Mutational analysis reveals multiple distinct sites within Fc gamma 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 immu ne response, and reducing the risk of autoimmunity. Previous studies conclu ded that inhibitory signal transduction by Fc gamma RIIB is mediated solely by its immunoreceptor tyrosine-based inhibition motif (ITIM) that, when ph osphorylated, recruits the SH2-containing inositol 5'-phosphatase SHIP and the SH2-containing tyrosine phosphatases SHP-1 and SHP-2. The mutational an alysis reported here reveals that the receptor's C-terminal 16 residues are also required for detectable Fc gamma RIIB association with SHIP in vivo a nd for Fc gamma RIIB-mediated phosphatidylinositol 3-kinase hydrolysis by S HIP. Although the ITIM appears to contain all the structural information re quired for receptor-mediated tyrosine phosphorylation of SHIP, phosphorylat ion is enhanced when the C-terminal sequence is present. Additionally, Fc g amma RIIB-mediated dephosphorylation of CD19 is independent of the cytoplas mic 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 uniquel y to transduction of Fc gamma RIIB-mediated inhibitory signals.