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) undergo rapid phosphorylation, followed by desensitization and internalization. In the case of the N-formyl peptide receptor (FPR), these latter two processing steps have been shown to be entirely dependent on phosphorylation of the receptor's carboxy terminus. We have previously demonstrated that FPR internalization can occur in the absence of receptor desensitization, indicating that FPR desensitization and internalization are regulated differentially. In this study, we have investigated whether human chemoattractant receptors internalize via clathrin-coated pits. Internalization of the FPR transiently expressed in HEK 293 cells was shown to be dependent upon receptor phosphorylation. Despite this, internalization of the FPR, as well as the C5a 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 competent 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 occurs in an internalization-independent manner.