Linkage of tyrosine kinase activity with transforming ability of the p185neu oncoprotein.

The neu oncogene encodes a 185 kd glycoprotein (p185neu) with intrinsic tyrosine kinase activity. Sequencing data has demonstrated that oncogenic p185neu differs from c-neu by a single point mutation within the transmembrane region of the glycoprotein. This mutation results in the substitution of a glutamic acid residue for a nonpolar valine residue at amino acid position 664 of the rat neu gene product. Recent studies have demonstrated that this mutation results in specific aggregation of the p185neu oncoprotein mimicking ligand induced dimerization events. The cellular consequences of the aggregated phenotype may include the enzymatic activation of p185neu. We demonstrate that the oncoprotein p185neu possesses higher intrinsic tyrosine kinase activity and that this increase in enzymatic activity is apparent within the plasma membrane, manifesting itself through the increased tyrosine phosphorylation of substrates. Furthermore, the neu oncoprotein itself is also phosphorylated on tyrosine to a higher extent than the proto-oncoprotein. These results strongly link enzymatic activation of p185neu to cellular transformation events. To test directly the effect of p185neu tyrosine kinase activity on cellular transformation we constructed mutant p185neu devoid of ATP binding ability. This mutant protein is expressed at high levels, but is unable to induce the transforming phenotype. The point mutation within the transmembrane region of p185neu mimics aspects of ligand induced activation events including increases in the specific tyrosine kinase activity of the molecule leading to cellular transformation.