title: N-formyl peptide receptor 3 (FPR3) departs from the homologous FPR2/ALX receptor with regard to the major processes governing chemoattractant receptor regulation, expression at the cell surface, and phosphorylation.
abstract: Among human N-formyl peptide chemoattractant receptors, FPR2/ALX and FPR3 share the highest degree of amino acid identity (83%), and trigger similar cell responses upon ligand binding. Although FPR2/ALX is a promiscuous receptor, FPR3 has only one specific high affinity ligand, F2L, and a more restricted tissue/cell distribution. In this study, we showed that FPR2/ALX behaved as the prototypical receptor FPR1. The agonist-dependent phosphorylation used a hierarchical mechanism with a prominent role of Ser(329), Thr(332), and Thr(335). Phosphorylation of FPR2/ALX was essential for its desensitization but the lack of phosphorylation did not result in enhanced or sustained responses. In contrast, resting FPR3 displayed a marked level of phosphorylation, which was only slightly increased upon agonist stimulation. Another noticeable difference between the two receptors was their subcellular distribution in unstimulated cells. Although FPR2/ALX was evenly distributed at the plasma membrane FPR3 was localized in small intracellular vesicles. By swapping domains between FPR2/ALX and FPR3, we uncovered the determinants involved in the basal phosphorylation of FPR3. Experiments aimed at monitoring receptor-bound antibody uptake showed that the intracellular distribution of FPR3 resulted from a constitutive internalization that was independent of C terminus phosphorylation. Unexpectedly, exchanging residues 1 to 53, which encompass the N-terminal extracellular region and the first transmembrane domain, between FPR2/ALX and FPR3 switched localization of the receptors from the plasma membrane to intracellular vesicles and vice versa. A clathrin-independent, possibly caveolae-dependent, mechanism was involved in FPR3 constitutive internalization. The peculiar behavior of FPR3 most probably serves distinct physiological functions that remain largely unknown.
authors.affiliation: INSERM, Unité 1036, Biology of Cancer and Infection, Grenoble F-38054, France. marie-josephe.rabiet@cea.fr
authors.forename: Marie-Josèphe
authors.initials: MJ
authors.lastname: Rabiet
authors.forename: Laurence
authors.initials: L
authors.lastname: Macari
authors.forename: Claes
authors.initials: C
authors.lastname: Dahlgren
authors.forename: François
authors.initials: F
authors.lastname: Boulay
mesh_terms: D021941:Caveolae / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000502:physiology*; D057809:HEK293 Cells; D018922:HL-60 Cells; D006801:Humans; D020125:Mutation, Missense; D010766:Phosphorylation / Q000502:physiology; D017434:Protein Structure, Tertiary; D044042:Receptors, Formyl Peptide / Q000235:genetics / Q000378:metabolism*; D044067:Receptors, Lipoxin / Q000235:genetics / Q000378:metabolism*
chemical_list: C105101:FPR2 protein, human; C469985:FPR3 protein, human; D044042:Receptors, Formyl Peptide; D044067:Receptors, Lipoxin