Site-directed mutagenesis of conserved charged residues in the helical region of the human C5a receptor. Arg2O6 determines high-affinity binding sites of C5a receptor

Structural biology of complement receptors

The complement system plays crucial roles in a wide breadth of immune and inflammatory processes and is frequently cited as an etiological or aggravating factor in many human diseases, from asthma to cancer. Complement receptors encompass at least eight proteins from four structural classes, orchestrating complement-mediated humoral and cellular effector responses and coordinating the complex cross-talk between innate and adaptive immunity. The progressive increase in understanding of the structural features of the main complement factors, activated proteolytic fragments, and their assemblies have spurred a renewed interest in deciphering their receptor complexes. In this review, we describe what is currently known about the structural biology of the complement receptors and their complexes with natural agonists and pharmacological antagonists. We highlight the fundamental concepts and the gray areas where issues and problems have been identified, including current research gaps. We seek to offer guidance into the structural biology of the complement system as structural information underlies fundamental and therapeutic research endeavors. Finally, we also indicate what we believe are potential developments in the field.

Investigating the Dynamic Binding Behavior of PMX53 Cooperating with Allosteric Antagonist NDT9513727 to C5a Anaphylatoxin Chemotactic Receptor 1 through Gaussian Accelerated Molecular Dynamics and Free-Energy Perturbation Simulations

C5a anaphylatoxin chemotactic receptor 1 (C5aR1) is an important target in anti-inflammatory therapeutics. The cyclic peptide antagonist PMX53 binds to the orthosteric site located in the extracellular vestibule of C5aR1, and the non-peptide antagonist NDT9513727 binds to the allosteric site formed by the middle region of TM3 (trans-membrane helix), TM4, and TM5. We catch a sight of the variational binding mode of PMX53 during the Gaussian accelerated molecular dynamic (GaMD) simulations. In the binary complex of C5aR1 and PMX53, the PMX53 takes a dynamic binding mechanism during the simulation. Namely, the side chain of Arg⁶ of PMX53 extends to TM6-TM7 (pose 1) or swings to TM5 (pose 2), forming a salt bridge with Glu199. Meanwhile, in the ternary complex of C5aR1 with PMX53 and NDT9513727, the side chain of Arg⁶ of PMX53 swings to TM5 (pose 2) from extending to TM6-TM7 (pose 1) at the beginning of the GaMD simulation. In subsequent simulation, PMX53 stabilizes in the pose 2 binding mode by forming a stable salt bridge with Glu199. The free-energy perturbation (FEP) calculations demonstrate that pose 1 (ΔG_binding = -10.94 kcal/mol) is more stable in the binary complex and pose 2 (ΔG_binding = -7.91 kcal/mol) is unstable because of highly dynamic TM5. NDT9513727 interacts directly with TM4 and TM5 and stabilizes the hydrophobic stack between the extracellular sides of the two helices. Therefore, pose 2 (ΔG_binding = -16.27 kcal/mol) is notably stable than pose 1 (ΔG_binding = -9.78 kcal/mol) in the ternary complex. The identification of a novel binding mode of PMX53 and the detailed structural information of PMX53 interacting with a receptor obtained by GaMD simulations will be helpful in designing potent antagonists of C5aR1.

Structural basis for recognition of N-formyl peptides as pathogen-associated molecular patterns

The formyl peptide receptor 1 (FPR1) is primarily responsible for detection of short peptides bearing N-formylated methionine (fMet) that are characteristic of protein synthesis in bacteria and mitochondria. As a result, FPR1 is critical to phagocyte migration and activation in bacterial infection, tissue injury and inflammation. How FPR1 distinguishes between formyl peptides and non-formyl peptides remains elusive. Here we report cryo-EM structures of human FPR1-Gi protein complex bound to S. aureus-derived peptide fMet-Ile-Phe-Leu (fMIFL) and E. coli-derived peptide fMet-Leu-Phe (fMLF). Both structures of FPR1 adopt an active conformation and exhibit a binding pocket containing the R201^5.38XXXR205^5.42 (RGIIR) motif for formyl group interaction and receptor activation. This motif works together with D106^3.33 for hydrogen bond formation with the N-formyl group and with fMet, a model supported by MD simulation and functional assays of mutant receptors with key residues for recognition substituted by alanine. The cryo-EM model of agonist-bound FPR1 provides a structural basis for recognition of bacteria-derived chemotactic peptides with potential applications in developing FPR1-targeting agents.

