Cbl-b, a RING-type E3 ubiquitin ligase, targets phosphatidylinositol 3-kinase for ubiquitination in T cells

Cbl-b is implicated in setting the threshold of T lymphocyte activation. In Cbl-b-deficient T cells, the activation of Vav, a guanine nucleotide excha nge factor, is significantly enhanced. The molecular mechanism underlying C bl-b-regulated Vav activation was unclear, Here it is shown that Cbl-b inte racts with and induces ubiquitin conjugation to the p85 regulatory subunit of phosphatidylinositol 3-kinase, an upstream regulator of Vav. A functiona l RING finger of Cbl-b was essential for p85 ubiquitination. However, a los s of function mutation at the well-conserved amino-terminal variant src hom ology (SH) 2 domain of Cbl-b did not affect its ligase activity. A distal c arboxyl-terminal proline-rich region in Cbl-b was mapped to contain the pri mary binding sequences for the SH3 domain of p85. Deletion of either the di stal proline-rich region in Cbl-b or the SH3 domain of p85 severely reduced ubiquitin conjugation to p85. The data suggest a molecular link for Cbl-b- mediated negative regulation of Vav, with phosphatidylinositol 3-kinase as a direct target for Cbl-b E3 ubiquitin ligase.