| The human papillomavirus (HPV) type 16 E5 protein is a small, hydrophobic protein that has been localized to the endoplasmic reticulum and Golgi apparatus. Historically, studies of E5 and interactions with cellular proteins have been complicated due to extremely low levels of expression. The E5 protein is considered to have weak oncogenic potential but it is controversial as to whether it assists in the transformation process or enhances the differentiation of its host cell, the keratinocyte. For many years the bovine papillomavirus (BPV) E5 protein has been used as the model system to study HPV E5 even though there are significant differences between the two as measured by transformation potential and activation of growth factor receptors. It has been shown that both proteins bind the 16kDa subunit of the vacuolar proton pump (V-ATPase). Mutants of BPV E5 that do not activate growth factor receptors still lead to transformation of NIH3T3 cells and this correlates with their ability to alkalize the Golgi. This may indicate a similar function for HPV E5 based on the fact it does not appear to activate growth factor receptors.; It was the aim of this study to increase the expression of the HPV E5 protein by altering the DNA sequence to encode for codons more frequently used in eukaryotic cells. Subsequently, we wanted to determine if HPV E5 causes a more alkaline pH in the Golgi and/or endosomes and correlate this with the biological function of E5. We were able to increase the expression of HPV E5 6--9 fold. The higher level of E5 expression did not increase cell growth but instead decreased the life span of the cells. It was also shown that E5 altered endosomal pH but not Golgi pH. The observation that HPV E5 inhibits the drop in endosomal pH and decreases the life span of keratinocytes indicates that, in the absence of E6 and E7, E5 may promote cellular senescence or terminal differentiation in primary keratinocytes. |
