E5 oncoprotein mutants activate phosphoinositide 3-kinase independently ofplatelet-derived growth factor receptor activation

The E5 oncoprotein of bovine papillomavirus type 1 is a Golgi-resident, 44- amino acid polypeptide that can transform fibroblast cell lines by activati ng endogenous platelet-derived growth factor receptor beta (PDGF-R), Howeve r, the recent discovery of E5 mutants that exhibit strong transforming acti vity but minimal PDGF-R tyrosine phosphorylation indicates that E5 can pote ntially use additional signal transduction pathway(s) to transform cells. W e now show that two classes of E5 mutants, despite poorly activating the PD GF-R, induce tyrosine phosphorylation and activation of phosphoinositide S- kinase (PI 3-K) and that this activation is resistant to a selective inhibi tor of PDGF-R kinase activity, tyrphostin AG1296, Consistent with this inde pendence from PDGF-R signaling, the E5 mutants fail to induce significant c ell proliferation in the absence of PDGF, unlike wild-type E5 or the sis on coprotein. Despite differences in growth factor requirements, however, both wild-type E5 and mutant E5 cell lines form colonies in agarose, Interestin gly, activation of PI 3-K occurs without concomitant activation of the ras- dependent mitogen-activated protein kinase pathway. The known ability of co nstitutively activated PI 3-K to induce anchorage-independent cell prolifer ation suggests a mechanism by which the mutant E5 proteins transform cells.