Mutation of the bovine papillomavirus E5 oncoprotein at amino acid 17 generates both high- and low-transforming varients

The E5 transforming protein of bovine papillomavirus type 1 is a 44-amino-acid, hydrophobic protein which localizes predominately to Golgi membranes. The E5 transmembrane domain contains a highly conserved glutamate residue at position 17 which, from previous limited mutagenic analysis, appeared essential for transforming activity. In order to determine the specific amino acid requirements at this position, we constructed a series of substitution mutants, representing all classes of amino acids, employing a vector which expressed E5 independently of other bovine papillomavirus gene products. All of the expressed E5 mutant proteins were stable, dimerized normally, and localized to the Golgi. Our results obtained with C127 mouse cell demonstrated that acidic amino acids (and serine) increased E5 transforming activity, whereas basic amino acids greatly inhibited E5 activity. Nonpolar amino acid substitutions were also defective. Interestingly, the relative transforming activities of these E5 mutant proteins changed dramatically when assayed with NIH 3T3 cells, suggesting that an auxiliary cellular protein(s) may modulate E5 transformation or that there are additional or different mechanisms of E5 transformation which are utilized in these two cell lines