Chemotactic Peptide Receptor Research Articles

Signal inhibitory receptor on leukocytes-1 recognizes bacterial and endogenous amphipathic α-helical peptides.

Signal inhibitory receptor on leukocytes-1 (SIRL-1) is a negative regulator of myeloid cell function and dampens antimicrobial responses. We here show that different species of the genus Staphylococcus secrete SIRL-1-engaging factors. By screening a library of single-gene transposon mutants in Staphylococcusaureus, we identified these factors as phenol-soluble modulins (PSMs). PSMs are amphipathic α-helical peptides involved in multiple aspects of staphylococcal virulence and physiology. They are cytotoxic and activate the chemotactic formyl peptide receptor2 (FPR2) on immune cells. Human cathelicidin LL-37 is also an amphipathic α-helical peptide with antimicrobial and chemotactic activities, structurally and functionally similar to α-type PSMs. We demonstrate that α-type PSMs from multiple staphylococcal species as well as human cathelicidin LL-37 activate SIRL-1, suggesting that SIRL-1 recognizes α-helical peptides with an amphipathic arrangement of hydrophobicity, although we were not able to show direct binding to SIRL-1. Upon rational peptide design, we identified artificial peptides in which the capacity to ligate SIRL-1 is segregated from cytotoxic and FPR2-activating properties, allowing specific engagement of SIRL-1. In conclusion, we propose staphylococcal PSMs and human LL-37 as a potential new class of natural ligands for SIRL-1.

Application of small molecule FPR1 antagonists in the treatment of cancers

The formylpeptide receptor-1 (FPR1) is a member of the chemotactic GPCR-7TM formyl peptide receptor family, whose principle function is in trafficking of various leukocytes into sites of bacterial infection and inflammation. More recently, FPR1 has been shown to be expressed in different types of cancer and in this context, plays a significant role in their expansion, resistance and recurrence. ICT12035 is a selective and potent (30 nM in calcium mobilisation assay) small molecule FPR1 antagonist. Here, we demonstrate the efficacy of ICT12035, in a number of 2D and 3D proliferation and invasion in vitro assays and an in vivo model. Our results demonstrate that targeting FPR1 by a selective small molecule antagonist, such as ICT12035, can provide a new avenue for the treatment of cancers.

Granulocyte Colony-Stimulating Factor-induced Terminal Maturation of Human Myeloid Cells Is Specifically Associated with Up-Regulation of Receptor-Mediated Function and CD10 Expression

The acute promyelocytic leukemia cell line NB4 was differentiated by all-trans retinoic acid (ATRA), which enhanced the superoxide-producing capacity stimulated by the chemotactic peptide and phorbol ester in this cell line. Granulocyte colony-stimulating factor (G-CSF) by itself had no effect on NB4 cells but exerted additional enhancing effects on the respiratory burst activity in the presence of ATRA. This finding was not due to the induction of G-CSF receptor by ATRA, because NB4 cells expressed abundant G-CSF receptor with or without ATRA. Unlike ATRA, G-CSF enhanced superoxide release stimulated by the chemotactic peptide but not by phorbol ester. In addition, G-CSF but not ATRA attenuated cell death and enhanced survival during differentiation. Cell surface expression of the chemotactic peptide receptors CD33 and CD10 but not of CD11b and CD11c was up-regulated by ATRA plus G-CSF far more profoundly than by ATRA alone. Fundamentally identical but slightly different phenomena for the cell surface expression of CD33 and CD10 were observed in the normal human bone marrow mononuclear cells; G-CSF induced CD10 even in the absence of ATRA and down-regulated CD33 in normal cells. The present results indicate that G-CSF-induced terminal maturation of human myeloid cells is associated with up-regulation of receptor-mediated function and CD10 expression.

