SH2 domain-containing inositol polyphosphate 5 '-phosphatase is the main mediator of the inhibitory action of the mast cell function-associated antigen

The mast cell function-associated Ag (MAFA) is a type H membrane glycoprote in originally found on the plasma membrane of rat mucosal-type mast cells ( RBL-2H3 line). A C-type lectin domain and an immunoreceptor tyrosine-based inhibitory motif (ITIM) are located in the extracellular and intracellular domains of MAFA, respectively. MAFA clustering has previously been shown to suppress the secretory response of these cells to the Fc epsilon RI stimul us. Here we show that the tyrosine of the ITIM undergoes phosphorylation, o n MAFA clustering, that is markedly enhanced on pervanadate treatment of th e cells. Furthermore, the Src homology 3 domain of the protein tyrosine kin ase Lyn binds directly to a peptide containing nonphosphorylated MAFA ITIM and PAAP motif. Results of both in vitro and in vivo experiments suggest th at Lyn is probably responsible for this ITIM phosphorylation, which increas es the Src homology domain 2 (SH2) affinity of Lyn for the peptide. In vitr o measurements established that tyrosine-phosphorylated MAFA ITIM peptides also bind the SH2 domains of inositol 5'-phosphatase (SHIP) as well as prot ein tyrosine phosphatase-2. However, the former single domain is bound 8-fo ld stronger than both of the latter. Further support for the role of SHIP i n the action of MAFA stems from in vivo experiments in which tyrosine-phosp horylated MAFA was found to bind primarily SHIP. In RBL-2H3 cells overexpre ssing wild-type SHIP, MAFA clustering causes markedly stronger inhibition o f the secretory response than in control cells expressing normal SHIP level s or cells overexpressing either wild-type protein tyrosine phosphatase-2 o r its dominant negative form. In contrast, on overexpression of the SH2 dom ain of SHIP, the inhibitory action of MAFA is essentially abolished. Taken together, these results suggest that SHIP is the primary enzyme responsible for mediating the inhibition by MAFA of RBL-2H3 cell response to the Fc ep silon RI stimulus.