Structural basis of FPR2 in recognition of Aß42 and neuroprotection by humanin.

Formyl peptide receptor 2 (FPR2) has been shown to mediate the cytotoxic effects of the ß amyloid peptide Aß42 and serves as a receptor for humanin, a peptide that protects neuronal cells from damage by Aß42, implying its involvement in the pathogenesis of Alzheimer's disease (AD). However, the interaction pattern between FPR2 and Aß42 or humanin remains unknown. Here we report the structures of FPR2 bound to Gi and Aß42 or N-formyl humanin (fHN). Combined with functional data, the structures reveal two critical regions that govern recognition and activity of Aß42 and fHN, including a polar binding cavity within the receptor helical bundle and a hydrophobic binding groove in the extracellular region. In addition, the structures of FPR2 and FPR1 in complex with different formyl peptides were determined, providing insights into ligand recognition and selectivity of the FPR family. These findings uncover key factors that define the functionality of FPR2 in AD and other inflammatory diseases and would enable drug development.

Caulerpin Mitigates Helicobacter pylori-Induced Inflammation via Formyl Peptide Receptors.

The identification of novel strategies to control Helicobacter pylori (Hp)-associated chronic inflammation is, at present, a considerable challenge. Here, we attempt to combat this issue by modulating the innate immune response, targeting formyl peptide receptors (FPRs), G-protein coupled receptors that play key roles in both the regulation and the resolution of the innate inflammatory response. Specifically, we investigated, in vitro, whether Caulerpin-a bis-indole alkaloid isolated from algae of the genus Caulerpa-could act as a molecular antagonist scaffold of FPRs. We showed that Caulerpin significantly reduces the immune response against Hp culture filtrate, by reverting the FPR2-related signaling cascade and thus counteracting the inflammatory reaction triggered by Hp peptide Hp(2-20). Our study suggests Caulerpin to be a promising therapeutic or adjuvant agent for the attenuation of inflammation triggered by Hp infection, as well as its related adverse clinical outcomes.

Mitocryptide-2: Identification of Its Minimum Structure for Specific Activation of FPR2-Possible Receptor Switching from FPR2 to FPR1 by Its Physiological C-terminal Cleavages.

Mitocryptides are a novel family of endogenous neutrophil-activating peptides originating from various mitochondrial proteins. Mitocryptide-2 (MCT-2) is one of such neutrophil-activating peptides, and is produced as an N-formylated pentadecapeptide from mitochondrial cytochrome b. Although MCT-2 is a specific endogenous ligand for formyl peptide receptor 2 (FPR2), the chemical structure within MCT-2 that is responsible for FPR2 activation is still obscure. Here, we demonstrate that the N-terminal heptapeptide structure of MCT-2 with an N-formyl group is the minimum structure that specifically activates FPR2. Moreover, the receptor molecule for MCT-2 is suggested to be shifted from FPR2 to its homolog formyl peptide receptor 1 (FPR1) by the physiological cleavages of its C-terminus. Indeed, N-terminal derivatives of MCT-2 with seven amino acid residues or longer caused an increase of intracellular free Ca2+ concentration in HEK-293 cells expressing FPR2, but not in those expressing FPR1. Those MCT-2 derivatives also induced ß-hexosaminidase secretion in neutrophilic/granulocytic differentiated HL-60 cells via FPR2 activation. In contrast, MCT-2(1-4), an N-terminal tetrapeptide of MCT-2, specifically activated FPR1 to promote those functions. Moreover, MCT-2 was degraded in serum to produce MCT-2(1-4) over time. These findings suggest that MCT-2 is a novel critical factor that not only initiates innate immunity via the specific activation of FPR2, but also promotes delayed responses by the activation of FPR1, which may include resolution and tissue regeneration. The present results also strongly support the necessity of considering the exact chemical structures of activating factors for the investigation of innate immune responses.

Barbadin selectively modulates FPR2-mediated neutrophil functions independent of receptor endocytosis.

