and wild-type receptors were transiently expressed in HEK293 cells and purified cell membranes were tested for the ability to bind VIP, while the receptor activity was measured as potency of cAMP production analysed on intact cells. Surface expression of the substituted conserved residues, W286A, I289A, W294A, and W295A, was evidently decreased to 20-30% compared to the wild-type expression. W286A also showed an significantly reduced potency of cAMP production. Substituted residues as F280A, E281A, and G284A showed a significant reduction in the potency of stimulated cAMP production amounting to 8-46-fold, compared to the wild-type with unaffected surface expression and VIP binding. These results indicate that some residues in the second extracellular loop of the human VPAC1R participate in the active mechanism of a ligand-mediated response without being directly involved in the binding of VIP.

Photoaffinity cross-linking identifies differences in the interactions of an agonist and an antagonist with the parathyroid hormone/parathyroid hormone-related protein receptor

Analogs of parathyroid hormone (PTH)-related protein (PTHrP), singularly substituted with a photoreactive L-p-benzoylphenylalanine (Bpa) at each of the first 6 N-terminal positions, were pharmacologically evaluated in human embryonic kidney cells stably expressing the recombinant human PTH/PTHrP receptor. Two of these analogs, in which the photoreactive residue is either in position 1 or 2 (Bpa(1)- and Bpa(2)-PTHrP, respectively) displayed high affinity binding. Bpa(1)-PTHrP also displayed high efficacy for the stimulation of increased cAMP levels. Surprisingly, Bpa(2)-PTHrP was found to be a potent antagonist, despite the presence of the principal activation domain (sequence 1-6). Analysis of the digestion profiles of the ligand-receptor photoconjugates revealed that both the agonist and the antagonist cross-link to the S-CH(3) group of Met(425) in transmembrane domain 6 of the human PTH/PTHrP receptor. However, the antagonist Bpa(2)-PTHrP also cross-links to a proximal site within the receptor domain Pro(415)-Met(425). Unlike the antagonist Bpa(2)-PTHrP, the potent agonist Bpa(2)-PTH, also bearing the Bpa residue in position 2, cross-links only to the S-CH(3) group of Met(425) (similar to Bpa(1)-PTHrP and Bpa(1)-PTH). Taken together, these results suggest that the antagonist Bpa(2)-PTHrP is able to distinguish between two distinct conformations of the receptor. The comparison between PTHrP analogs substituted by Bpa at two consecutive positions and across PTH and PTHrP reveals insights into the PTH/PTHrP ligand-receptor bimolecular interaction at the level of a single amino acid.

Receptors for PTH and PTHrP: their biological importance and functional properties

The type 1 receptor (PTH1R) for parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) is a G protein-coupled receptor that is highly expressed in bone and kidney and mediates in these tissues the PTH-dependent regulation of mineral ion homeostasis. The PTH1R also mediates the paracrine actions of PTHrP, which play a particularly vital role in the process of endochondral bone formation. These important functions, the likely involvement of the PTH1R in certain genetic diseases affecting skeletal development and calcium homeostasis, and the potential utility of PTH in treating osteoporosis have been the driving force behind intense investigations of both the receptor and its peptide ligands. Recent lines of work have led to the identification of constitutively active PTH1Rs in patients with Jansen's metaphyseal chondrodysplasia, the demonstration of inverse agonism by certain ligand analogs, and the discovery of the PTH-2 receptor subtype that responds to PTH but not PTHrP. As reviewed herein, a detailed exploration of the receptor-ligand interaction process is currently being pursued through the use of site-directed mutagenesis and photoaffinity cross-linking methods; ultimately, such work could enable the development of novel PTH receptor ligands that have therapeutic value in treating diseases such as osteoporosis and certain forms of hypercalcemia.

Domains determining agonist selectivity in chimaeric VIP2 (VPAC2)/PACAP (PAC1) receptors

1 The VPAC2 and PAC1 receptors are closely related members of the Group II G protein-coupled receptor family. At the VPAC2 receptor, VIP is equipotent to PACAP-38 in stimulating cyclic AMP production, whereas at the PAC1 receptor PACAP-38 is many fold more potent than VIP. In this study, domains which confer this selectivity were investigated by constructing four chimaeric receptors in which segments of the VPAC2 receptor were exchanged with the corresponding segment from the PAC1 receptor. 2 When expressed in COS 7 cells all the chimaeric receptors bound the common ligand [125I]PACAP-27 and produced cyclic AMP in response to agonists. 3 Relative selectivity for agonists was determined primarily by the amino terminal extracellular domain of the PAC1 receptor and the VPAC2 receptor. The interchange of other domains had little effect on the potency of PACAP-38 or PACAP-27. 4 For chimaeric constructs with a PAC1 receptor amino terminal domain, the substitution of increasing portions of the VPAC2 receptor decreased the potency of VIP yet increased that of helodermin. 5 This suggests that the interaction of VIP/helodermin but not PACAP with the PAC1 receptor may be influenced (and differentially so) by additional receptor domains.

