FCGRIII (CD16) is a low affinity Fc gamma receptor and is encoded by two genes (A and B), the transmembrane form FCGRIIIA and the GPI anchored FCGRIIIB (Edberg et al. 1989). Between the two isoforms FCGRIIIA is involved in phagocytosis whereas FCGRIIIB is capable of inducing calcium signalling and actin polymerisation, but its role in phagocytosis is still not clear (Garcia Garcia & Rosales 2002). FCGRIIIA is expressed in macrophages and natural killer cells as a multi chain complex consisting of a single alpha chain containing IgG binding domains and a signal transducing gamma and/or zeta dimer (Wirthmuller et al. 1992, Lanier et al. 1989). Both gamma and zeta chains contain a conserved immunoreceptor tyrosine based activation motif (ITAM), which has 2 copies of the YXXL sequence (Isakov 1997). However, the gamma chain of FCGRIIIA is approximately sixfold more efficient in mediating phagocytosis than the zeta subunit (Park & Schreiber 1995). Phosphorylation of the conserved tyrosine residues of the ITAM in these accessory proteins is required for the phagocytic signal mediated by FCRGIIIA.
The first step in Fc-gamma receptor (FCGR) phagocytosis is binding and clustering of FCGRs by IgG-coated foreign particles. FCGR are clustered at the cell surface by multivalent antigen-antibody complexes and recruited to lipid raft micro domains; monovalent ligand binding is insufficient to generate a signal.
This cross linking results in the localisation of FCGRs into lipid rafts and this may aid in their recruiting and complexing with additional signalling proteins associated with lipid rafts (Kono et al. 2002). This is followed by phosphorylation of the tyrosine residues within the ITAM located on the cytoplasmic portion of accessory gamma/zeta chains by membrane associated tyrosine kinases of the Src family (Park et al. 1993).