Efficient cell transformation by the Tpr-Met oncoprotein is dependent upon tyrosine 489 in the carboxy-terminus

The receptor for hepatocyte growth factor/scatter factor (HGF/SF) was originally identified as an oncogene, Tpr-Met, which consists of the cytoplasmic tyrosine kinase domain of the HGF/SF receptor (Met) fused down-stream of sequences encoded by the tpr gene. As a consequence of this rearrangement the Tpr-Met fusion oncoprotein is localized to the cytoplasm and is a constitutively activated kinase. To identify signalling pathways important for Tpr-Met-mediated cell transformation we have generated tyrosine to phenylalanine mutants of Tpr-Met that are compromised in their ability to transform Fischer rat 3T3 (Fr3T3) cells in culture. We show that a single tyrosine residue in the carboxy terminus of Tpr-Met (residue 489) is essential for efficient transformation of Fr3T3 cells by this oncoprotein. Mutation of tyrosine 489 to phenylalanine does not affect the exogenous kinase activity of the Tpr-Met oncoprotein toward casein, but it impairs the ability of the mutant protein to bind to and activate phosphatidylinositol 3 kinase in vivo and completely abolishes the in vivo association with the Grb2 adaptor protein as well as the association and/or phosphorylation of an unknown protein of 110 kDa. These data are consistent with a single tyrosine residue in the Tpr-Met oncoprotein being essential for the activation of several signalling pathways which lead to the transformation of Fr3T3 fibroblasts.