Quantitative cell membrane-based radioligand binding assays for parathyroid hormone receptors

Most current assays of PTH receptor ligand binding employ whole cells as the vehicle for receptor. Whole cell binding does not easily permit the estimation of physically meaningful binding parameters, the detection of multiple receptor states, or the evaluation of the effects of receptor modulators such as guanine nucleotides. We have developed quantitative methods for the measurement of equilibrium ligand binding parameters at cloned parathyroid hormone (PTH) receptors in cell membrane preparations. Centrifugation is used to separate bound and free [125I]-labeled peptide radioligands, and nonfat dried milk is used as a blocking agent to reduce nonspecific binding. This method is useful for measurement of agonist and antagonist radioligand binding at the PTH-1 receptor and binding of [125I]PTH(1-34) at the PTH-2 receptor. Less than 25% of [125I]PTH(1-34) or [125I]PTHrP(1-36) is degraded during the assay incubation. We demonstrated the utility of the assay using measurements of ligand binding properties at the PTH-1 receptor. (1) Homologous displacement experiments provided estimates of Kd and Bmax for the radioligands. (2) Displacement of radiolabeled antagonist binding ([125I]PTH(3-34)) by an unlabeled agonist (RS-66271) revealed multiple affinity states of agonist-receptor interaction. (3) Comparison of RS-66271 displacement in the presence and absence of GTP-gammaS demonstrated that the highest affinity state is guanine nucleotide-sensitive, suggesting that this state requires stabilization by G-protein. This assay thus allows more mechanistic interpretation of binding data than PTH binding assays in current use. A more convenient rapid-filtration method was also developed for measurement of radioligand binding at PTH-1 and PTH-2 receptors.

Role of the extracellular loops of G protein-coupled receptors in ligand recognition: a molecular modeling study of the human P2Y1 receptor

The P2Y1 receptor is a G protein-coupled receptor (GPCR) and is stimulated by extracellular ADP and ATP. Site-directed mutagenesis of the three extracellular loops (ELs) of the human P2Y1 receptor indicates the existence of two essential disulfide bridges (Cys124 in EL1 and Cys202 in EL2; Cys42 in the N-terminal segment and Cys296 in EL3) and several specific ionic and H-bonding interactions (involving Glu209 and Arg287). Through molecular modeling and molecular dynamics simulations, an energetically sound conformational hypothesis for the receptor has been calculated that includes transmembrane (TM) domains (using the electron density map of rhodopsin as a template), extracellular loops, and a truncated N-terminal region. ATP may be docked in the receptor, both within the previously defined TM cleft and within two other regions of the receptor, termed meta-binding sites, defined by the extracellular loops. The first meta-binding site is located outside of the TM bundle, between EL2 and EL3, and the second higher energy site is positioned immediately underneath EL2. Binding at both the principal TM binding site and the lower energy meta-binding sites potentially affects the observed ligand potency. In meta-binding site I, the side chain of Glu209 (EL2) is within hydrogen-bonding distance (2.8 A) of the ribose O3', and Arg287 (EL3) coordinates both alpha- and beta-phosphates of the triphosphate chain, consistent with the insensitivity in potency of the 5'-monophosphate agonist, HT-AMP, to mutation of Arg287 to Lys. Moreover, the selective reduction in potency of 3'NH2-ATP in activating the E209R mutant receptor is consistent with the hypothesis of direct contact between EL2 and nucleotide ligands. Our findings support ATP binding to at least two distinct domains of the P2Y1 receptor, both outside and within the TM core. The two disulfide bridges present in the human P2Y1 receptor play a major role in the structure and stability of the receptor, to constrain the loops within the receptor, specifically stretching the EL2 over the opening of the TM cleft and thus defining the path of access to the binding site.

