Phosphotyrosine Kinase Assays as a Prescreen for Inhibitors of EGFr

The epidermal growth factor receptor (EGFr) is a 170-kDa glycosylated transmembrane protein found in a wide variety of tissues (1 ). It is the receptor for epidermal growth factor (EGF), transforming growth factor α (TGFα), and other related growth factor peptides (2 ,3 ). In 1980, EGF binding activity of receptor preparations was shown to be associated with a protein kinase activity. Subsequent studies demonstrated that the EGF binding activity and kinase activity were properties of the same protein (4 ). In the absence of EGF, tyrosine kinase activity is low. Binding of EGF to the receptor domain results in receptor dimerisation and subsequent autophosphorylation of the cytoplasmic domain of EGFr at specific tyrosine residues. This autophosphorylation results in an activation of the tyrosine kinase activity of the EGFr. The phosphorylated tyrosine residues of the EGFr are involved in binding enzymes containing SH2 or SH3 domains (5). Enzymes identified to date that bind tightly to EGFr phosphotyrosine residues include phosphatidyl inositol 3 kinase, phospholipase Cγ and ras GTPase activating protein (6 ). Binding of these enzymes occurs at highly specific phosphotyrosine residues of the receptor and is believed to be a mechanism for signal transduction from the receptor to the final intracellular target.