THE SH3 DOMAIN-BINDING T-CELL TYROSYL PHOSPHOPROTEIN P120 - DEMONSTRATION OF ITS IDENTITY WITH THE C-CBL PROTOONCOGENE PRODUCT AND IN-VIVO COMPLEXES WITH FYN, GRB2, AND PHOSPHATIDYLINOSITOL 3-KINASE

Previously, we have identified p120 as a Fyn/Lck SH3 and SH2 domain-binding protein that is tyrosine phosphorylated rapidly after T cell receptor triggering. Here, we used direct protein purification, amino acid sequence analysis, reactivity with antibodies, and two-dimensional gel analyses to identify p120 as the human c-cbl protooncogene product. We demonstrate in, vivo complexes of p120(cbl) with Fyn tyrosine kinase, the adaptor protein Grb2, and the p85 subunit of phosphatidylinositol (PI) 3-kinase, The association of p120(cbl) with Fyn and the p85 subunit of PI 3-kinase (together with PI 3-kinase activity) was markedly increased by T cell activation, consistent with in vitro binding of p120(cbl) to their SH2 as well as SH3 domains, In contrast, a large fraction of p120(cbl) was associated with Grb2 prior to activation, and this association did not change upon T cell activation. In vitro, p120(cbl) interacted with Grb2 exclusively through its SH3 domains, These results demonstrate a novel Grb2-p120(cbl) signaling complex in T cells, distinct from the previously analyzed Grb2-Sos complex. The association of p120(cbl) with ubiquitous signaling proteins strongly suggests a general signal transducing function for this enigmatic protooncogene with established leukemogenic potential but unknown physiological function.