Chondrocyte-matrix attachment complexes mediate survival and differentiation

Integrin mediated cell-extracellular matrix interactions are required for survival and differentiation of many cell types. In this review, the cell-matrix attachment complex (CMAX) is described for chondrocytes. The evidence that integrin-mediated signal transduction is necessary for normal chondrocyte differentiation and survival in various culture conditions and in vivo are reviewed. The possible signal transduction pathways stimulated by the extracellular matrix components are discussed with a review of current data from chondrocyte experiments. In addition, the influence of parathyroid hormone and transforming growth factor beta on chondrocyte survival has been included as they may function in concert with integrin mediated signal transduction. Finally, specific changes in gene expression preceding apoptosis are discussed. The current understanding of how integrin-mediated signals prevent apoptosis and implications of anchorage-dependent survival for development and differentiation of the chondrocyte phenotype are discussed.

A conserved threonine in the second extracellular loop of the human EP2 and EP4 receptors is required for ligand binding

G protein coupled receptors for prostaglandins are activated when agonists are bound to a binding pocket formed in part by the seven transmembrane domains. Recent studies have determined that substitution of a conserved threonine in the second extracellular loop of the prostaglandin EP3 receptor resulted in increased affinity for ligands with a C1 methyl ester moiety. The homologous threonine in the second extracellular loop of the human prostaglandin EP2 and EP4 receptors was mutated to alanine. When expressed in COS1 cells, detectable radioligand binding at both of these receptors bearing the threonine to alanine substitution (EP2T185A; EP4T168A) was abolished, as well as the receptors' ability to stimulate intracellular [cAMP]. In contrast, EP2 and EP4 receptors bearing conservative threonine to serine mutations (EP2T185S; EP4T168S) displayed Kd values for [3H]prostaglandin E2 similar to wild type receptors: 8.8 +/- 0.7 nM for EP2T185S compared to 12.9 +/- 1.2 nM for EP2 wild type; 2.0 +/- 0.8 nM for EP4T168S compared to 0.9 +/- 0.3 nM for the EP4 wild type receptor. The EC50 values for cAMP stimulation were 1.3 +/- 0.6 nM for EP2 wild type; 2.7 +/- 1.3 nM for EP2T185S; 1.1 +/- 0.3 nM for EP4 wild type; and 1.4 +/- 0.33 nM for EP4T168S. These studies suggest a critical role for the hydroxyl moiety on these conserved threonine residues at position 168/185 of the second extracellular loop in prostaglandin receptor-ligand interactions.

Evidence for a ligand interaction site at the amino-terminus of the parathyroid hormone (PTH)/PTH-related protein receptor from cross-linking and mutational studies

Low resolution mutational studies have indicated that the amino-terminal extracellular domain of the rat parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor (rP1R) interacts with the carboxyl-terminal portion of PTH-(1-34) or PTHrP-(1-36). To further define ligand-receptor interactions, we prepared a fully functional photoreactive analog of PTHrP, [Ile5,Bpa23,Tyr36]PTHrP-(1-36)-amide ([Bpa23]PTHrP, where Bpa is p-benzoyl-L-phenylalanine). Upon photolysis, radioiodinated [Bpa23]PTHrP covalently and specifically bound to the rP1R. CNBr cleavage of the broad approximately 80-kDa complex yielded a radiolabeled approximately 9-kDa non-glycosylated protein band that could potentially be assigned to rP1R residues 23-63, Tyr23 being the presumed amino-terminus of the receptor. This assignment was confirmed using a mutant rP1R (rP1R-M63I) that yielded, upon photoligand binding and CNBr digestion, a broad protein band of approximately 46 kDa, which was reduced to a sharp band of approximately 20 kDa upon deglycosylation. CNBr digestion of complexes formed with two additional rP1R double mutants (rP1R-M63I/L40M and rP1R-M63I/L41M) yielded non-glycosylated protein bands that were approximately 6 kDa in size, indicating that [Bpa23]PTHrP cross-links to amino acids 23-40 of the rP1R. This segment overlaps a receptor region previously identified by deletion mapping to be important for ligand binding. Alanine scanning of this region revealed two residues, Thr33 and Gln37, as being functionally involved in ligand binding. Thus, the convergence of photoaffinity cross-linking and mutational data demonstrates that the extreme amino-terminus of the rP1R participates in ligand binding.

