Chemotactic Receptor Research Articles

Leukocyte adhesion dynamics in shear flow.

Of the white blood cells that traverse the circulation, the polymorphonuclear leukocytes commonly called neutrophils, are the most numerous, numbering approximately 5000 per microliter of blood. In a sense, these cells are the emergency response unit within the body's circulatory highway since they are the first to be recruited at sites of tissue trauma, infection, or inflammation. The chief function of neutrophils is the capture, phagocytosis, and degradation of foreign invaders. To carry out this critical function, neutrophils sense infection via chemotactic receptors that trigger cellular activation upon ligation of as few as 10-100 bacterial or chemokine peptides. Despite this acute sensitivity to stimuli, neutrophils circulate in healthy individuals largely in a passive state, with a very low efficiency of capture and arrest on quiescent endothelium. Here, we define the efficiency as the fraction of all cells passing by a given length of vessel wall that is captured and achieves firm adhesion. Within seconds of chemotactic signaling, adhesion molecules expressed on the plasma membrane of neutrophils are activated and a rapid boost in the efficiency of stable adhesion is detected. This occurs for both homotypic neutrophil-neutrophil adhesion and neutrophil capture and adhesion on inflamed endothelium. With such large numbers of circulating neutrophils, and with their inherent capability to rapidly adhere to postcapillary venular endothelium, the body has evolved a diverse set of mechanisms to regulate the inflammatory cascade that begins with intracellular signaling and leads to cell arrest and extravasation at sites of tissue insult. In this article we will focus on the interplay between particle-fluid dynamics that involve the shear and normal forces that transport cells transversely and axially within the vessel, and the activation and interaction of adhesion molecules that involve receptor-ligand bond formation that enables the neutrophils to resist wall shear stress and adhere to specific sites of inflammation.

Probing structural requirements of fMLP receptor: On the size of the hydrophobic pocket corresponding to residue 2 of the tripeptide

The conformationally constrained f-L-Met-Acnc-L-Phe-OMe (n = 4,9–12) tripeptides, analogues of the chemoattractant f-L-Met-L-Leu-L-Phe-OH, were synthesized in solution by classical methods and fully characterized. These compounds and the published f-L-Met-Xxx-L-Phe-OMe (Xxx = Aib and Acnc where n = 3, 5–8) analogues were compared to determine the combined effect of backbone preferred conformation and side-chain bulkiness at position 2 on the relation of 3D-structure to biological activity. A conformational study of all the analogues was performed in solution by FT-IR absorption and 1H-NMR techniques. In parallel, each peptide was tested for its ability to induce chemotaxis, superoxide anion production and lysozyme secretion from human neutrophils. The biological and conformational data are discussed in relation to the proposed model of the chemotactic receptor on neutrophils, in particular of the hydrophobic pocket accommodating residue 2 of the tripeptide. Copyright © 2002 European Peptide Society and John Wiley & Sons, Ltd.

The fibrinolytic receptor for urokinase activates the G protein-coupled chemotactic receptor FPRL1/LXA4R.

The function of urokinase and its receptor is essential for cell migration in pathological conditions, as shown by the analysis of knockout mice phenotypes. How a protease of a fibrinolytic pathway can induce migration is not understood and no link between this protease and migration-promoting G protein-coupled receptors has been described. We now show that FPRL1/LXA4R, a G protein-coupled receptor for a number of polypeptides and for the endogenous lipoxin A4 (LXA4), is the link between urokinase-type plasminogen activator (uPA) and migration as it directly interacts with an activated, soluble, cleaved form of uPA receptor (uPAR) (D2D3(88-274)) to induce chemotaxis. In this article we show that (i) both uPAR and FPRL1/LXA4R are necessary for the chemotactic activity of uPA whereas FPRL1/LXA4R is sufficient to mediate D2D3(88-274)-induced cell migration. (ii) Inhibition or desensitization of FPRL1/LXA4R by antibodies or specific ligands specifically prevents chemotaxis induced by D2D3(88-274) in THP-1 cells and human peripheral blood monocytes. (iii) Desensitization of FPRL1/LXA4R prevents the activation of tyrosine kinase Hck induced by D2D3(88-274). (iv) D2D3(88-274) directly binds to FPRL1/LXA4R and is competed by two specific FPRL1/LXA4R agonists, the synthetic MMK-1 peptide and a stable analog of LXA4. Thus, a naturally produced cleaved form of uPAR is a unique endogenous chemotactic agonist for FPRL1/LXA4R receptor and its activity can be antagonized by specific ligands. These results provide the first direct link, to our knowledge, between the fibrinolytic machinery and the inflammatory response, demonstrating that uPA-derived peptide fragments can activate a specific chemotactic receptor.