Up-Regulation of FPR2, a Chemotactic Receptor for Amyloid β 1–42 (Aβ 42), in Murine Microglial Cells by TNFα

Human FPRL1 and its mouse homologue FPR2 are functional receptors for several exogenous and host-derived chemotactic peptides, including amyloid β 42 (Aβ 42), a critical pathogenic factor in Alzheimer's disease. We investigated the effect of TNFα on the expression and function of FPR2 in mouse microglial cells, a crucial inflammatory cell type in the CNS. Primary murine microglia and a cell line N9 in resting state expressed low levels of FPR2 gene and lacked the response to chemotactic agonists for this receptor. Incubation with TNFα, however, increased microglial expression of FPR2 gene, in association with potent chemotactic responses to FPR2-specific agonists including Aβ 42. The effect of TNFα was dependent on the p55 TNFα receptor and activation of MAP kinase p38. TNFα concomitantly down-regulated microglial response to the chemokine SDF-1α. Thus, by selectively up-regulating FPR2 in microglia, TNFα has the capacity to amplify host response in inflammatory diseases in the CNS.

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.

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.

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.

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

Interactions 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. (Blood. 2000;96:2887-2894)

Novel pathophysiological role of classical chemotactic peptide receptors and their communications with chemokine receptors.

The bacterial N-formylpeptides, such as N-formyl-Met-Leu-Phe (fMLF), are some of the first identified and most potent chemoattractants for phagocytic leukocytes. Two fMLF receptors, the high affinity formyl peptide receptor (FPR) and its low affinity variant FPR-like 1 (FPRL1), belong to the seven-transmembrane, Gi protein-coupled receptor superfamily which also includes chemokine receptors. Despite their reaction with bacterial chemotactic peptides, the physiological role of these receptors in humans remains unclear. Our recent studies have identified novel exogenous as well as host-derived agonists for FPR and FPRL1. Furthermore, activation of these receptors by their agonists results in desensitization of the receptors for other chemoattractants, including two chemokine receptors, CCR5 and CXCR4, which serve as major co-receptors for HIV-1. These results suggest that FPR and FPRL1 may play important roles not only in host defense and immunological responses but also in the fine tuning of cell activation in the presence of multiple stimuli.

Activation of the Chemotactic Peptide Receptor FPRL1 in Monocytes Phosphorylates the Chemokine Receptor CCR5 and Attenuates Cell Responses to Selected Chemokines

FPRL1 is a seven-transmembrane (STM), G-protein coupled receptor which was originally identified as a low affinity receptor for the bacterial chemotactic formyl peptide and a high affinity receptor for the lipid metabolite lipoxin A4. We recently discovered that a number of peptides, including several synthetic domains of the HIV-1 envelope proteins and the serum amyloid A, use FPRL1 to induce migration and calcium mobilization in human monocytes and neutrophils. In this study, we report that a synthetic peptide domain of the V3 region of the HIV-1 envelope gp120, activates the FPRL1 receptor in monocytes and neutrophils. Furthermore, monocytes prestimulated with V3 peptide showed reduced response to several chemokines that use multiple cell receptors. This is associated with a rapid phosphorylation of the chemokine receptor CCR5 on the serine residues. Our study suggests that FPRL1, as a classical chemoattractant receptor, may play an important role in modulating monocyte activation in the presence of multiple stimuli.

T21/DP107, A Synthetic Leucine Zipper-Like Domain of the HIV-1 Envelope gp41, Attracts and Activates Human Phagocytes by Using G-Protein-Coupled Formyl Peptide Receptors

A leucine zipper-like domain, T21/DP107, located in the amino terminus of the ectodomain of gp41, is crucial to the formation of fusogenic configuration of the HIV-1 envelope protein gp41. We report that the synthetic T21/DP107 segment is a potent stimulant of migration and calcium mobilization in human monocytes and neutrophils. The activity of T21/DP107 on phagocytes was pertussis toxin-sensitive, suggesting this peptide uses Gi-coupled seven-transmembrane receptor(s). Since the bacterial chemotactic peptide fMLP partially desensitized the calcium-mobilizing activity of T21/DP107 in phagocytes, we postulated that T21/DP107 might preferentially use a lower affinity fMLP receptor. By using cells transfected to express cloned prototype chemotactic N-formyl peptide receptor (FPR) or its variant, FPR-like 1 (FPRL1), we demonstrate that T21/DP107 activates both receptors but has a much higher efficacy for FPRL1. In addition, T21/DP107 at nM concentrations induced migration of FPRL1-transfected human embryonic kidney 293 cells. In contrast, fMLP did not induce significant chemotaxis of the same cells at a concentration as high as 50 microM. Although a lipid metabolite, lipoxin A4, was a high-affinity ligand for FPRL1, it was not reported to induce Ca2+ mobilization or chemotaxis in FPRL1-transfected cells. Therefore, T21/DP107 is a first chemotactic peptide agonist identified thus far for FPRL1. Our results suggest that this peptide domain of the HIV-1 gp41 may have the potential to activate host innate immune response by interacting with FPR and FPRL1 on phagocytes.