Detection of invading bacteria is key to immunity. Here, the authors report cryo-electron microscopy structures of agonist-bound formyl peptide receptor 1 (FPR1), that reveal structural basis for recognition of bacteria-derived formyl peptides.

Complement Receptors and Their Role in Leukocyte Recruitment and Phagocytosis

The complement system is deeply embedded in our physiology and immunity. Complement activation generates a multitude of molecules that converge simultaneously on the opsonization of a target for phagocytosis and activation of the immune system via soluble anaphylatoxins. This response is used to control microorganisms and to remove dead cells, but also plays a major role in stimulating the adaptive immune response and the regeneration of injured tissues. Many of these effects inherently depend on complement receptors expressed on leukocytes and parenchymal cells, which, by recognizing complement-derived molecules, promote leukocyte recruitment, phagocytosis of microorganisms and clearance of immune complexes. Here, the plethora of information on the role of complement receptors will be reviewed, including an analysis of how this functionally and structurally diverse group of molecules acts jointly to exert the full extent of complement regulation of homeostasis.

GPCR-I-TASSER: A Hybrid Approach to G Protein-Coupled Receptor Structure Modeling and the Application to the Human Genome

Experimental structure determination remains difficult for G protein-coupled receptors (GPCRs). We propose a new hybrid protocol to construct GPCR structure models that integrates experimental mutagenesis data with ab initio transmembrane (TM) helix assembly simulations. The method was tested on 24 known GPCRs where the ab initio TM-helix assembly procedure constructed the correct fold for 20 cases. When combined with weak homology and sparse mutagenesis restraints, the method generated correct folds for all the tested cases with an average Cα root-mean-square deviation 2.4 Å in the TM regions. The new hybrid protocol was applied to model all 1,026 GPCRs in the human genome, where 923 have a high confidence score and are expected to have correct folds; these contain many pharmaceutically important families with no previously solved structures, including Trace amine, Prostanoids, Releasing hormones, Melanocortins, Vasopressin, and Neuropeptide Y receptors. The results demonstrate new progress on genome-wide structure modeling of TM proteins.

Structural and functional characterization of human and murine C5a anaphylatoxins

Complement is an ancient part of the innate immune system that plays a pivotal role in protection against invading pathogens and helps to clear apoptotic and necrotic cells. Upon complement activation, a cascade of proteolytic events generates the complement effectors, including the anaphylatoxins C3a and C5a. Signalling through their cognate G-protein coupled receptors, C3aR and C5aR, leads to a wide range of biological events promoting inflammation at the site of complement activation. The function of anaphylatoxins is regulated by circulating carboxypeptidases that remove their C-terminal arginine residue, yielding C3a-desArg and C5a-desArg. Whereas human C3a and C3a-desArg adopt a canonical four-helix bundle fold, the conformation of human C5a-desArg has recently been described as a three-helix bundle. Here, the crystal structures of an antagonist version of human C5a, A8(Δ71-73), and of murine C5a and C5a-desArg are reported. Whereas A8(Δ71-73) adopts a three-helix bundle conformation similar to human C5a-desArg, the two murine proteins form a four-helix bundle. A cell-based functional assay reveals that murine C5a-desArg, in contrast to its human counterpart, exerts the same level of activition as murine C5a on its cognate receptor. The role of the different C5a conformations is discussed in relation to the differential activation of C5a receptors across species.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.

The role of the anaphylatoxins in health and disease

The anaphylatoxin (AT) C3a, C5a and C5a-desArg are generally considered pro-inflammatory polypeptides generated after proteolytic cleavage of C3 and C5 in response to complement activation. Their well-appreciated effector functions include chemotaxis and activation of granulocytes, mast cells and macrophages. Recent evidence suggests that ATs are also generated locally within tissues by pathogen-, cell-, or contact system-derived proteases. This local generation of ATs is important for their pleiotropic biologic effects beyond inflammation. The ATs exert most of the biologic activities through ligation of three cognate receptors, i.e. the C3a receptor, the C5a receptor and the C5a receptor-like, C5L2. Here, we will discuss recent findings suggesting that ATs regulate cell apoptosis, lipid metabolism as well as innate and adaptive immune responses through their impact on antigen-presenting cells and T cells. As we will outline, such regulatory functions of ATs and their receptors play important roles in the pathogenesis of allergy, autoimmunity, neurodegenerative diseases, cancer and infections with intracellular pathogens.