Probing structural requirements of fMLP receptor: on the size of the hydrophobic pocket corresponding to residue 2 of the tripeptide.

The conformationally constrained f-L-Met-Ac(n)c-L-Phe-OMe (n = 4,9-12) tripeptides, analogues of the chemoattractant f-L-Met-L-Leu-L-Phe-OH, were synthesized in solution by classical methods and fully characterized. These compounds and the published f-L-Met-Xxx-L-Phe-OMe (Xxx = Aib and Ac(n)c where n = 3, 5-8) analogues were compared to determine the combined effect of backbone preferred conformation and side-chain bulkiness at position 2 on the relation of 3D-structure to biological activity. A conformational study of all the analogues was performed in solution by FT-IR absorption and 1H-NMR techniques. In parallel, each peptide was tested for its ability to induce chemotaxis, superoxide anion production and lysozyme secretion from human neutrophils. The biological and conformational data are discussed in relation to the proposed model of the chemotactic receptor on neutrophils, in particular of the hydrophobic pocket accommodating residue 2 of the tripeptide.

Activation of CD8 T cells induces expression of CD4, which functions as a chemotactic receptor

It was previously shown that costimulation of CD8(+) lymphocytes results in de novo expression of CD4. This study expanded on this observation to investigate the function of CD4 on CD8 cells. The ability of costimulated CD8 cells to respond to interleukin 16 (IL-16), a ligand that binds CD4 and induces cellular chemotaxis, was examined. IL-16-mediated ligation of CD4 expressed on CD8 T cells was found to induce an intracellular signal that directs migration of these cells in vitro. Thus, expression of CD4 on a CD8 lymphocyte has functional importance and may serve to control distribution of newly activated CD8 T cells in vivo.

Detection of differentially expressed genes in particle disease using array-filter analysis.

The precise cellular mechanism of osteolysis in particle disease is still unknown. The aim of the study was to screen for new gene products in macrophages during particle contact. In an established macrophage model THP1-cells (human monocytic cells) were differentiated under the influence of vitamin D3 and GM-CSF into macrophage-like cells (MLC). MLCs were incubated each with different concentrations of polyethylene particles, Lipopolysaccharids (LPS) and controls. Isolated RNA was transcribed into complementary radioactive 32P labeled cDNA. This probe was hybridised on an human cDNA expression array and analysed by autoradiography. To obtain a more reliable method quantifying mRNA, the reverse transcriptase polymerase chain reaction (RT-PCR) was used. The arrays showed an upregulation of the following genes by particles: TNF-Rezeptor 2, IL-1 Receptor Antagonist, Bone Morphogenic Protein 4 and HM 145. This was proven three times using RT-PCR and statistically significant in comparison to the controls. LPS induced the same upregulation except for HM145 whereas particles caused downregulation of this mRNA expression. Our results prove that the model of differentiated THP-1 cells treated with PE particles is a suitable system to analyse differential gene expression patterns, since the induction of the major positive control genes TNF alpha and IL1 beta were detected by this approach. BMP 4 is known as signal protein which mediates ectopic bone formation and can also be interpreted as a contra regulatory gene. HM 145 belongs to the leukocyte chemotactic peptide receptor family. HM 145 seems to be one of the first genes that is enhanced along the septical pathway but less expressed by contact with particles. Analysis of HM 145 expression might help to diagnose septic versus aseptic loosening of prosthesis.