Synergistic activation of a family of phosphoinositide 3-kinase via G-protein coupled and tyrosine kinase-related receptors

Phosphoinositide 3-kinase (PI 3-kinase) is a key signaling enzyme implicated in a variety of receptor-stimulated cell responses. Stimulation of receptors possessing (or coupling to) protein-tyrosine kinase activates heterodimeric PI 3-kinases, which consist of an 85-kDa regulatory subunit (p85) containing Src-homology 2 (SH2) domains and a 110-kDa catalytic subunit (p110α or p110β). Thus, this form of PI 3-kinases could be activated in vitro by a phosphotyrosyl peptide containing a YMXM motif that binds to the SH2 domains of p85. Receptors coupling to αβγ-trimeric G proteins also stimulate the lipid kinase activity of a novel p110γ isoform, which is not associated with p85, and thereby is not activated by tyrosine kinase receptors. The activation of p110γ PI 3-kinase appears to be mediated through the βγ subunits of the G protein (Gβγ). In addition, rat liver heterodimeric PI 3-kinases containing the p110β catalytic subunit are synergistically activated by the phosphotyrosyl peptide plus Gβγ. Such enzymatic properties were also observed with a recombinant p110β/p85α expressed in COS-7 cells. In contrast, another heterodimeric PI 3-kinase consisting of p110α and p85 in the same rat liver, together with a recombinant p110α/p85α, was not activated by Gβγ, though their activities were stimulated by the phosphotyrosyl peptide. Synergistic activation of PI 3-kinase by the stimulation of the two major receptor types was indeed observed in intact cells, such as chemotactic peptide ( N-formyl–Met–Leu–Phe) plus insulin (or Fcγ II) receptors in differentiated THP-1 and CHO cells and adenosine (A1) plus insulin receptors in rat adipocytes. Thus, PI 3-kinase isoforms consisting of p110β catalytic and SH2-containing (p85 or its related) regulatory subunits appeared to function as a ‘cross-talk’ enzyme between the two signal transduction pathways mediated through tyrosine kinase and G protein-coupled receptors.

Nitrous oxide impairs the neutrophil oxidative response.

Nitrous oxide has been shown inconsistently to impair the oxidative function of neutrophils. The choice of the stimulus, receptor agonists, or stimuli acting independent of receptors seems to determine whether nitrous oxide impairs the oxidative functions, suggesting an interference with the cytosolic signaling of neutrophils. Production of hydrogen peroxide by neutrophils was assessed using flow cytometric analysis. N-formyl-methionyl-leucyl-phenylalanine (FMLP), C5a, dioctanylglycerol, and phorbol-12-myristate-13-acetate were used as stimuli. In addition, the expression of receptors for FMLP and the cytosolic-free calcium response of cells were measured. Nitrous oxide depresses C5a- or FMLP-induced generation of reactive oxygen derivatives in a concentration-dependent manner. However, the response with direct activation of protein kinase C was unaffected. Further, the number of FMLP receptors and the cytosolic calcium response were unaffected. Inhibition of the oxidative response was not reversible within the observation period of 4 h. Nitrous oxide inhibited the intracellular signaling of the investigated G-protein-coupled receptors for chemotactic peptides. No interference of nitrous oxide with reduced nicotinamide adenine dinucleotide phosphate oxidase, the oxidative enzyme system of neutrophils, nor with its activation through protein kinase C was detected.

Requirements for both Rac1 and Cdc42 in membrane ruffling and phagocytosis in leukocytes.