Identification of binding domains of the growth hormone-releasing hormone receptor by analysis of mutant and chimeric receptor proteins

The hypothalamic peptide GH-releasing hormone (GHRH) stimulates the release of GH from the pituitary through binding and activation of the GHRH receptor, which belongs to the family of G protein-coupled receptors. The objective of this study was to identify regions of the receptor critical for interaction with the ligand by expressing and analyzing truncated and chimeric epitope-tagged GHRH receptors. Two truncated receptors, GHRHdeltaN, in which part of the N-terminal domain between the putative signal sequence and the first transmembrane domain was deleted, and GHRHdeltaC, which was truncated downstream of the first intracellular loop, were generated. Both the receptors were deficient in ligand binding, indicating that neither the N-terminal extracellular domain (N terminus) nor the membrane-spanning domains with the associated extracellular loops (C terminus) are alone sufficient for interaction with GHRH. In subsequent studies, chimeric proteins between the receptors for GHRH and vasoactive intestinal peptide (VIP) or secretin were generated, using the predicted start of the first transmembrane domain as the junction for the exchange of the N terminus between receptors. The chimeras having the N terminus of the GHRH receptor and the C terminus of either the VIP or secretin receptor (GNVC and GNSC) did not bind GHRH or activate adenylate cyclase after GHRH treatment. The reciprocal chimeras having the N terminus of either the VIP or secretin receptors and the C terminus of the GHRH receptor (VNGC and SNGC) bound GHRH and stimulated cAMP accumulation after GHRH treatment. These results suggest that although the N-terminal extracellular domain is essential for ligand binding, the transmembrane domains and associated extracellular loop regions of the GHRH receptor provide critical information necessary for specific interaction with GHRH.

Transmembrane residues together with the amino terminus limit the response of the parathyroid hormone (PTH) 2 receptor to PTH-related peptide

The mechanisms of ligand binding and receptor activation for G-protein-coupled receptors in the secretin/parathyroid hormone (PTH) receptor subfamily are not understood. The PTH1 receptor (PTH1R) signals in response to both PTH and parathyroid hormone-related peptide (PTHrP), whereas the PTH2 receptor (PTH2R) responds only to PTH, not to PTHrP. To locate PTHrP discriminatory domains in the PTH2R, we generated PTH1R/PTH2R chimeras in which the extracellular amino-terminal domains were exchanged. Production of cAMP in response to 1 microM PTHrP or PTH was identical in cells expressing the PTH1R with the PTH2R amino terminus and in cells expressing the PTH2R with the PTH1R amino terminus. The ability of the chimeric receptor with the PTH2R amino terminus to respond fully to PTHrP showed that the body of the PTH2R must contain sites that limit the response to PTHrP. Mutations to PTH1R sequence were therefore made in each of the seven transmembrane domains of the PTH2R. Mutations in transmembrane domains 3 and 7 resulted in receptors able to respond to PTHrP. Thus, residues in more than one domain form a barrier or filter, allowing the receptor to discriminate between different ligands.

Multiple regions of ligand discrimination revealed by analysis of chimeric parathyroid hormone 2 (PTH2) and PTH/PTH-related peptide (PTHrP) receptors

PTH and PTH-related peptide (PTHrP) bind to the PTH/PTHrP receptor and stimulate cAMP accumulation with similar efficacy. Only PTH activates the PTH2 receptor. To examine the structural basis for this selectivity, we analyzed receptor chimeras in which the amino terminus and third extracellular domains of the two receptors were interchanged. All chimeric receptors bound radiolabeled PTH with high affinity. Transfer of the PTH2 receptor amino terminus to the PTH/PTHrP receptor eliminated high-affinity PTHrP binding and significantly decreased activation by PTHrP. A PTH/PTHrP receptor N terminus modified by deletion of the nonhomologous E2 domain transferred weak PTHrP interaction to the PTH2 receptor. Introduction of the PTH2 receptor third extracellular loop into the PTH/PTHrP receptor increased the EC50 for PTH and PTHrP, while preserving high-affinity PTH binding and eliminating high-affinity PTHrP binding. Similarly, transfer of the PTH/PTHrP receptor third extracellular loop preserved high-affinity PTH binding by the PTH2 receptor but decreased its activation. Return of Gln440 and Arg394, corresponding residues in the PTH/PTHrP and PTH2 receptor third extracellular loops, to the parent residue restored function of these receptors. Simultaneous interchange of wild-type amino termini and third extracellular loops eliminated agonist activation but not binding for both receptors. Function was restored by elimination of the E2 domain in the receptor with a PTH/PTHrP receptor N terminus and return of Gln440/Arg394 to the parent sequence in both receptors. These data suggest that the amino terminus and third extracellular loop of the PTH2 and PTH/PTHrP receptors interact similarly with PTH, and that both domains contribute to differential interaction with PTHrP.