FPR2, a member of the family of G protein-coupled receptors (GPCRs), mediates neutrophil migration, a response that has been linked to ß-arrestin recruitment. ß-Arrestin regulates GPCR endocytosis and can also elicit non-canonical receptor signaling. To determine the poorly understood role of ß-arrestin in FPR2 endocytosis and in NADPH-oxidase activation in neutrophils, Barbadin was used as a research tool in this study. Barbadin has been shown to bind the clathrin adaptor protein (AP2) and thereby prevent ß-arrestin/AP2 interaction and ß-arrestin-mediated GPCR endocytosis. In agreement with this, AP2/ß-arrestin interaction induced by an FPR2-specific agonist was inhibited by Barbadin. Unexpectedly, however, Barbadin did not inhibit FPR2 endocytosis, indicating that a mechanism independent of ß-arrestin/AP2 interaction may sustain FPR2 endocytosis. This was confirmed by the fact, that FPR2 also underwent agonist-promoted endocytosis in ß-arrestin deficient cells, albeit at a diminished level as compared to wild type cells. Dissection of the Barbadin effects on FPR2-mediated neutrophil functions including NADPH-oxidase activation mediated release of reactive oxygen species (ROS) and chemotaxis revealed that Barbadin had no effect on chemotactic migration whereas the release of ROS was potentiated/primed. The effect of Barbadin on ROS production was reversible, independent of ß-arrestin recruitment, and similar to that induced by latrunculin A. Taken together, our data demonstrate that endocytic uptake of FPR2 occurs independently of ß-arrestin, while Barbadin selectively augments FPR2-mediated ROS production independently of receptor endocytosis. Given that Barbadin binds to AP2 and prevents the AP2/ß-arrestin interaction, our results indicate a role for AP2 in FPR2-mediated ROS release from neutrophils.

Synergetic Roles of Formyl Peptide Receptor 1 Oligomerization in Ligand-Induced Signal Transduction.

G protein-coupled receptors (GPCRs) transduce extracellular signals into cells by interacting with G proteins and arrestins. Emerging evidence suggests that GPCRs on the plasma membrane are in a dynamic equilibrium among monomers, dimers, and larger oligomers. Nevertheless, the role of the oligomer formation in the GPCR signal transduction remains unclear. Using multicolor single-molecule live-cell imaging, we show a dynamic interconversion between small and large oligomer states of a chemoattractant GPCR, Formyl Peptide Receptor 1 (FPR1), and its binding affinity with G protein. Full agonist stimulation increased a fraction of large FPR1 oligomers, which allowed for prolonged FPR1-G protein interaction. The G protein interaction with FPR1 was most stabilized at the full agonist-bound large FPR1 oligomers. Based on these results, we propose that G protein-mediated signal transduction may be regulated synergistically by the ligand-binding and FPR1 oligomerization. Cooperative signal control induced by receptor oligomerization is anticipated as a target for drug discovery.

Dual modulation of formyl peptide receptor 2 by aspirin-triggered lipoxin contributes to its anti-inflammatory activity.

The eicosanoid lipoxin A4 and aspirin-triggered 15-epi-lipoxin A4 (ATL) are potent anti-inflammatory agents. How their anti-inflammatory effects are mediated by receptors such as the formyl peptide receptor 2 (FPR2/ALX) remains incompletely understood. In the present study, fluorescent biosensors of FPR2/ALX were prepared and ATL-induced conformational changes were recorded. A biphasic dose curve consisting of a descending phase and an ascending phase was observed, with the descending phase corresponding to diminished FPR2 response such as Ca2+ mobilization induced by the potent synthetic agonist WKYMVm. Preincubation of FPR2-expressing cells with 100 pM of ATL also lowered the threshold for WKYMVm to induce ß-arrestin-2 membrane translocation, and inhibited WKYMVm-induced interleukin 8 secretion, suggesting signaling bias favoring anti-inflammatory activities. At 100 pM and above, ATL-induced receptor conformational changes resembling that of the WKYMVm along with a weak but measurable inhibition of forskolin-induced cAMP accumulation. However, no Ca2+ mobilization was induced by ATL until its concentration reached 1 µM. Taken together, these results suggest a dual regulatory mechanism by which ATL exerts anti-inflammatory effects through FPR2/ALX.

Structural basis of ligand binding modes at the human formyl peptide receptor 2.

The human formyl peptide receptor 2 (FPR2) plays a crucial role in host defense and inflammation, and has been considered as a drug target for chronic inflammatory diseases. A variety of peptides with different structures and origins have been characterized as FPR2 ligands. However, the ligand-binding modes of FPR2 remain elusive, thereby limiting the development of potential drugs. Here we report the crystal structure of FPR2 bound to the potent peptide agonist WKYMVm at 2.8 Å resolution. The structure adopts an active conformation and exhibits a deep ligand-binding pocket. Combined with mutagenesis, ligand binding and signaling studies, key interactions between the agonist and FPR2 that govern ligand recognition and receptor activation are identified. Furthermore, molecular docking and functional assays reveal key factors that may define binding affinity and agonist potency of formyl peptides. These findings deepen our understanding about ligand recognition and selectivity mechanisms of the formyl peptide receptor family.

