THE E5 ONCOPROTEIN OF HUMAN PAPILLOMAVIRUS TYPE-16 INHIBITS THE ACIDIFICATION OF ENDOSOMES IN HUMAN KERATINOCYTES

The human papillomavirus type 16 E5 oncoprotein possesses mitogenic ac tivity that acts synergistically with epidermal growth factor (EGF) in human keratinocytes and inhibits the degradation of the EGF receptor in endosomal compartments after ligand-stimulated endocytosis. One pot ential explanation for these observations is that E5 inhibits the acid ification of endosomes. This may be mediated through the 16-kDa compon ent of the vacuolar proton-ATPase, since animal and human papillomavir us E5 proteins bind this subunit protein. Using a ratio-imaging techni que to determine endosomal pH, we found that the acidification of endo somes in ES expressing keratinocytes was delayed at least fourfold com pared with normal human keratinocytes and endosomes in some cells neve r completely acidified. Furthermore, E5 expression increased the resis tance of keratinocytes to protein synthesis inhibition by diphtheria t oxin, a process dependent on efficient endosomal acidification. Finall y, artificially inhibiting endosomal acidification with chloroquine du ring the endocytosis of EGF receptors in keratinocytes demonstrated ma ny of the same effects as the expression of human papillomavirus type 16 E5, including prolonged retention of undegraded EGF receptors in in tracellular vesicles.