A 77-KDA PROTEIN ASSOCIATES WITH PP125(FAK) IN MAST-CELLS AND BECOMESTYROSINE-PHOSPHORYLATED BY HIGH-AFFINITY IGE RECEPTOR AGGREGATION

The focal adhesion kinase, pp125(FAK), is a novel nonreceptor protein tyrosine kinase expressed in different cells including mast cells. Her e we report that a 77-kDa protein associates with pp125(FAK) in the ma st cell analog, rat basophilic leukemia (RBL-2H3) cells. When pp125(FA K) immunoprecipitates were subjected to an in vitro kinase assay, ther e was prominent phosphorylation on tyrosine of pp125(FAK) and of a 77- kDa protein. By V8 protease digestion mapping and by immunoblotting wi th two different anti-pp125(FAK) antibodies, the 77-kDa protein was di stinct from pp125(FAK). This Fak Associated Protein or FAP was detecte d in RBL-2H3 cells but not in fibroblasts. The aggregation of the high affinity IgE receptor, Fc epsilon RI, induced the in vivo tyrosine ph osphorylation of FAP. However, there was a marked decrease in the in v itro phosphorylation of FAP in the immunoprecipitates from Fc epsilon RI aggregated cells. Both of these Fc epsilon RI-mediated effects were enhanced by cell adhesion. There was strong association of FAP with n on-tyrosine-phosphorylated pp125(FAK). Thus this interaction does not appear to be mediated by the Src homology 2 domain. Together the data indicate that FAP associates with pp125(FAK) and suggest that FAP may play a role in Fc epsilon RI signaling.