Structure of formylpeptide receptor 2-Gi complex reveals insights into ligand recognition and signaling.

Formylpeptide receptors (FPRs) as G protein-coupled receptors (GPCRs) can recognize formylpeptides derived from pathogens or host cells to function in host defense and cell clearance. In addition, FPRs, especially FPR2, can also recognize other ligands with a large chemical diversity generated at different stages of inflammation to either promote or resolve inflammation in order to maintain a balanced inflammatory response. The mechanism underlying promiscuous ligand recognition and activation of FPRs is not clear. Here we report a cryo-EM structure of FPR2-Gi signaling complex with a peptide agonist. The structure reveals a widely open extracellular region with an amphiphilic environment for ligand binding. Together with computational docking and simulation, the structure suggests a molecular basis for the recognition of formylpeptides and a potential mechanism of receptor activation, and reveals conserved and divergent features in Gi coupling. Our results provide a basis for understanding the molecular mechanism of the functional promiscuity of FPRs.

Structure-function relationship of an Urokinase Receptor-derived peptide which inhibits the Formyl Peptide Receptor type 1 activity.

The interaction between the short 88Ser-Arg-Ser-Arg-Tyr92 sequence of the urokinase receptor (uPAR) and the formyl peptide receptor type 1 (FPR1) elicits cell migration. We generated the Ac-(D)-Tyr-(D)-Arg-Aib-(D)-Arg-NH2 (RI-3) peptide which inhibits the uPAR/FPR1 interaction, reducing migration of FPR1 expressing cells toward N-formyl-methionyl-leucyl-phenylalanine (fMLF) and Ser-Arg-Ser-Arg-Tyr (SRSRY) peptides. To understand the structural basis of the RI-3 inhibitory effects, the FPR1/fMLF, FPR1/SRSRY and FPR1/RI-3 complexes were modeled and analyzed, focusing on the binding pocket of FPR1 and the interaction between the amino acids that signal to the FPR1 C-terminal loop. We found that RI-3 shares the same binding site of fMLF and SRSRY on FPR1. However, while fMLF and SRSRY display the same agonist activation signature (i.e. the series of contacts that transmit the conformational transition throughout the complex), translating binding into signaling, RI-3 does not interact with the activation region of FPR1 and hence does not activate signaling. Indeed, fluorescein-conjugated RI-3 prevents either fMLF and SRSRY uptake on FPR1 without triggering FPR1 internalization and cell motility in the absence of any stimulus. Collectively, our data show that RI-3 is a true FPR1 antagonist and suggest a pharmacophore model useful for development of compounds that selectively inhibit the uPAR-triggered, FPR1-mediated cell migration.

Identification of Residues Critical for FPR2 Activation by the Cryptic Peptide Mitocryptide-2 Originating from the Mitochondrial DNA-Encoded Cytochrome b.

Similar to bacteria, synthesis of mitochondrial DNA-encoded proteins requires an N-formylated methionine to initiate translation. Thus, the N-formylated methionine peptides originating from mitochondria should be recognized as danger signals. To date, only one such peptide, denoted as mitocryptide-2 (MCT-2), originating from the N-terminal of the mitochondrial cytochrome b, has been isolated from mammalian tissues. Human neutrophils express FPR1 and FPR2 that detect formyl peptides, and the precise structural determinants for receptor recognition remain to be elucidated. MCT-2 is known to activate neutrophils through FPR2 but not FPR1. The aim of this study was to elucidate the structural determinants of importance for receptor preference and human neutrophil activation in MCT-2 by generating a series of MCT-2 variants. We show that there is an absolute requirement for the N-formyl group and the side chain of Met1 at position 1 of MCT-2 but also the C terminus is of importance for MCT-2 activity. We also uncovered individual side chains that positively contribute to MCT-2 activity as well as those suppressed in the response. The MCT-2 peptide and its two polymorphic variants ([Thr7]MCT-2 and [Ser8]MCT-2) all activated neutrophils, but MCT-2 containing Ile7 and Asn8 was the most potent. We also show that some peptide variants displayed a biased FPR2-signaling property related to NADPH oxidase activation and ß-arrestin recruitment, respectively. In conclusion, we disclose several critical elements in MCT-2 that are required for neutrophil activation and disclose structural insights into how FPR2 recognition of this mitochondrial DNA-derived peptide may increase our understanding of the role of FPR2 in aseptic inflammation.