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

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

Random mutagenesis of the complement factor 5a (C5a) receptor N terminus provides a structural constraint for C5a docking

The N terminus of G protein-coupled receptors has been implicated in binding to peptide hormones. We have used random saturation mutagenesis to identify essential residues in the N terminus of the human complement factor 5a receptor (C5aR). In a library of N-terminal mutant C5aR molecules screened for activation by C5a, residues 24-30 of the C5aR showed a marked propensity to mutate to cysteine, most likely indicating that sulfhydryl groups at these positions are appropriately situated to form disulfide interactions with the unpaired Cys(27) of human C5a. This presumptive spatial constraint allowed the ligand to be computationally docked to the receptor to form a model of the C5a/C5aR interaction. When the N-terminal mutant C5aR library was rescreened with C5a C27R, a ligand incapable of disulfide interactions, no individual position in the N terminus was essential for receptor signaling. However, the region 19-29 was relatively highly conserved in the functional mutants, further demonstrating that this region of the C5aR makes a productive physiologic interaction with the C5a ligand.

Genetic analysis of the first and third extracellular loops of the C5a receptor reveals an essential WXFG motif in the first loop

The extracellular loops of G protein-coupled receptors (GPCRs) frequently contain binding sites for peptide ligands. However, the mechanism of receptor activation following ligand binding and the influence of the extracellular loops in other aspects of receptor function are poorly understood. Here we report a structure-function analysis of the first and third extracellular loops of the human C5a receptor, a GPCR that binds a 74-amino acid peptide ligand. Amino acid substitutions were randomly incorporated into each loop, and functional receptors were identified in yeast. The first extracellular loop contains a large number of positions that cannot tolerate amino acid substitutions, especially residues within the WXFG motif found in many rhodopsin-like GPCRs, yet disruption of these residues does not alter C5a binding affinity. These results demonstrate an unanticipated role for the first extracellular loop, and the WXFG motif in particular, in ligand-mediated activation of the C5a receptor. This motif likely serves a similar role in other GPCRs. The third extracellular loop, in contrast, contains far fewer preserved residues and appears to play a less essential role in receptor activation.

Molecular characterization of the gerbil C5a receptor and identification of a transmembrane domain V amino acid that is crucial for small molecule antagonist interaction

Anaphylatoxin C5a is a potent inflammatory mediator associated with pathogenesis and progression of several inflammation-associated disorders. Small molecule C5a receptor (C5aR) antagonist development is hampered by species-specific receptor biology and the associated inability to use standard rat and mouse in vivo models. Gerbil is one rodent species reportedly responsive to small molecule C5aR antagonists with human C5aR affinity. We report the identification of the gerbil C5aR cDNA using a degenerate primer PCR cloning strategy. The nucleotide sequence revealed an open reading frame encoding a 347-amino acid protein. The cloned receptor (expressed in Sf9 cells) bound recombinant human C5a with nanomolar affinity. Alignment of the gerbil C5aR sequence with those from other species showed that a Trp residue in transmembrane domain V is the only transmembrane domain amino acid unique to small molecule C5aR antagonist-responsive species (i.e. gerbil, human, and non-human primate). Site-directed mutagenesis was used to generate human and mouse C5aRs with a residue exchange of this Trp residue. Mutation of Trp to Leu in human C5aR completely eliminated small molecule antagonist-receptor interaction. In contrast, mutation of Leu to Trp in mouse C5aR enabled small molecule antagonist-receptor interaction. This crucial Trp residue is located deeper within transmembrane domain V than residues reportedly involved in C5a- and cyclic peptide C5a antagonist-receptor interaction, suggesting a novel interaction site(s) for small molecule antagonists. These data provide insight into the basis for small molecule antagonist species selectivity and further define sites critical for C5aR activation and function.

