Clustering the mast cell function-associated antigen (MAFA) leads to tyrosine phosphorylation of p62Dok and SHIP and...

The mast cell function-associated antigen (MAFA) is a type II membranal glycoprotein expressed by rat mast cells and basophils. MAFA clustering by its specific monoclonal antibody, (mAb) G63, efficiently inhibits the FcvarepsilonRI induced secretory response of mucosal-type mast cells of the RBL-2H3 line, as well as bone marrow-derived mast cells. Here we present results which suggest that MAFA has also a capacity of modulating the cell cycle of the RBL-2H3 line. We found that MAFA clustering, by mAb G63 or by its F(ab')2 fragments, reduces the cell proliferation rate. Cell cycle analysis by flow cytometry revealed that the number of cells in sub-G phase is considerably higher for cells on which MAFA was clustered. Results of biochemical experiments established that MAFA clustering leads to a marked increase in the transient tyrosine phosphorylation of the adaptor protein p62(Dok) and the inositol phosphatase SHIP. Concomitantly, their respective binding to RasGAP and Shc was increased. Furthermore, the GTP binding protein Sos1 was found to dissociate from Shc upon MAFA clustering, suggesting that SHIP and Sos1 compete for Shc binding. We therefore suggest that MAFA has also a role in regulating RBL-2H3 cell proliferation rate by inhibiting RasGTP formation in the Ras signaling pathway.