DesMol2, an Effective Tool for the Construction of Molecular Libraries and Its Application to QSAR Using Molecular Topology.

A web application, DesMol2, which offers two main functionalities, is presented: the construction of molecular libraries and the calculation of topological indices. These functionalities are explained through a practical example of research of active molecules to the formylpeptide receptor (FPR), a receptor associated with chronic inflammation in systemic amyloidosis and Alzheimer's disease. Starting from a data(base) of 106 dioxopiperazine pyrrolidin piperazine derivatives and their respective constant values of binding affinity to FPR, multilinear regression and discriminant analyses are performed to calculate several predictive topological-mathematical models. Next, using the DesMol2 application, a molecular library consisting of 6,120 molecules is built and performed for each predictive model. The best potential active candidates are selected and compared with results from other previous works.

Studies on acid stability and solid-phase block synthesis of peptide-peptoid hybrids: ligands for formyl peptide receptors.

α-Peptoids as well as peptide/α-peptoid hybrids and peptide/ß-peptoid hybrids constitute major classes of proteolytically stable peptidomimetics that have been extensively investigated as mimetics of biologically active peptides. Representatives of lipidated peptide/ß-peptoid hybrids have been identified as promising immunomodulatory lead compounds, and hence access to these via protocols suitable for gram-scale synthesis is warranted to enable animal in vivo studies. Recent observations indicated that several byproducts appear in crude mixtures of relatively short benzyl-based peptide/ß-peptoid oligomers, and that these were most predominant when the ß-peptoid units displayed an α-chiral benzyl side chain. This prompted an investigation of their stability under acidic conditions. Simultaneous deprotection and cleavage of peptidomimetics containing either α-chiral α- or ß-peptoid residues required treatment with strong acid only for a short time to minimize the formation of partially debenzylated byproducts. The initial work on peptide/ß-peptoid oligomers with an alternating design established that it was beneficial to form the amide bond between the carboxyl group of the α-amino acid and the congested amino functionality of the ß-peptoid residue in solution. To further simplify oligomer assembly on solid phase, we now present a protocol for purification-free solid-phase synthesis of tetrameric building blocks. Next, syntheses of peptidomimetic ligands via manual solid-phase methodologies involving tetrameric building blocks were found to give more readily purified products as compared to those obtained with dimeric building blocks. Moreover, the tetrameric building blocks could be utilized in automated synthesis with microwave-assisted heating, albeit the purity of the crude products was not increased.

The endogenous antimicrobial cathelicidin LL37 induces platelet activation and augments thrombus formation.

Platelet-associated complications including thrombosis, thrombocytopenia, and hemorrhage are commonly observed during various inflammatory diseases such as sepsis, inflammatory bowel disease, and psoriasis. Despite the reported evidence on numerous mechanisms/molecules that may contribute to the dysfunction of platelets, the primary mechanisms that underpin platelet-associated complications during inflammatory diseases are not fully established. Here, we report the discovery of formyl peptide receptor 2, FPR2/ALX, in platelets and its primary role in the development of platelet-associated complications via ligation with its ligand, LL37. LL37 acts as a powerful endogenous antimicrobial peptide, but it also regulates innate immune responses. We demonstrate the impact of LL37 in the modulation of platelet reactivity, hemostasis, and thrombosis. LL37 activates a range of platelet functions, enhances thrombus formation, and shortens the tail bleeding time in mice. By utilizing a pharmacological inhibitor and Fpr2/3 (an ortholog of human FPR2/ALX)-deficient mice, the functional dependence of LL37 on FPR2/ALX was determined. Because the level of LL37 is increased in numerous inflammatory diseases, these results point toward a critical role for LL37 and FPR2/ALX in the development of platelet-related complications in such diseases. Hence, a better understanding of the clinical relevance of LL37 and FPR2/ALX in diverse pathophysiological settings will pave the way for the development of improved therapeutic strategies for a range of thromboinflammatory diseases.

A novel MtHSP70-FPR1 fusion protein enhances cytotoxic T lymphocyte responses to cervical cancer cells by activating human monocyte-derived dendritic cells via the p38 MAPK signaling pathway.

