N-Formyl met-leu-phe receptors, including FPR and FPRL1 expressed by neutrophils and macrophages, respectively, recognize bacterial peptides containing
N-formylmethionyl residues and stimulate directed movement of the cells. Because all bacterial proteins and few mammalian proteins (only those synthesized within mitochondria) are initiated by N-formylmethionine, FPR and FPRL1 allow phagocytes to detect and respond preferentially to bacterial proteins. The bacterial peptide ligands that bind these receptors are some of the first identified and most potent chemoattractants for leukocytes. Chemoattractants include several types of diffusible molecules, often produced at sites of infection, that bind to specific receptors on cells and direct their movement toward the source of the chemoattractant. Other chemoattractants, such as the chemokines discussed in Chapter 3, are made by host cells. FPR and FPRL1, along with all other chemoattractant receptors, belong to the seven-transmembrane, guanosine triphosphate (GTP)-binding (G) protein-coupled receptor (GPCR) superfamily. These receptors initiate intracellular responses through associated trimeric G proteins (see Chapter 7). The G proteins stimulate many types of cellular responses, including cytoskeletal changes, resulting in increased cell motility.
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