EFFICIENT CELLULAR-TRANSFORMATION BY THE MET ONCOPROTEIN REQUIRES A FUNCTIONAL GRB2 BINDING-SITE AND CORRELATES WITH PHOSPHORYLATION OF THEGRB2-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 followi ng genomic rearrangement and has constitutive kinase activity, We have previously demonstrated that a single carboxyl-terminal tyrosine resi due, Tyr(489), is essential for efficient transformation of Fr3T3 fibr oblasts by Tpr-Met and forms a multisubstrate binding site for Grb2, p hosphatidylinositol 3' kinase, phospholipase C gamma, SHP2, and an unk nown protein of 110 kDa, A mutant Tpr-Met protein that selectively fai ls to bind Grba has reduced transforming activity, implicating pathway s downstream of Grba in Tpr-Met mediated cell transformation, We show here that the 110-kDa Tpr-Met substrate corresponds to the recently id entified Grb2-associated protein, Gab1, Moreover, we show that tyrosin e phosphorylation of the Cbl protooncogene product as well as Gab1 req uired Tyr(489) and correlated with the ability of Tpr-Met to associate with Grba and to transform cells, providing evidence that pathways do wnstream of Gab1 and/or Cbl may Flay a role in Tpr-Met-mediated cell t ransformation.