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Regulation of the Association of p120cbl with Grb2 in Jurkat T Cells*

The c-cbl protooncogene product (p120cbl) is a known substrate of multiple tyrosine kinases. It is found in complexes with critical signal transduction molecules, including the linker protein Grb2. Here, we demonstrate using an immobilized Grb2-binding peptide that the Grb2-p120cbl complex dissociates in vivo following engagement of the T-cell antigen receptor in Jurkat T-cells. The early kinetics of this dissociation correlate with the known time course of tyrosine phosphorylation of p120cbl and other substrates. This dissociation persists in vivo even when p120cbl becomes dephosphorylated to basal levels. However, this decreased association is not observed in protein overlay assays on nitrocellulose membranes in which a Grb2 fusion protein is used to detect p120cbl from stimulated or unstimulated cells. These data suggest that the tyrosine phosphorylation of p120cbl does not completely account for the regulation of its association with Grb2. Additionally, we used truncation mutations of p120cbl to map the p120cbl-Grb2 interaction to amino acids 481-528 of p120cbl; this interaction is stronger in longer constructs that include additional proline-rich motifs. The in vivo regulation of the Grb2-p120cbl complex further supports the idea of a significant role for p120cbl in receptor-mediated signaling pathways.