Identification of Determinants of Ligand Binding Affinity and Selectivity in the Prostaglandin D2 Receptor CRTH2

The chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) is a G protein-coupled receptor that mediates the pro-inflammatory effects of prostaglandin D(2) (PGD(2)) generated in allergic inflammation. The CRTH2 receptor shares greatest sequence similarity with chemoattractant receptors compared with prostanoid receptors. To investigate the structural determinants of CRTH2 ligand binding, we performed site-directed mutagenesis of putative mCRTH2 ligand-binding residues, and we evaluated mutant receptor ligand binding and functional properties. Substitution of alanine at each of three residues in the transmembrane (TM) helical domains (His-106, TM III; Lys-209, TM V; and Glu-268, TM VI) and one in extracellular loop II (Arg-178) decreased PGD(2) binding affinity, suggesting that these residues play a role in binding PGD(2). In contrast, the H106A and E268A mutants bound indomethacin, a nonsteroidal anti-inflammatory drug, with an affinity similar to the wild-type receptor. HEK293 cells expressing the H106A, K209A, and E268A mutants displayed reduced inhibition of intracellular cAMP and chemotaxis in response to PGD(2), whereas the H106A and E268A mutants had functional responses to indomethacin similar to the wild-type receptor. Binding of PGE(2) by the E268A mutant was enhanced compared with the wild-type receptor, suggesting that Glu-268 plays a role in determining prostanoid ligand selectivity. Replacement of Tyr-261 with phenylalanine did not affect PGD(2) binding but decreased the binding affinity for indomethacin. These results provided the first details of the ligand binding pocket of an eicosanoid-binding chemoattractant receptor.

Site-specific disulfide capture of agonist and antagonist peptides on the C5a receptor

The manner by which peptidic ligands bind and activate their corresponding G-protein-coupled receptors is not well understood. One of the better characterized peptidic ligands is the chemotactic cytokine complement factor 5a (C5a), a 74-amino acid helical bundle. Previous studies showed 6-mer peptide analogs derived from the C terminus of the C5a ligand can bind to C5aR (Kd values approximately 0.1-1 microm) and either agonize or antagonize the receptor (Gerber, B. O., Meng, E. C., Dotsch, V., Baranski, T. J., and Bourne, H. R. (2001) J. Biol. Chem. 276, 3394-3400). Here, we provide direct biochemical data using disulfide trapping to support a model that these peptides bind within a transmembrane helical triad formed by alpha-helices III, VI, and VII. We show that the three amino acids on the C terminus of the peptide analogs bind too weakly to exert a functional effect themselves. However, when a cysteine residue is placed on their N terminus they can be trapped by disulfide interchange to specific cysteines in helix III and VI and not to other cysteines, engineered into the C5aR. The trapped peptides function as agonists or partial antagonists, similar to the non-covalent parents from which they were derived. These data help to further refine the binding mode for C5a to the C5aR and suggest an approach and a binding site that may be applicable to studying other peptide binding receptors.

The dark side of C5a in sepsis

Sepsis is a major clinical problem for which therapeutic interventions have been largely unsuccessful, in spite of promising strategies that were successful in animals, especially rodents. There is new evidence that sepsis causes excessive activation of the complement system and that this induces paralysis of innate immune functions in phagocytic cells due to effects of the powerful complement-activation product, C5a. This review describes our present understanding of how and why sepsis is a life-threatening condition and how it might be more effectively treated.

Characterisation of C5a receptor agonists from phage display libraries

C5a des-Arg(74) has a 10- to 100-fold lower receptor binding affinity than intact C5a and is only a partial agonist. We have used phage display selection from randomly mutated C5a des-Arg(74) libraries to isolate variant proteins that can activate C5a receptors with similar potency to C5a. Here we explore the interactions of three variants (V1-3) with C5aR mutated at residues involved in the differential response. The mutant Asp(282)Arg-C5aR is preferentially activated by C5a des-Arg(74), probably due to repulsion between Arg(74) of C5a and the substituent Arg(282). In accordance with this hypothesis, V2 (with a polar C-terminus which has no Arg residue) but not V1 (with a C-terminal Arg residue at position 73) could activate Asp(282)Arg-C5aR. V3, with a very hydrophobic C-terminus, was the most potent agonist at Asp(282)Arg-C5aR. Arg(175) is a potential counterion for the C-terminal carboxylate of C5a. C5aR mutated to either Ala or Asp at this position lost nearly all responsiveness to both C5a and C5a des-Arg(74), suggesting that mutation of Arg(175) caused a non-specific loss of receptor conformation and a loss of signalling capacity. However, V3 could still activate Arg(175)Asp/Ala-C5aR with the same potency as wild-type C5aR, demonstrating that the mutant receptors retained high signalling capability and showed a specific loss of responsiveness. Thus C5a des-Arg(74) variants produced by phage display are potentially useful tools for the dissection of ligand-receptor interactions.