Kinetics of the biphasic modulation of phagocytosis

A kinetic model for the biphasic modulation of phagocytosis, fulfilling the requirements of biochemistry of ligand‐binding reactions, was constructed on the basis of a biocybernetic notion of the feedback loop‐containing (autocalytic) process. Using boundary conditions for parameters of possible kinetic models, a single model was selected, in which the biphasic modulatory effect exerted on a phagocytic activity of human neutrophils by the peptide preparation Immax A1 was described as a result of mutual counteraction of two antagonistic compounds (stimulator and inhibitor of phagocytosis) competing for bacterial chemotactic peptide receptors on neutrophils. This model, in which the integrated luminescence‐based normalised measure of inhibition of phagocytosis stands for the reaction rate, was found to have a form of a 2:3 rational function of the peptide preparation concentration. A corresponding stoichiometric scheme, describing the binding both of the inhibitor and of the stimulator to neutrophils, was constructed on the assumption that inhibition was not total when connected with the generation of three‐component complexes, stimulator‐neutrophil‐inhibitor.

Increased neutrophil motility by beta-glucan in the absence of chemoattractant.

Systemic candidasis is a life-threatening complication of antibiotic and immunosuppressive therapies and can alter host defense mechanisms through pathways that are poorly understood. Promotion of polymorphonuclear leukocyte (PMN) chemotaxis by beta-glucan towards fMLP or IL-8 gradients demonstrates a fundamental effect on host defenses by pathogenic fungi. The aim of the present study was to determine whether recognition of beta-glucan is sufficient to alter PMN motility in the absence of agonists of G-coupled protein chemotactic receptors. Present findings demonstrate a profound increase in PMN motility by beta-glucan supplementation of a fibronectin substratum in an underagarose migration assay. Motility on beta-glucan included a 3-fold increase in distance of migration, as well as a 5-fold increase in the number of PMNs recruited into the motile phase as compared to motility on fibronectin alone. This promotion of motility is determined by the beta2 integrin complement receptor 3 (CR3) (CD11b/CD18) rather than the beta1 integrin very late antigen 3 (VLA-3), which mediates chemotaxis on beta-glucan-supplemented matrix towards fMLP. PMN motility on beta-glucan-supplemented fibronectin was selectively decreased by inhibitors of pp60 src and ras, whereas motility was promoted by inhibition of p38-MAPK. No effect of these inhibitors was seen on PMNs migrating on fibronectin alone. Migration on beta-glucan-supplemented fibronectin, but not on fibronectin alone, was negatively regulated by protein kinase C (PKC) or cAMP activation. These findings indicate that beta-glucan is sufficient to alter the migratory capacity of PMN in the absence of costimulation by fMLP. Enhanced PMN migration on beta-glucan is mediated through specific integrins and second messenger pathways that are distinct from those utilized by PMNs migrating in the absence of beta-glucan.

A Sense of Direction

Intracellular signaling events that regulate chemotaxis are thought to be restricted to the edge of the cell that faces the highest concentration of attractant. However, a chemotactic Dictyostelium amoeba displays a constant and uniform surface distribution of its chemotactic receptors. Ueda et al. have used single-molecule imaging on live Dictyostelium cells to determine that the ligand binding kinetics of chemotactic receptors at the leading edge is different from other locations on the cell surface. Thus, the mechanism for directional sensing relies on differences in the signaling state of the receptors. M. Ueda, Y. Sako, T. Tanaka, P. Devreotes, T. Yanagida , Single-molecule analysis of chemotactic signaling in Dictyostelium cells. Science 294 , 864-867 (2001). [Abstract] [Full Text]

