Efficient cellular transformation by the Met oncoprotein requires a functional Grb2 binding site and correlates with phosphorylation of the Grb2-associated proteins, Cbl and Gab1.

The Tpr-Met oncoprotein consists of the catalytic kinase domain of the hepatocyte growth factor/scatter factor receptor tyrosine kinase (Met) fused downstream from sequences encoded by the tpr gene. Tpr-Met is a member of a family of tyrosine kinase oncoproteins generated following genomic rearrangement and has constitutive kinase activity. We have previously demonstrated that a single carboxyl-terminal tyrosine residue, Tyr489, is essential for efficient transformation of Fr3T3 fibroblasts by Tpr-Met and forms a multisubstrate binding site for Grb2, phosphatidylinositol 3' kinase, phospholipase Cgamma, SHP2, and an unknown protein of 110 kDa. A mutant Tpr-Met protein that selectively fails to bind Grb2 has reduced transforming activity, implicating pathways downstream of Grb2 in Tpr-Met mediated cell transformation. We show here that the 110-kDa Tpr-Met substrate corresponds to the recently identified Grb2-associated protein, Gab1. Moreover, we show that tyrosine phosphorylation of the Cbl protooncogene product as well as Gab1 required Tyr489 and correlated with the ability of Tpr-Met to associate with Grb2 and to transform cells, providing evidence that pathways downstream of Gab1 and/or Cbl may play a role in Tpr-Met-mediated cell transformation.