C5a mutants are potent antagonists of the C5a receptor (CD88) and of C5L2: position 69 is the locus that determines agonism or antagonism

The anaphylatoxin C5a exerts a plethora of biologic activities critical in the pathogenesis of systemic inflammatory diseases. Recently, we reported on a C5a mutant, jun/fos-A8, as a potent antagonist for the human and mouse C5a receptor (CD88). Addressing the molecular mechanism accounting for CD88 receptor antagonism by site-directed mutagenesis, we found that a positively charged amino acid at position 69 is crucial. Replacements by either hydrophobic or negatively charged amino acids switched the CD88 antagonist jun/fos-A8 to a CD88 agonist. In addition to CD88, the seven-transmembrane receptor C5L2 has recently been found to provide high affinity binding sites for C5a and its desarginated form, C5adesArg74. A jun/fos-A8 mutant in which the jun/ fos moieties and amino acids at positions 71-73 were deleted, A8Delta71-73, blocked C5a and C5adesArg74 binding to CD88 and C5L2. In contrast, the cyclic C5a C-terminal analog peptide AcF-[OP-d-ChaWR] inhibited binding of the two anaphylatoxins to CD88 but not to C5L2, suggesting that the C5a core segment is important for high affinity binding to C5L2. Both receptors are coexpressed on human monocytes and the human mast cell line HMC-1; however, C5L2 expression on monocytes is weaker as compared with HMC-1 cells and highly variable. In contrast, no C5L2 expression was found on human neutrophils. A8Delta71-73 is the first antagonist that blocks C5a and C5adesArg74 binding to both C5a receptors, CD88 and C5L2, making it a valuable tool for studying C5L2 functions and for blocking the biological activities of C5a and C5adesArg74 in mice and humans.

Structure-function relationships of human C5a and C5aR

Using peptides that represent linear regions of the powerful complement activation product, C5a, or loops that connect the four alpha helices of C5a, we have defined the ability of these peptides to reduce binding of (125)I-C5a to human neutrophils, inhibit chemotactic responses of neutrophils to C5a, and reduce H(2)O(2) production in neutrophils stimulated with PMA. The data have defined likely sites of interaction of C5a with C5aR. The peptides had no functional activity per se on neutrophils and did not interfere with neutrophil responses to the unrelated chemotactic peptide, N-formyl-Met-Leu-Phe. Although previous data have suggested that there are two separate sites on C5a reactive with C5aR, the current data suggest that C5a interacts with C5aR in a manner that engages three discontinuous regions of C5a.

Constitutive activation and endocytosis of the complement factor 5a receptor: evidence for multiple activated conformations of a G protein-coupled receptor

Serpentine receptors relay hormonal or sensory stimuli to heterotrimeric guanine nucleotide-binding proteins (G proteins). In most G protein-coupled receptors (GPCRs), binding of the agonist ligand elicits both stimulation of the G protein and endocytosis of the receptor. We have begun to address whether these responses reflect the same sets of conformational changes in the receptor using constitutively active mutants of the human complement factor 5a receptor (C5aR). Two different mutant receptors both constitutively activate G protein-mediated responses, but one (F251A) is endocytosed only in response to ligand stimulation, while the other (NQ) is constitutively internalized in the absence of ligand. Both the constitutive and ligand-dependent endocytosis are accompanied by recruitment of beta-arrestin to the receptor. An inactivating mutation (N296A) complements the NQ mutation, producing a receptor that is activated only upon exposure to agonist; this revertant receptor (NQ/N296A) is nevertheless constitutively endocytosed. Thus one mutant (F251A) requires agonist for triggering endocytosis but not for activation of the downstream G protein signal, while another (NQ/N296A) behaves in the opposite fashion. Dissociation of two responses normally dependent on agonist binding indicates that the corresponding functions of an activated GPCR reflect different sets of changes in the receptor's conformation.

