SRC FAMILY TYROSINE KINASE LYN BINDS SEVERAL PROTEINS INCLUDING PAXILLIN IN RAT BASOPHILIC LEUKEMIA-CELLS

Aggregation of the high affinity IgE receptors on rat basophilic leuke mia (RBL-2H3) cells results in protein tyrosine phosphorylation althou gh the receptor has no intrinsic enzymatic activity. The Src related p rotein tyrosine kinase p53/56(lyn) present in RBL-2H3 cells could play a role in this reaction. Here we have isolated the cDNA for rat Lyn a nd found it to be very homologous at the amino acid level to both the human and mouse proteins. A bacterially expressed maltose binding prot ein-Lyn (MBP-Lyn) fusion protein was already tyrosine phosphorylated a nd had tyrosine kinase activity. In a filter-binding assay, MBP-Lyn fu sion protein (at 0.1 mu M) specifically bound to several proteins of R BL-2H3 cells. In lysates of IgE receptor-activated cells, there was in creased binding of MBP-Lyn to 65, 72, 78 and 110 kDa tyrosine phosphor ylated proteins. The 72, 78 and 110 kDa tyrosine phosphorylated protei ns were precipitated by a fusion protein containing the Lyn Src Homolo gy 2 (SH2) domain. The 72 kDa Lyn binding protein was different from p 72(syk). Furthermore, paxillin, a cytoskeletal protein, was identified as one of the Lyn binding proteins. Thus Fc epsilon RI mediated signa l transduction in RBL-2H3 cells may result from the interaction of p53 /56(lyn) with paxillin, pp72, pp110 and other proteins.