Tyrosine phosphorylation of p62^dok by p210^bcr-abl inhibits RasGAP activity

The t(9;22) chromosomal translocation is found in almost all patients with chronic myelogenous leukemia. The resultant Bcr-Abl fusion gene expresses a chimeric fusion protein p210^bcr-abl with increased tyrosine kinase activity. Hematopoietic progenitors isolated from chronic myelogenous leukemia patients in the chronic phase contain constitutively tyrosine-phosphorylated p62^dok protein. p62^dok associates with the Ras GTPase-activating protein (RasGAP), but only when p62^dok is tyrosine phosphorylated. Here we have investigated the interaction between p62^dok and RasGAP and the consequences of p62^dok tyrosine phosphorylation on the activity of RasGAP. We have found that p62^dok is directly tyrosine phosphorylated by p210^bcr-abl, and the sites of phosphorylation are located in the C-terminal half of the p62^dok molecule. We have identified five tyrosine residues that are involved in in vitro RasGAP binding and have found that tyrosine-phosphorylated p62^dok inhibits RasGAP activity. Our results suggest that p210^bcr-abl might lead to the activation of the Ras signaling pathway by inhibiting a key down-regulator of Ras signaling.