The oncoprotein 70Z/3 Cbl signals in an autonomous fashion or through blockade of endogenous c-Cbl, a negative regulator of signaling. The mechanism of 70Z/3 Cbl-induced signaling was investigated by comparing the molecular requirements for 70Z/3 Cbl- and TCR-induced phospholipase Cγ1 (PLCγ1) activation. 70Z/3 Cbl-induced PLCγ1 tyrosine phosphorylation required, in addition to the PLCγ1 N-terminal SH2 domain, the C-terminal SH2 and SH3 domains that were dispensable for TCR-induced phosphorylation. Deletion of the leucine zipper of 70Z/3 Cbl did not eliminate 70Z/3 Cbl-induced PLCγ1 phosphorylation, suggesting that blockage of c-Cbl via dimerization with 70Z/3 Cbl cannot fully explain 70Z/3 Cbl activating characteristics. The complete elimination of PLCγ1 phosphorylation required deleting the SH3 domain-binding region of 70Z/3 Cbl, consistent with 70Z/3 Cbl binding the PLCγ1 SH3 domain. 70Z/3 Cbl-induced PLCγ1 phosphorylation required Zap-70, as for the TCR, and the tyrosine kinase binding domain of 70Z/3 Cbl, which binds Zap-70, but did not require PLCγ1 binding to Lat, a crucial interaction in TCR-induced PLCγ1 phosphorylation. Furthermore, 70Z/3 Cbl-induced activation of NFAT, a PLCγ1/Ca2+-dependent transcriptional event, required Zap-70, but was independent of Slp-76, an adapter required for TCR-induced NFAT activation. These results suggest that 70Z/3 Cbl and PLCγ1 form a TCR-, Lat- and Slp-76-independent complex that leads to PLCγ1 phosphorylation and activation.