A Sense of Direction

Intracellular signaling events that regulate chemotaxis are thought to be restricted to the edge of the cell that faces the highest concentration of attractant. However, a chemotactic Dictyostelium amoeba displays a constant and uniform surface distribution of its chemotactic receptors. Ueda et al. have used single-molecule imaging on live Dictyostelium cells to determine that the ligand binding kinetics of chemotactic receptors at the leading edge is different from other locations on the cell surface. Thus, the mechanism for directional sensing relies on differences in the signaling state of the receptors. M. Ueda, Y. Sako, T. Tanaka, P. Devreotes, T. Yanagida , Single-molecule analysis of chemotactic signaling in Dictyostelium cells. Science 294, 864-867 (2001).[Abstract][Full Text]

Unique chemotactic response profile and specific expression of chemokine receptors CCR4 and CCR8 by CD4(+)CD25(+) regulatory T cells.

Chemokines dictate regional trafficking of functionally distinct T cell subsets. In rodents and humans, a unique subset of CD4(+)CD25(+) cytotoxic T lymphocyte antigen (CTLA)-4(+) regulatory T cells (Treg) has been proposed to control peripheral tolerance. However, the molecular basis of immune suppression and the trafficking properties of Treg cells are still unknown. Here, we determined the chemotactic response profile and chemokine receptor expression of human blood-borne CD4(+)CD25(+) Treg cells. These Treg cells were found to vigorously respond to several inflammatory and lymphoid chemokines. Treg cells specifically express the chemokine receptors CCR4 and CCR8 and represent a major subset of circulating CD4(+) T cells responding to the chemokines macrophage-derived chemokine (MDC)/CCL22, thymus and activation-regulated chemokine (TARC)/CCL17, I-309/CCL1, and to the virokine vMIP-I (ligands of CCR4 and CCR8). Blood-borne CD4(+) T cells that migrate in response to CCL1 and CCL22 exhibit a reduced alloproliferative response, dependent on the increased frequency of Treg cells in the migrated population. Importantly, mature dendritic cells preferentially attract Treg cells among circulating CD4(+) T cells, by secretion of CCR4 ligands CCL17 and CCL22. Overall, these results suggest that CCR4 and/or CCR8 may guide Treg cells to sites of antigen presentation in secondary lymphoid tissues and inflamed areas to attenuate T cell activation.

Expression of the Ym2 lectin-binding protein is dependent on interleukin (IL)-4 and IL-13 signal transduction: identification of a novel allergy-associated protein.

Asthma pathophysiology is intimately regulated by CD4(+) Th2 lymphocytes and the cytokines interleukin (IL)-4 and IL-13. However, the mechanisms by which these cytokines promote disease have not been fully elucidated. In order to identify novel molecular mediators of allergy, a comparison was made of the bronchoalveolar lavage, which demonstrated that the Ym2 protein was abundantly up-regulated in the lung during the development of allergy. Low levels of the Ym1 isomer were also detected. Importantly, neither Ym1 nor Ym2 has been characterized previously in the context of allergic pulmonary inflammation. Western immunoblot showed that enhanced expression of these proteins was dependent on CD4(+) T cells and IL-4 or IL-13 signaling via the IL-4Ralpha subunit. In addition, intratracheal instillation of IL-13 into naive mice was sufficient to induce expression. Ym1 is homologous to eosinophil chemotactic factor L. However, only weak eosinophil chemotaxis was observed in response to Ym protein in both in vitro and in vivo assays. By contrast, the homology of Ym1 and Ym2 to proteins associated with tissue remodeling, together with the previous findings that Ym1 is homologous to chitinase and binds heparin sulfate and GlcN oligomers (chitobiose, chitotriose, and chitotetraose), strongly suggests these proteins play an important role in airway wall remodeling in the allergic lung.

