TRANSFORMATION-SPECIFIC INTERACTION OF THE BOVINE PAPILLOMAVIRUS E5 ONCOPROTEIN WITH THE PLATELET-DERIVED GROWTH-FACTOR RECEPTOR TRANSMEMBRANE DOMAIN AND THE EPIDERMAL GROWTH-FACTOR RECEPTOR CYTOPLASMIC DOMAIN

The bovine papillomavirus E5 transforming protein appears to activate both the epidermal growth factor receptor (EGF-R) and the platelet-der ived growth factor receptor (PDGF-R) hy a ligand-independent mechanism . To further investigate the ability of E5 to activate receptors of di fferent classes and to determine whether this stimulation occurs throu gh the extracellular domain required for ligand activation, we constru cted chimeric genes encoding PDGF-R and EGF-R by interchanging the ext racellular, membrane, and cytoplasmic coding domains. Chimeras were tr ansfected into NIH 3T3 and CHO(LR73) cells. All chimeras expressed sta ble protein which, upon addition of the appropriate ligand, could be a ctivated as assayed by tyrosine autophosphorylation and biological tra nsformation. Cotransfection of E5 with the wild-type and chimeric rece ptors resulted in the ligand-independent activation of receptors, prov ided that a receptor contained either the transmembrane domain of the PDGF-R or the cytoplasmic domain of the EGF-R. Chimeric receptors that contained both of these domains exhibited the highest level of E5-ind uced biochemical and biological stimulation. These results imply that E5 activates the PDGF-R and EGR-R by two distinct mechanisms, neither of which specifically involves the extracellular domain of the recepto r. Consistent with the biochemical and biological activation data, coi mmunoprecipitation studies demonstrated that E5 formed a complex with any chimera that contained a PDGF-R transmembrane domain or an EGF-R c ytoplasmic domain, with those chimeras containing both domains demonst rating the greatest efficiency of complex formation. These results sug gest that although different domains of the PDGF-R and EGF-R are requi red for E5 activation, both receptors are activated directly by format ion of an E5-containing complex.