Residues in the membrane-spanning and extracellular loop regions of the parathyroid hormone (PTH)-2 receptor determine signaling selectivity for PTH and PTH-related peptide

The parathyroid hormone (PTH)-2 receptor displays strong ligand selectivity in that it responds fully to PTH but not at all to PTH-related peptide (PTHrP). In contrast, the PTH-1 receptor (PTH/PTHrP receptor) responds fully to both ligands. Previously it was shown that two divergent residues in PTH and PTHrP account for PTH-2 receptor selectivity; position 23 (Trp in PTH and Phe in PTHrP) determines binding selectivity and position 5 (Ile in PTH and His in PTHrP) determines signaling selectivity. To identify sites in the PTH-2 receptor involved in discriminating between His5 and Ile5, we constructed PTH-2 receptor/PTH-1 receptor chimeras, expressed them in COS-7 cells, and tested for cAMP responsiveness to [Trp23] PTHrP-(1-36), and to the nondiscriminating peptide [Ile5, Trp23]PTHrP-(1-36) (the Phe23 --> Trp modification enabled high affinity binding of each ligand to the PTH-2 receptor). The chimeras revealed that the membrane-spanning/loop region of the receptor determined His5/Ile5 signaling selectivity. Subsequent analysis of smaller cassette substitutions and then individual point mutations led to the identification of two single residues that function as major determinants of residue 5 signaling selectivity. These residues, Ile244 at the extracellular end of transmembrane helix 3, and Tyr318 at the COOH-terminal portion of extracellular loop 2, are replaced by Leu and Ile in the PTH-1 receptor, respectively. The results thus indicate a functional interaction between two residues in the core region of the PTH-2 receptor and residue 5 of the ligand.

The second extracellular loop of the prostaglandin EP3 receptor is an essential determinant of ligand selectivity

The prostaglandin EP3 receptor binds Prostaglandin E2 in a ligand binding pocket formed in part by seven transmembrane alpha-helices. The present studies demonstrate that the second extracellular loop of the receptor is involved in prostanoid ligand recognition as well. Site-directed mutagenesis of seven conserved residues clustered in the amino portion of the second extracellular loop was performed. Receptors with single amino acid substitutions at each of these positions were transiently transfected into HEK293tsA201 cells, their ligand binding profiles assessed, and each receptor was tested for its ability to decrease intracellular cAMP levels. Substitution of Trp199 or Thr202 with alanine resulted in receptors with increases in affinity up to 128-fold for prostanoid compounds with a C1 methyl ester but wild type affinities for natural prostanoid ligands that have a carboxylate moiety at the C1 position. In contrast, substitution of Pro200 with serine caused a loss of selectivity up to 20-fold for naturally occurring prostanoid agonists as compared with the wild type EP3 receptor: the PS200 receptor displayed a decrease in affinity for E-ring compounds and an increase in affinity for F- and D-ring compounds. The EC50 for inhibition of cAMP remained unchanged for each receptor tested.

Chimeric proteins can exceed the sum of their parts: implications for evolution and protein design

Chimeric analogs derived from pairs of homologous proteins routinely exhibit activities found in one or both parents. We describe chimeras of two glycoprotein hormones, human chorionic gonadotropin (hCG) and human follitropin (hFSH), that exhibit activity unique to a third family member, human thyrotropin (hTSH). The results show that biological activity can be separated from hormone-specific amino acid residues. This is consistent with a model for the evolution of homologous ligand-receptor pairs involving gene duplication and the creation of inhibitory determinants that restrict binding. Disruption of these determinants can unmask activities characteristic of other members of a protein family. Combining portions of two ligands to create analogs with properties of a third family member can facilitate identifying key determinants of protein-protein interaction and may be a useful strategy for creating novel therapeutics. In the case of the glycoprotein hormones, this showed that two different hormone regions (i.e., the seat-belt and the intersubunit groove) appear to limit inappropriate contacts with receptors for other members of this family. These observations also have important caveats for chimera-based protein design because an unexpected gain of function may limit the therapeutic usefulness of some chimeras.

Full activation of chimeric receptors by hybrids between parathyroid hormone and calcitonin. Evidence for a common pattern of ligand-receptor interaction

Calcitonin (CT) and parathyroid hormone (PTH), whose receptors belong to the same family of G protein-coupled receptors, share no amino acid sequence homology and selectively activate either CT or PTH receptors. We now show, however, that reciprocal hybrid ligands (CT/PTH and PTH/CT), which do not activate the "wild-type" receptors, activate PTH/CT and CT/PTH receptor chimeras, respectively. Our findings indicate that PTH and CT share a similar architecture with at least two functional, receptor-specific domains. These domains are sufficiently independent to permit synthetic hybrid ligands to efficiently activate appropriate receptor chimeras. Therefore, both ligands follow, despite their very different primary sequences, a common pattern of ligand-receptor interaction.