IDENTIFICATION OF A HUMAN EPIDERMAL GROWTH-FACTOR RECEPTOR-ASSOCIATEDPROTEIN-KINASE AS A NEW MEMBER OF THE MITOGEN-ACTIVATED PROTEIN-KINASE EXTRACELLULAR SIGNAL-REGULATED PROTEIN-KINASE FAMILY

A putative mitogen-activated protein kinase (MAPK) has recently been i dentified, which potentially phosphorylates the human epidermal growth factor (EGF) receptor at a physiological site (Thr-669) and is distin guished from other MAPKs/extracellular signal-regulated protein kinase s (ERKs) on the basis of chromatographic, immunological, and kinetic d ata. Here we report that this newly discovered MAPK is physically asso ciated with the EGF receptor in A431 cells and with the related recept or/tyrosine kinase HER2 (encoded by c-neu) in enzyme preparations obta ined from Wilm's tumors. This human EGF receptor-associated kinase is characterized as a 40-kDa Thr-669 kinase that exists in a high molecul ar mass complex with the respective growth factor receptor. EGF treatm ent of A431 cells stimulates the tyrosine phosphorylation of p40 and i ncreases Thr-669 kinase activity in p40-containing fractions. The 40-k Da kinase is recognized by affinity-purified polyclonal antibodies dir ected against the sea star p44mpk and a Pan-ERK antibody directed agai nst the conserved subdomain VIII of MAPKs/ERKs, but is not recognized by antibodies selective for the rat p44erk1 and/or the p42mapk/erk2 is oforms, thus identifying the EGF receptor-associated kinase as a novel MAPK that may regulate receptor function in vivo.