Specific pathways linking heterotrimeric G proteins and Fcgamma receptors to the actin-based cytoskeleton are poorly understood. To test a requirement for Rho family members in cytoskeletal events mediated by structurally diverse receptors in leukocytes, we transfected the full-length human chemotactic peptide receptor in RAW 264.7 cells and examined cytoskeletal alterations in response to the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (FMLP), colony stimulating factor-1 (CSF-1), IgG-coated particles, and phorbol 12-myristate 13-acetate (PMA). Expression of Rac1 N17, Cdc42 N17, or the GAP domain of n-chimaerin inhibited cytoskeletal responses to FMLP and CSF-1, and blocked phagocytosis. Accumulation of F-actin- rich "phagocytic cups" was partially inhibited by expression of Rac1 N17 or Cdc42 N17. In contrast, PMA-induced ruffling was not inhibited by expression of Rac1 N17, but was blocked by expression of Cdc42 N17, indicating that cytoskeletal inhibition by these constructs was nonoverlapping. These results demonstrate differential requirements for Rho family GTPases in leukocyte motility, and indicate that both Rac1 and Cdc42 are required for Fcgamma receptor- mediated phagocytosis and for membrane ruffling mediated by structurally distinct receptors in macrophages.

Subtle Differences between Human and Rabbit Neutrophil Receptors Shown by the Secretagogue Activity of Constrained Formyl Peptides

Stereochemically constrained extended β-antiparallel and folded β-turn analogs of the chemotactic agentN-formyl-Met-Leu-Phe-OH were tested for their ability to induce the release of β-glucuronidase from human and rabbit neutrophils. Selected biologically active peptides were further examined for their capacity to inhibit the binding of f-Met-Leu-[3H]Phe to whole human neutrophils at 4°C. The results suggest that Dpg2analogs with the extended backbone are significantly more potent in human peripheral blood neutrophils than the folded β-turn analogs. Surprisingly, in rabbit peritoneal neutrophils, the extended Dpg2analog appears to be marginally less active than the flexible parent peptide and the folded Ac6c2analog. In human neutrophils, the secretagogue activity increases in the following order with alteration in the C-terminal functions: -CONH2< -COOMe < -COOH << -COOBzl. However, this order of potency differs from that observed for the rabbit formyl peptide receptor (-COOH < -COOMe < -CONH2<< -COOBzl). In human neutrophils, the peptides’ ability to compete for the receptor binding site of f-Met-Leu-[3H]Phe correlates well with their secretagogue potency. The results provide convincing evidence for the existence of subtle differences between human peripheral blood neutrophils and rabbit peritoneal neutrophils with regard to ligand–receptor interactions of constrained chemotactic peptides. What is new and novel in this report is that constrained peptides can distinguish between the rabbit and human chemotactic peptide receptors which have so far been believed to have similar response to secretagogue agents. The data emphasize that directly relating the secretagogue activity observed in rabbit neutrophils to that observed in human neutrophils may not be unequivocal.

Peptides from the Amino-Terminus of Mouse Mitochondrially Encoded NADH Dehydrogenase Subunit 1 Are Potent Chemoattractants

Binding of N-formylated chemotactic peptides to specific cell surface receptors on polymorphonuclear leukocytes initiates a wide range of biological responses including migration of inflammatory cells, superoxide release, lysosomal enzyme secretion, calcium mobilization, and cellular activation. We previously established that the mouse MHC class I-b molecule H-2M3a binds peptides from the NH2-terminus of the mitochondrially encoded NADH dehydrogenase subunit 1 (ND1). Inasmuch as the N-formyl group is essential for peptide binding both to the chemotactic peptide receptor and to H-2M3a, we sought to test whether ND1 peptides can induce chemotaxis. We now show that fND11-12, fND11-8, fND11-6, fND11-5, fND11-4 and fND11-3 trigger the chemotactic receptor. Although all tested ND I peptide derivatives were chemotactic, we found an inverse relationship between peptide length and chemotactic potency (ED50). Our data establish that mitochondrially derived peptides are potent chemotactic ligands. The release of N-formylated peptides from disintegrating mitochondria may play an important role in the inflammatory response resulting from tissue injury. By attracting the host phagocytic cells to sites of tissue breakdown, these peptides could mediate an essential first step in tissue repair and healing.