OBJECTIVE: To evaluate the potential effects of recombinant mycobacterium tuberculosis heat shock protein 70-formyl peptide receptor 1 (MtHSP70-FPR1) fusion protein on human monocyte-derived dendritic cell (moDC) maturation; cytotoxic T lymphocyte (CTL) responses to cervical cancer (CC) cells; and the roles of the p38 MAPK, ERK, and JNK pathways in its transition. METHODS: Monocytes were positively selected with a MACS column with antiCD14 antibody-conjugated microbeads from umbilical cord blood. MoDCs were stimulated with MtHSP70-FPR1, MtHSP70, a mix of MtHSP70 and FPR1, FPR1, or phosphate buffer solution (PBS) as control. Flow cytometry was used to analyze the surface molecule expression of moDCs and IFN-γ-producing CD8+ T cells. T cell proliferation was assessed using [3][H]-thymidine assays. The cytotoxicity of moDC-activated T cells against CC cells was evaluated by MTT assays. Cytokine production was determined by enzyme-linked immunosorbent assay. Western blotting was used to investigate protein expression. RESULTS: Compared with MtHSP70, MtHSP70 + FPR1, FPR1, or PBS-mediated moDCs, MtHSP70-FPR1-pulsed moDCs expressed higher levels of CD80, CD86, CD83, HLA-DR, and CCR7; secreted more IL-12p70, TNF-ɑ and IL-1ß; and elicited stronger CTL priming and proliferation, resulting in an effective, HLA-I-dependent killing effect on CC cells. The p38 MAPK, ERK, and JNK pathways were all activated in MtHSP70-FPR1-mediated moDC maturation, but the p38 MAPK pathway played a vital role. CONCLUSIONS: The excellent capability of MtHSP70-FPR1 fusion protein to induce phenotypical and functional maturation of moDCs and CC-specific CTL responses partly illustrates the potential clinical benefits of DC-based immunotherapy for CC.

Role of spatial inhomogenity in GPCR dimerisation predicted by receptor association-diffusion models.

G protein-coupled receptor (GPCR) association is an emerging paradigm with far reaching implications in the regulation of signalling pathways and therapeutic interventions. Recent super resolution microscopy studies have revealed that receptor dimer steady state exhibits sub-second dynamics. In particular the GPCRs, muscarinic acetylcholine receptor M1 (M1MR) and formyl peptide receptor (FPR), have been demonstrated to exhibit a fast association/dissociation kinetics, independent of ligand binding. In this work, we have developed a spatial kinetic Monte Carlo model to investigate receptor homo-dimerisation at a single receptor resolution. Experimentally measured association/dissociation kinetic parameters and diffusion coefficients were used as inputs to the model. To test the effect of membrane spatial heterogeneity on the simulated steady state, simulations were compared to experimental statistics of dimerisation. In the simplest case the receptors are assumed to be diffusing in a spatially homogeneous environment, while spatial heterogeneity is modelled to result from crowding, membrane micro-domains and cytoskeletal compartmentalisation or 'corrals'. We show that a simple association-diffusion model is sufficient to reproduce M1MR association statistics, but fails to reproduce FPR statistics despite comparable kinetic constants. A parameter sensitivity analysis is required to reproduce the association statistics of FPR. The model reveals the complex interplay between cytoskeletal components and their influence on receptor association kinetics within the features of the membrane landscape. These results constitute an important step towards understanding the factors modulating GPCR organisation.

On-resin N-formylation of peptides: a head-to-head comparison of reagents in solid-phase synthesis of ligands for formyl peptide receptors.

4-Nitrophenyl formate was found to be the most convenient reagent in solid-phase formylation of peptides with a high formylation degree within 20 min to 3 h depending on reaction temperature and length of peptide.

Formyl Peptide Receptor Polymorphisms: 27 Most Possible Ways for Phagocyte Dysfunction.

Formyl peptide receptors (FPRs) expressed by mammalian myeloid cells are the important part of innate immunity. They belong to the seven-transmembrane domain class of receptors coupled to heterotrimeric GTP-binding proteins. Binding of the receptor with a wide spectrum of exogenous and endogenous ligands triggers such defensive phagocyte reactions as chemotaxis, secretory degranulation, and respiratory burst, keeping a balance of inflammatory and antiinflammatory processes in the organism. The association between single nucleotide polymorphisms in the gene of FPR1 receptor resulting in disruption of the receptor structure and the development of certain pathologies accompanied with inflammation, such as aggressive periodontitis, macular degeneration, and even gastric cancer (Maney, P., and Walters, J. D. (2009) J. Periodontol., 80, 1498-1505; Liang, X. Y., et al. (2014) Eye, 28, 1502-1510; Otani, T., et al. (2011) Biochem. Biophys. Res. Commun., 405, 356-361) has been shown. In this review, we matched the missense mutation of formyl-peptide receptors with their known functional domains and classified them according to their potential significance in pathology.

