CHARACTERIZATION OF CBL TYROSINE PHOSPHORYLATION AND A CBL-SYK COMPLEX IN RBL-2H3 CELLS

Tyrosine phosphorylation of the Cbl protooncogene has been shown to oc cur after engagement of a number of different receptors on hematopoiet ic cells. However, the mechanisms by which these receptors induce Cbl tyrosine phosphorylation are poorly understood. Here we demonstrate th at engagement of the high affinity IgE receptor (Fc epsilon R1) leads to the tyrosine phosphorylation of Cbl and analyze how this occurs. We show that at least part of Fc epsilon RI-induced Cbl tyrosine phospho rylation is mediated by the Syk tyrosine kinase, and that the Syk-depe ndent tyrosine phosphorylation of Cbl occurs mainly distal to the Cbl proline-rich region within the COOH-terminal 250 amino acids. Furtherm ore, we show by coprecipitation that Cbl is present in a complex with Syk before receptor engagement, that the proline-rich region of Cbl an d a region of Syk comprised of the two SH2 domains and intradomain lin ker are required for formation of the complex, and that little. or no tyrosine-phosphorylated Cbl is detected in complex with Syk. Overexpre ssion of truncation mutants of Cbl capable of binding Syk has the effe ct of blocking tyrosine phosphorylation of endogenous Cbl. These resul ts define a potentially important intramolecular interaction in mast c ells and suggest a complex function for Cbl in intracellular signaling pathways.