The Synthetic Peptide Trp-Lys-Tyr-Met-Val-Met-NH2 Specifically Activates Neutrophils through FPRL1/Lipoxin A4 Receptors and Is an Agonist for the Orphan Monocyte-expressed Chemoattractant Receptor FPRL2

Neutrophils express the G protein-coupled N-formyl peptide receptor (FPR) and its homologue FPRL1, whereas monocytes express FPR, FPRL1, and FPRL2, an orphan receptor sharing 83% amino acid identity with FPRL1. FPRL1 is a promiscuous receptor activated by serum amyloid A and by different synthetic peptides, including the hexapeptide Trp-Lys-Tyr-Met-Val-d-Met-NH(2) (WKYMVm). By measuring calcium flux in HL-60 cells transfected with FPR, FPRL1, or FPRL2, we show that WKYMVm activated all three receptors, whereas the l-conformer WKYMVM activated exclusively FPRL1 and FPRL2. The functionality of FPRL2 was further assessed by the ability of HL-60-FPRL2 cells to migrate toward nanomolar concentrations of hexapeptides. The half-maximal effective concentrations of WKYMVM for calcium mobilization in HL-60-FPRL1 and HL-60-FPRL2 cells were 2 and 80 nm, respectively. Those of WKYMVm were 75 pm and 3 nm. The tritiated peptide WK[3,5-(3)H(2)]YMVM bound to FPRL1 (K(D) approximately 160 nm), but not to FPR. The two conformers similarly inhibited binding of (125)I-labeled WKYMVm to FPRL2-expressing cells (IC(50) approximately 2.5-3 micrometer). Metabolic labeling with orthophosphoric acid revealed that FPRL1 was differentially phosphorylated upon addition of the l- or d-conformer, indicating that it induced different conformational changes. In contrast to FPRL1, FPRL2 was already phosphorylated in the absence of agonist and not evenly distributed in the plasma membrane of unstimulated cells. However, both receptors were internalized upon addition of either of the two conformers. Taken together, the results indicate that neutrophils are activated by WKYMVM through FPRL1 and that FPRL2 is a chemotactic receptor transducing signals in myeloid cells.

Polymorphism in the fractalkine receptor CX3CR1 as a genetic risk factor for coronary artery disease

AbstractCoronary atherosclerosis is a major cause of death in industrialized countries. Monocytes, which play a key role in atherosclerosis, migrate into the vessel wall, presumably guided by specific chemoattractant and adhesion molecules. A compelling candidate for this role is the chemokine receptor CX3CR1, which is expressed on monocytes and acts as either a chemotactic receptor or an adhesion molecule, depending on whether its ligand, fractalkine, is presented free or membrane bound. A common variant of CX3CR1 was recently identified, encoded by the alleles I249 and M280, which form a common I249M280 haplotype. When CX3CR1 genotypes were analyzed in 151 patients with acute coronary syndromes and in 249 healthy controls, CX3CR1 I249 heterozygosity was associated with a markedly reduced risk of acute coronary events, independent of established acquired coronary risk factors (eg, smoking, diabetes). The adjusted odds ratio for this allele was 0.43 (95% confidence interval, 0.26-0.72; P = .001). Consistent with this, functional analysis of peripheral blood mononuclear cells showed that CX3CR1 I249 heterozygosity was associated with a significant decrease in the number of fractalkine binding sites per cell. The results show that CX3CR1 I249 is an independent genetic risk factor for coronary artery disease and that CX3CR1 may be involved in the pathogenesis of atherosclerotic disease.

Differential regulation of formyl peptide receptor-like 1 expression during the differentiation of monocytes to dendritic cells and macrophages.