Induction of G protein-coupled peptide receptor EBI 1 by human herpesvirus 6 and 7 infection in CD4+ T cells

EBI 1, a putative lymphocyte-specific G protein-coupled peptide receptor, was induced by human herpesvirus 6 or 7 infection in CD4+ T cells, and its expression increased early after infection and reached a plateau at 48 h. The induction of the EBI 1 gene by human herpesvirus 6 or 7 infection was not mediated by soluble factors but by the virus itself. Deduced from comparisons of the amino acid sequences among members of the G protein-coupled receptor superfamily, these findings suggest that EBI 1 may be a member of the leukocyte chemotactic peptide receptor family.

Regulatory interaction of N-formyl peptide chemoattractant receptors with the membrane skeleton in human neutrophils.

The cytoskeleton and/or membrane skeleton has been implicated in the regulation of N-formyl peptide receptors. The coupling of these chemotactic receptors to the membrane skeleton was investigated in plasma membranes from unstimulated and desensitized human neutrophils using the photoreactive agonist N-formyl-met-leu-phe-lys-N epsilon-[125I]2(p-azidosalicylamido)ethyl-1,3'- dithiopropionate (fMLFK-[125I]ASD). When membranes of unstimulated cells were solubilized in Triton-X 100, a detergent that does not disrupt actin filaments, only 50% of the photoaffinity-labeled receptors were solubilized sedimenting in sucrose density gradients at a rate consistent with previous reports. The remainder were found in the pellet fraction along with the membrane skeletal actin. Solubilization of the membranes in the presence of p-chloromercuriphenylsulfonic acid, elevated concentrations of KCl, or deoxyribonuclease I released receptors in parallel with actin. When membranes from neutrophils, desensitized by incubation with fMLFK-[125I]ASD at 15 degrees C, were solubilized, nearly all receptors were recovered in the pellet fraction. Incubation of cells with the ligand at 4 degrees C inhibited desensitization partially and prevented the conversion of a significant fraction of receptors to the form associated with the membrane skeletal pellet. In these separations the photoaffinity-labeled receptors not sedimenting to the pellet cosedimented with actin. Approximately 25% of these receptors could be immunosedimented with antiactin antibodies suggesting that N-formyl peptide receptors may interact directly with actin. These results are consistent with a regulatory role for the interaction of chemotactic N-formyl peptide receptors with actin of the membrane skeleton.

A carboxyl-terminal tail peptide of neutrophil chemotactic receptor disrupts its physical complex with G protein.

The binding of G protein to the N-formyl peptide receptor of human neutrophils was investigated with site-specific synthetic peptides. Peptide CT336(322) (322RALTEDSTQTSDTAT336) from the carboxyl-terminal tail region of the receptor competed with the receptor for binding to bovine Gi protein. The peptide competition was assayed by dissociation of a GTP-sensitive, rapidly sedimenting (7S) form of receptor-G protein complex as analyzed by velocity sedimentation on linear sucrose density gradients. An IC50 of 590 microM was determined for CT336(322) peptide. A control peptide, with the reverse sequence, rCT322(336) (336TATDSTQTSDETLAR322), did not perturb the sedimentation of the reconstituted receptor-G protein complex up to the highest tested concentration, 3 mM. Other peptides tested, corresponding to central portions of the predicted intracellular loop regions CII140(127) (127VLHPVWTQNHRTVS140) and CIII239(227) (227KIHKQGLIKSSRP239) of the receptor, failed to dissociate the reconstituted receptor-G protein complex. Control peptides from the extracellular region EII184(170) (170KTGTVACTFNFSPWT184) and an unrelated sequence matching a portion of neutrophil cytochrome b, CYT306(296) (296KVVITKVVTHPFKTIE306), were also ineffective. Our results suggest that the cytoplasmic tail of the formyl chemotactic peptide receptor is involved in its coupling to the signal-transducing G protein.