Mapping the ligand-binding site on the C5a receptor: arginine74 of C5a contacts aspartate282 of the C5a receptor

The interaction between the anaphylatoxin C5a and its receptor involves two distinct sites. One site is formed by acidic residues at the receptor N-terminus and contributes to only ligand binding. The second site, responsible for activation, is less well defined. In this study, we demonstrate that the receptor residue D(282), near the extracellular face of transmembrane domain VII, is a component of the second ligand-binding site. Mutation of D(282) to A decreases the sensitivity of the receptor to activation by intact C5a but not by its less potent metabolite, C5adR(74), which lacks the C-terminal arginine(74). The mutation of the R(74) residue of C5a to A causes a 60-fold decrease in wild-type receptor sensitivity, but only a 2-fold decrease for the receptor mutated at D(282). In contrast, the mutation of R(74) to D makes C5a completely inactive on both wild-type and A(282) C5a receptors. The mutation of D(282) to R partly restores the response to C5a[D(74)], which is a more effective ligand than C5a at the mutant receptor. A peptide mimic of the C5a activation domain with a C-terminal R potently activates the wild type but is only a weak agonist at the mutant D(282)R-C5a receptor. Conversely, a peptide with D at the C-terminus is a more effective activator of D(282)R than wild-type C5a receptors. These data indicate that the R(74) side chain of C5a makes an interaction with receptor D(282) that is responsible for the higher potency of intact C5a versus that of C5adR(74).

Species dependence for binding of small molecule agonist and antagonists to the C5a receptor on polymorphonuclear leukocytes

This study investigated the receptor binding affinities of a C5a agonist and cyclic antagonists for polymorphonuclear leukocytes (PMNs) isolated from human, sheep, pig, dog, rabbit, guinea pig, rat and mouse. The affinities of the two small molecule antagonists, F-[OPdChaWR] and AcF-[OPdChaWR], and the agonist, YSFKPMPLaR, revealed large differences in C5a receptor (C5aR) affinities between species. The antagonists bound to human, rat and dog PMNs with similar high affinities, but with lower affinities to PMNs from all other species. The C5a agonist also bound with varying affinities between species, but showed a different affinity profile to the antagonists. In contrast, recombinant human C5a had similar affinity for PMNs of all species investigated. The low correlation between the affinities of the antagonists and the agonist between species either suggests that different receptor residues are important for distinguishing between agonist/antagonist binding, or that the agonist and antagonist peptides bind to two distinct sites within the C5aR.

Identification of ligand effector binding sites in transmembrane regions of the human G protein-coupled C3a receptor

The human C3a anaphylatoxin receptor (C3aR) is a G protein-coupled receptor (GPCR) composed of seven transmembrane alpha-helices connected by hydrophilic loops. Previous studies of chimeric C3aR/C5aR and loop deletions in C3aR demonstrated that the large extracellular loop2 plays an important role in noneffector ligand binding; however, the effector binding site for C3a has not been identified. In this study, selected charged residues in the transmembrane regions of C3aR were replaced by Ala using site-directed mutagenesis, and mutant receptors were stably expressed in the RBL-2H3 cell line. Ligand binding studies demonstrated that R161A (helix IV), R340A (helix V), and D417A (helix VII) showed no binding activity, although full expression of these receptors was established by flow cytometric analysis. C3a induced very weak intracellular calcium flux in cells expressing these three mutant receptors. H81A (helix II) and K96A (helix III) showed decreased ligand binding activity. The calcium flux induced by C3a in H81A and K96A cells was also consistently reduced. These findings suggest that the charged transmembrane residues Arg161, Arg340, and Asp417 in C3aR are essential for ligand effector binding and/or signal coupling, and that residues His81 and Lys96 may contribute less directly to the overall free energy of ligand binding. These transmembrane residues in C3aR identify specific molecular contacts for ligand interactions that account for C3a-induced receptor activation.

Ligand binding sites on guinea pig C3aR: point and deletion mutations in the large extracellular loop and vicinity

Human C3a receptor (huC3aR) belongs to the G-protein coupled receptor family chacterized by having seven transmembrane domains. The huC3aR is a unique member of this family having a large extracellular (EC) loop of 175 amino acids between the 4th and 5th transmembrane domains. Based on a comparison of C3aR sequences from several species, a number of charged and conserved amino acids (Asp182, Asp309, Asp310, and Arg331) in and near the large EC loop of guinea pig C3aR were replaced using site-directed mutagenesis. Competitive binding assays showed that changing Arg331 in guinea pig C3aR to Ala (or Gln), but not changing Asp182, Asp309, or Asp310 to Ala, resulted in complete loss of ligand binding activity. These results and major EC loop deletions demonstrated that an essential C3a binding site is present in the transmembrane portion of C3aR, but not in the large EC loop. Replacement of Arg331 by a noncharged residue was sufficient to eliminate ligand-receptor interactions.