Monocytes are the common precursors for myeloid dendritic cells (DC) and macrophages. Identification of chemotactic receptors expressed by myeloid DC, macrophages, and their precursors in the course of differentiation and maturation is important not only for elucidation of their in vivo trafficking, but also for understanding of the functional distinction between DC and macrophages. We chose to study formyl peptide receptor like-1 (FPRL1), a chemotactic receptor known to interact with several endogenous agonists that are involved in inflammatory and host defense responses. Here we show that FPRL1 is down-regulated as monocytes differentiate into DC. This down-regulation occurs at both mRNA and functional levels. Therefore, the interaction of FPRL1 with its agonists is more likely to regulate the in vivo trafficking of DC precursors than DC. In contrast, FPRL1 expression is maintained at both mRNA and functional levels as monocytes differentiate into macrophages. Thus, our results demonstrate further distinctions between myeloid DC and macrophages, albeit they share a common precursor. The fact that macrophages rather than myeloid DC express functional FPRL1 suggests that this chemotactic receptor may be more involved in inflammatory reactions and innate host defense than in adaptive immune responses.

Differential roles of the NPXXY motif in formyl peptide receptor signaling.

The NPXXY motif (X represents any amino acid) in the seventh transmembrane domain of the chemotactic formyl peptide receptor (FPR) is highly conserved among G protein-coupled receptors. Recent work suggested that this motif contributes to G protein-coupled receptor internalization and signal transduction; however, its role in FPR signaling remains unclear. In this study we replaced Asn(297) and Tyr(301) in the NPXXY motif of the human FPR with Ala (N297A) and Ala/Phe (Y301A/Y301F), respectively, and determined the effects of the substitutions on FPR functions in transfected rat basophilic leukemia cells. Whereas all the mutant receptors were expressed on the cell surface, the N297A receptor exhibited reduced binding affinity and was unable to mediate activation of phospholipase C-beta and the p42/44 mitogen-activated protein kinase (MAP kinase). The Y301F receptor displayed significantly decreased ligand-stimulated internalization and MAP kinase activation, suggesting that the hydrogen bonding at Tyr(301) is critical for these functions. The Y301F receptor showed a chemotactic response similar to that of wild-type FPR, indicating that cell chemotaxis does not require receptor internalization and hydrogen bonding at the Tyr(301) position. In contrast, the Y301A receptor displayed a left-shifted, but overall reduced, chemotaxis response that peaked at 0.1-1 nM. Finally, using a specific MAP kinase kinase inhibitor, we found that activation of MAP kinase is required for efficient FPR internalization, but is not essential for chemotaxis. These findings demonstrate that residues within the NPXXY motif differentially regulate the functions of FPR.

Chemotactic transduction in biomining microorganisms

The oxidative attack of ores by bioleaching microorganisms requires that the cells adhere to specific sites on the surface of the minerals. This attachment will depend on the sensing by the microorganisms of a dissolved ion concentration gradient present in the immediate vicinity of the solid. Chemotactic responses towards metal ions and other compounds have been described for Thiobacillus ferrooxidans (now Acidithiobacillus ferrooxidans) and Leptospirillum ferrooxidans. More recently, we isolated and characterized a gene from L. ferrooxidans coding for a putative chemotactic receptor (LcrI). I review here what is known about the chemotactic behaviour of biomining microorganisms, and analyze further the characteristics of LcrI. The LcrI-deduced protein contained two hydrophobic regions most likely corresponding to the two transmembrane regions present in all the methyl-accepting chemotaxis proteins (MCPs) which produce a folded protein with both a periplasmic and a cytoplasmic domain. We have proposed that the cytoplasmic domain of LcrI also contains a highly conserved domain (HCD region) present in all bacterial chemotactic receptors, and two probable methylation sites. The periplasmic domain of LcrI showed a very high positive charge at acidic pH when compared with periplasmic domains of a non-acidophilic bacterium. This charge difference may represent a special adaptation of acidophilic microorganisms such as T. ferrooxidans and L. ferrooxidans that have to sense effectors at the very low pH of their periplasms. On the other hand, the presence in LcrI of two putative amphipathic helices, characteristic of the cytoplasmic domains of MCPs, that may have a role in transmembrane signal transduction, suggests that the signaling mechanism mediated by the cytoplasmic region of LcrI would be essentially the same in the acidophilic chemolithoautotrophic biomining microorganisms when compared with other microorganisms.

