The protein tyrosine kinase Syk activity is reduced by clustering the mastcell function-associated antigen

The mast cell function-associated antigen (MAFA) is a glycoprotein first id entified on the membrane of rat mucosal-type mast cells (RBL-2H3 line). MAF A clustering causes a dose-dependent inhibition of these cells' secretory r esponse to the type I Fee receptor (Fc epsilon RI) stimulus. The inhibition has earlier been shown to take place upstream to the production step of in ositol phosphates in the Fc epsilon RI coupling cascade. To resolve further the mechanism of action of MAFA, we have investigated the events prior to the activation of phospholipase C. Activities of the non-receptor protein t yrosine kinases Lyn and Syk in untreated cells were compared with those whe re the Fc epsilon RI, MAFA or both were clustered. Syk tyrosine phosphoryla tion and activation, as well as LAT (linker for activation of T cells) tyro sine phosphorylation, both induced by Fc epsilon RI clustering, were found to be reduced upon MAFA clustering. In contrast, the activity of the Src ho mology domain 2 (SH2)-containing protein tyrosine phosphatase (SHP-2) incre ased. MAFA clustering also enhanced the cc-isolation of SHP-2 and Syk with tyrosine-phosphorylated MAFA in both untreated and Fc epsilon RI-stimulated cells. SHP-2 caused a decline in the Fc epsilon RI-induced tyrosine phosph orylation of Syk, at least under in vitro conditions. Taken together, these results suggest that one possible mechanism by which MAFA affects the Fc e psilon RI stimulation cascade is suppression of Syk activity, i.e. MAFA clu stering leads SHP-2 to act on Syk, thereby reducing its tyrosine phosphoryl ation and its activity.