Mutational analysis reveals multiple distinct sites within Fcγ receptor IIB that function in inhibitory signaling

The low-affinity receptor for IgG, Fc gamma RIIB, functions broadly in the immune system, blocking mast cell degranulation, dampening the humoral immune response, and reducing the risk of autoimmunity. Previous studies concluded that inhibitory signal transduction by Fc gamma RIIB is mediated solely by its immunoreceptor tyrosine-based inhibition motif (ITIM) that, when phosphorylated, recruits the SH2-containing inositol 5'-phosphatase SHIP and the SH2-containing tyrosine phosphatases SHP-1 and SHP-2. The mutational analysis reported here reveals that the receptor's C-terminal 16 residues are also required for detectable Fc gamma RIIB association with SHIP in vivo and for Fc gamma RIIB-mediated phosphatidylinositol 3-kinase hydrolysis by SHIP. Although the ITIM appears to contain all the structural information required for receptor-mediated tyrosine phosphorylation of SHIP, phosphorylation is enhanced when the C-terminal sequence is present. Additionally, Fc gamma RIIB-mediated dephosphorylation of CD19 is independent of the cytoplasmic tail distal from residue 237, including the ITIM. Finally, the findings indicate that tyrosines 290, 309, and 326 are all sites of significant Fc gamma RIIB1 phosphorylation following coaggregation with B cell Ag receptor. Thus, me conclude that multiple sites in Fc gamma RIIB contribute uniquely to transduction of Fc gamma RIIB-mediated inhibitory signals.