Monocyte chemotactic protein-1 receptor CCR2B is a glycoprotein that has tyrosine sulfation in a conserved extracellular N-terminal region.

Monocyte chemotactic protein-1 (MCP-1) binding to its receptor, CCR2B, plays an important role in a variety of diseases involving infection, inflammation, and/or injury. In our effort to understand the molecular basis of this interaction and its biological consequences, we recognized a conserved hexad of amino acids at the N-terminal extracellular domain of several chemokine receptors, including CCR2B. Human embryonic kidney 293 cells expressing Flag-tagged CCR2B containing site-directed mutations in this region, 21-26, including a consensus tyrosine sulfation site were used to determine MCP-1 binding and its biological consequences. The results showed that several of these amino acids are important for MCP-1 binding and consequent lamellipodium formation, chemotaxis, and signal transduction involving adenylate cyclase inhibition and Ca(2+) influx into cytoplasm. Mutations that prevented adenylate cyclase inhibition and Ca(2+) influx did not significantly inhibit lamellipodium formation and chemotaxis, suggesting that these signaling events are not involved in chemotaxis. CCR2B was found to be sulfated at Tyr(26); this sulfation was abolished by the substitution of Tyr with Ala and severely reduced by substitution of Asp(25), a part of the consensus sulfation site. The expressed CCR2B was found to be N:-glycosylated, as N:-glycosidase F treatment of the receptor or growth of the cells in tunicamycin reduced the receptor size to the same level, from 50 to 45 kDa. Thus, CCR2B is the first member of the CC chemokine receptor family shown to be a glycoprotein that is sulfated at the N-terminal Tyr. These post-translational modifications probably have significant biological functions.

Cache – a signaling domain common to animal Ca 2+-channel subunits and a class of prokaryotic chemotaxis receptors

Interactions of proteins with small molecules are an important aspect of cellular regulatory processes. These interactions include those between surface receptors and their extracellular ligands, and those of various intracellular allosteric effectors with their target proteins. Furthermore, some proteins can bind small molecules and use them as sensors of other stimuli, such as light and redox status. In intracellular contexts, small-molecule recognition is achieved in part through specific domains such as PAS (domain found in Drosophila p̱eriod clock protein, a̱ryl hydrocarbon receptor and s̱ingle-minded proteins),1,2 GAF (domain found in cG̱MP-phosphodiesterases, Anabena a̱denylate cyclases and Escherichia coli F̱hlA)3 and ACT (a̱spartokinase, c̱horismate mutase, ṮyrA) (Ref. 4). Extracellular ligand binding is less well understood, but, in certain cases such as the binding proteins of ABC transporters5 and different prokaryotic chemotactic receptors,6 it is known to involve specific families of ligand-binding domains. We report here the identification of a novel extracellular domain that is predicted to have a role in small-molecule recognition in a wide range of proteins, including the animal dihydropyridine-sensitive voltage-gated Ca2+ channel α-2δ subunit and various bacterial chemotaxis receptors.

Down-regulation of the chemokine receptor CCR5 by activation of chemotactic formyl peptide receptor in human monocytes

AbstractInteractions between cell surface receptors are important regulatory elements in the complex host responses to infections. In this study, it is shown that a classic chemotactic factor, the bacterial chemotactic peptide N-formyl-methionyl-leucylphenyl-alanine (fMLF), rapidly induced a protein-kinase-C–mediated serine phosphorylation and down-regulation of the chemokine receptor CCR5, which serves as a major human immunodeficiency virus (HIV)-1 coreceptor. The fMLF binding to its receptor, formyl peptide receptor (FPR), resulted in significant attenuation of cell responses to CCR5 ligands and in inhibition of HIV-1-envelope-glycoprotein–mediated fusion and infection of cells expressing CD4, CCR5, and FPR. The finding that the expression and function of CCR5 can be regulated by peptides that use an unrelated receptor may provide a novel approach to the design of anti-inflamatory and antiretroviral agents.