Receptor activation by human C5a des Arg74 but not intact C5a is dependent on an interaction between Glu199 of the receptor and Lys68 of the ligand

Despite the expression of only one type of receptor, there is great variation in the ability of different cell types to discriminate between C5a and its more stable metabolite, C5a des Arg74. The mechanism that underlies this phenomenon is not understood but presumably involves differences in the interaction with the C5a receptor. In this paper, we have analyzed the effects of a substitution mutation of the receptor (Glu199 --> Lys199) and the corresponding reciprocal mutants (Lys68 --> Glu68) of C5a, C5a des Arg74 and peptide analogues of the C-terminus of C5a on the ability of the C5a receptor to discriminate between ligands with and without Arg74. The use of these mutants indicates that the Lys68/Glu199 interaction is essential for activation of receptor by C5a des Arg74 but not for activation by intact C5a. The substitution of Asp for Arg74 of C5a [Lys68] produces a ligand with equal potency on both the wild-type and mutant receptors, suggesting that it is the C-terminal carboxyl group rather than the side chain of Arg74 that controls the responsiveness of the receptor to Lys68. In contrast, the mutation of Lys68 to Glu(68) has little effect on the ability of either C5a or C5a des Arg(74) to displace [(125)I]C5a from the receptors, indicating that binding of ligand and receptor activation are distinct but interdependent events. C5a and the truncated ligand, C5a des Arg74, appear to have different modes of interaction with the receptor and the ability of the human C5a receptor to discriminate between these ligands is at least partly dependent on an interaction with the receptor residue, Glu199.

C5a receptor activation. Genetic identification of critical residues in four transmembrane helices

Hormones and sensory stimuli activate serpentine receptors, transmembrane switches that relay signals to heterotrimeric guanine nucleotide-binding proteins (G proteins). To understand the switch mechanism, we subjected 93 amino acids in transmembrane helices III, V, VI, and VII of the human chemoattractant C5a receptor to random saturation mutagenesis. A yeast selection identified 121 functioning mutant receptors, containing a total of 523 amino acid substitutions. Conserved hydrophobic residues are located on helix surfaces that face other helices in a modeled seven-helix bundle (Baldwin, J. M., Schertler, G. F., and Unger, V. M. (1997) J. Mol. Biol. 272, 144-164), whereas surfaces predicted to contact the surrounding lipid tolerate many substitutions. Our analysis identified 25 amino acid positions resistant to nonconservative substitutions. These appear to comprise two distinct components of the receptor switch, a surface at or near the extracellular membrane interface and a core cluster in the cytoplasmic half of the bundle. Twenty-one of the 121 mutant receptors exhibit constitutive activity. Amino acids substitutions in these activated receptors predominate in helices III and VI; other activating mutations truncate the receptor near the extracellular end of helix VI. These results identify key elements of a general mechanism for the serpentine receptor switch.

Chimeric receptors of the human C3a receptor and C5a receptor (CD88)

Chimeras were generated between the human anaphylatoxin C3a and C5a receptors (C3aR and C5aR, respectively) to define the structural requirements for ligand binding and discrimination. Chimeric receptors were generated by systematically exchanging between the two receptors four receptor modules (the N terminus, transmembrane regions 1 to 4, the second extracellular loop, and transmembrane region 5 to the C terminus). The mutants were transiently expressed in HEK-293 cells (with or without Galpha-16) and analyzed for cell surface expression, binding of C3a and C5a, and functional responsiveness (calcium mobilization) toward C3a, C5a, and a C3a as well as a C5a analogue peptide. The data indicate that in both anaphylatoxin receptors the transmembrane regions and the second extracellular loop act as a functional unit that is disrupted by any reciprocal exchange. N-terminal substitution confirmed the two-binding site model for the human C5aR, in which the receptor N terminus is required for high affinity binding of the native ligand but not a C5a analogue peptide. In contrast, the human C3a receptor did not require the original N terminus for high affinity binding of and activation by C3a, a result that was confirmed by N-terminal deletion mutants. This indicates a completely different binding mode of the anaphylatoxins to their corresponding receptors. The C5a analogue peptide, but not C5a, was an agonist of the C3aR. Replacement of the C3aR N terminus by the C5aR sequence, however, lead to the generation of a true hybrid C3a/C5a receptor, which bound and functionally responded to both ligands, C3a and C5a.

