Internalization of the human N-formyl peptide and C5a chemoattractant receptors occurs via clathrin-independent mechanisms

After stimulation by ligand, most G protein-coupled receptors (GPCRs) under go rapid phosphorylation, followed by desensitization and internalization. In the case of the N-formyl peptide receptor (FPR), these latter two proces sing steps have been shown to be entirely dependent on phosphorylation of t he receptor's carboxy terminus. We have previously demonstrated that FPR in ternalization can occur in the absence of receptor desensitization, indicat ing that FPR desensitization and internalization are regulated differential ly. In this study, we have investigated whether human chemoattractant recep tors internalize via clathrin-coated pits. Internalization of the FPR trans iently expressed in HEK 293 cells was shown to be dependent upon receptor p hosphorylation. Despite this, internalization of the FPR, as well as the C5 a receptor, was demonstrated to be independent of the actions of arrestin, dynamin, and clathrin. In addition, we utilized fluorescence microscopy to visualize the FPR and beta (2)-adrenergic receptor as they internalized in the same cell, revealing distinct sites of internalization. Last, we found that a nonphosphorylatable mutant of the FPR, unable to internalize, was co mpetent to activate p44/42 MAP kinase. Together, these results demonstrate not only that the FPR internalizes via an arrestin-, dynamin-, and clathrin -independent pathway but also that signal transduction to MAP kinases occur s in an internalization-independent manner.