The Formyl Peptide Receptors: Diversity of Ligands and Mechanism for Recognition.

The formyl peptide receptors (FPRs) are G protein-coupled receptors that transduce chemotactic signals in phagocytes and mediate host-defense as well as inflammatory responses including cell adhesion, directed migration, granule release and superoxide production. In recent years, the cellular distribution and biological functions of FPRs have expanded to include additional roles in homeostasis of organ functions and modulation of inflammation. In a prototype, FPRs recognize peptides containing N-formylated methionine such as those produced in bacteria and mitochondria, thereby serving as pattern recognition receptors. The repertoire of FPR ligands, however, has expanded rapidly to include not only N-formyl peptides from microbes but also non-formyl peptides of microbial and host origins, synthetic small molecules and an eicosanoid. How these chemically diverse ligands are recognized by the three human FPRs (FPR1, FPR2 and FPR3) and their murine equivalents is largely unclear. In the absence of crystal structures for the FPRs, site-directed mutagenesis, computer-aided ligand docking and structural simulation have led to the identification of amino acids within FPR1 and FPR2 that interact with several formyl peptides. This review article summarizes the progress made in the understanding of FPR ligand diversity as well as ligand recognition mechanisms used by these receptors.

Crystal structure of pentapeptide-independent chemotaxis receptor methyltransferase (CheR) reveals idiosyncratic structural determinants for receptor recognition.

Chemotactic methyltransferase, CheR catalyse methylation of specific glutamate residues in the cytoplasmic domain of methyl-accepting chemotactic protein receptors (MCPRs). The methylation of MCPRs is essential for the chemical sensing and chemotactic bacterial mobility towards favorable chemicals or away from unfavorable ones. In this study, crystal structure of B. subtilis CheR (BsCheR) in complex with S-adenosyl-l-homocysteine (SAH) has been determined to 1.8Å resolution. This is the first report of crystal structure belonging to the pentapeptide-independent CheR (PICheR) class. Till date, only one crystal structure of CheR from S. typhimurium (StCheR) belonging to pentapeptide-dependent CheR (PDCheR) class is available. Structural analysis of BsCheR reveals a helix-X-helix motif (HXH) with Asp53 as the linker residue in the N-terminal domain. The key structural features of the PDCheR ß-subdomain involved in the formation of a tight complex with the pentapeptide binding motif in MCPRs were found to be absent in the structure of BsCheR. Additionally, isothermal titration calorimetry (ITC) experiments were performed to investigate S-adenosyl-(l)-methionine (SAM) binding affinity and KD was determined to be 0.32mM. The structure of BsCheR reveals that mostly residues of the large C-terminal domain contribute to SAH binding, with contributions of few residues from the linker region and the N-terminal domain. Structural investigations and sequence analysis carried out in this study provide critical insights into the distinct receptor recognition mechanism of the PDCheR and PICheR methyltransferase classes.

The sensing of bacteria: emerging principles for the detection of signal sequences by formyl peptide receptors.

The ability to detect specific chemical signatures released by bacteria and other microorganisms is a fundamental feature of immune defense against pathogens. There is increasing evidence that chemodetection of such microorganism-associated molecular patterns (MAMPs) occurs at many places in the body including specific sets of chemosensory neurons in the mammalian nose. Formyl peptide receptors (FPRs) are a unique family of G protein-coupled receptors (GPCRs) that can detect the presence of bacteria and function as chemotactic receptors. Here, we highlight the recent discovery of a vast family of natural FPR agonists, the bacterial signal peptides (or signal sequences), thus providing new insight into the molecular mechanisms of bacterial sensing by human and mouse FPRs. Signal peptides in bacteria are formylated, N-terminal protein signatures required for directing the transfer of proteins through the plasma membrane. After their cleavage and release, signal peptides are available for FPR detection and thus provide a previously unrecognized MAMP. With over 170 000 predicted sequences, bacterial signal peptides represent one of the largest families of GPCR ligands and one of the most complex classes of natural activators of the innate immune system. By recognizing a conserved three-dimensional peptide motif, FPRs employ an unusual detection mechanism that combines structural promiscuity with high specificity and sensitivity, thus solving the problem of detecting thousands of distinct sequences yet maintaining selectivity. How signal peptides are released by bacteria and sensed by GPCRs and how these processes shape the responses of other cells and whole organisms represents an important topic for future research.