NMR of drugs and ligands bound to membrane receptors

NMR methods are now able to give detailed structural, dynamic and electronic information about drugs and ligands while constrained at their site of action in membrane-embedded receptors, information which is essential for mechanistic descriptions of their action and design of new ligands. Using solid state NMR methods, a peptic ulcer drug analogue has been described at atomic resolution (to +/- 0.3 A between two atoms) at its site of action in the gastric H+/K+-ATPase, and the aromaticity of the agonist binding site of the nicotinic acetylcholine receptor has been demonstrated, with both targets in functionally competent membranes under conditions similar to those used in screening assays. G-protein-coupled receptor ligands and prosthetic groups are also being resolved using NMR methods.

Residues 21-30 within the extracellular N-terminal region of the C5a receptor represent a binding domain for the C5a anaphylatoxin

The functions of the C5a anaphylatoxin are expressed through its interaction with a cell-surface receptor with seven transmembrane helices. The interaction of C5a with the receptor has been explained by a two-site model whereby recognition and effector sites on C5a bind, respectively, to recognition and effector domains on the receptor, leading to receptor activation (Chenoweth, D. E., and Hugli, T. E. (1980) Mol. Immunol. 17, 151-161. In addition, the extracellular N-terminal region of the C5a receptor has been implicated as the recognition domain for C5a, responsible for approximately 50% of the binding energy of the C5a-receptor complex (Mery, L., and Boulay, F. (1994) J. Biol. Chem. 269, 3457-3463; DeMartino, J. A., Van Riper, G., Siciliano, S. J., Molineaux, C. J., Konteatis, Z. D., Rosen, H., and Springer, M. S. (1994) J. Biol. Chem. 269, 14446-14450). In this work, the interactions of C5a with the N-terminal domain of the C5a receptor were examined by use of recombinant human C5a molecules and peptide fragments M1NSFN5YTTPD10YGHYD15DKDTL20DLNTP25VDKTS30NTLR(hC5aRF-1-34), acetyl-HYD15DKDTL20DLNTP25VDKTS30NTLR (hC5aRF-13-34), and acetyl-TL20DLNTP25VDKTS30N-amide (hC5aRF-19-31) derived from human C5a receptor. Binding induced resonance perturbations in the NMR spectra of the receptor fragments and the C5a molecules indicated that the isolated Nterminal domain or residues 1-34 of the C5a receptor retain specific binding to C5a and to a C5a analog devoid of the agonistic C-terminal tail in the intact C5a. Residues of C5a perturbed by the binding of the receptor peptides are localized within the helical core of the C5a structure, in agreement with the results from functional studies employing mutated C5a and intact receptor molecules. All three receptor peptides, hC5aRF-1-34, hC5aRF-13-34, and hC5aRF-19-31, responded to the binding of C5a through the 21-30 region containing either hydrophobic, polar, or positively charged residues such as Thr24, Pro25, Val26, Lys28, Thr29, and Ser30. The 21-30 segment of all three receptor fragments was found to have a partially folded conformation in solution, independent of residues 1-18. These results indicate that a short peptide sequence, or residues 21-30, of the C5a receptor N terminus may constitute the binding domain for the recognition site on C5a.

The human C3a receptor is expressed on neutrophils and monocytes, but not on B or T lymphocytes

The pathophysiological relevance of the complement split product C3a as a proinflammatory mediator is still ill defined. The expression pattern of the human C3a receptor (C3aR) can provide important clues for the role of this anaphylatoxin in inflammation. There is strong evidence for C3aR expression on basophils, and eosinophils, but additionally, only on tumor cell lines of leukemic or hepatic origin. It is unclear whether neutrophils also express the C3aR, but need a costimulus provided by eosinophils for certain biological responses, or whether neutrophils lack the C3aR and respond to C3a via a secondary stimulus generated by eosinophils, i.e., by an indirect mode. In the present study, polyclonal antiserum raised against the second extracellular loop of the C3aR was used to characterize C3aR expression on peripheral blood leukocytes. For high degree purification of neutrophils, a negative selection method was established that decreased the contamination with CD9(bright+) eosinophils down to <0.2%. Flow cytometric analyses, functional assays, and binding assays on highly purified neutrophils confirmed C3aR expression and coupling. Monocytes were identified as an additional C3aR-positive cell population of the peripheral blood. The expression of the C3aR on eosinophils could be confirmed. In contrast, the receptor could not be detected on unchallenged B or T lymphocytes (or lymphocyte-derived Raji cells).