C3G IS TYROSINE-PHOSPHORYLATED AFTER INTEGRIN-MEDIATED CELL-ADHESION IN NORMAL BUT NOT IN BCR ABL EXPRESSING CELLS/

The SH2-SH3 adaptor protein Crkl has been implicated in the signal tra nsduction pathways of several membrane-bound receptors. Tyrosine phosp horylation of proteins associated with such signalling complexes can g enerate binding sites for the Crkl SH2-domain and can recruit proteins constitutively bound to Crkl via the Crkl SH3 domain into such comple xes. In the current study we show that Crkl, but only a minor amount o f the related Crk, form constitutive complexes in vivo with guanine nu cleotide exchange factor C3G in 3T3 fibroblasts. Adhesion of both norm al and transformed cells to fibronectin or other extracellular matrix proteins consistently induces the tyrosine-phosphorylation of C3G. Adh esion-induced tyrosine phosphorylation of C3G is dependent on an intac t cytoskeleton and peaks at 5-10 min after attachment. In contrast, 3T 3 cells stably transfected with Bcr/Abl P210 show a prominent reductio n in the amount of C3G complexed to Crkl and do not exhibit tyrosine-p hosphorylation of C3G upon spreading and attachment. These data establ ish that integrin-mediated cell adhesion results in Crkl-mediated tyro sine phosphorylation of C3G, a pathway which can be disrupted by Bcr/A bl.