The Human Papillomavirus Type 16 E5 Protein Alters Vacuolar H+-ATPase Function and Stability in Saccharomyces cerevisiae

Abstract

The human papillomavirus 16 (HPV-16) E5 oncoprotein is a small integral membrane protein that binds to the 16-kDa subunit of the vacuolar H+-ATPase (v-ATPase). Conservation within the family of v-ATPases prompted us to look to Saccharomyces cerevisiae as a potential model organism for E5 study. The E5 open reading frame, driven by a galactose-inducible promoter, was integrated into the yeast genome, and the resulting strain demonstrated a nearly complete growth arrest at neutral pH, consistent with defects associated with yeast v-ATPase mutants. Furthermore, this strain demonstrated a severe reduction in pH-dependent and v-ATPase-dependent vacuolar localization of fluorescent markers. Overexpression of the yeast 16-kDa subunit homolog partially suppressed E5-associated growth defects. E5 expression was correlated with a disassociation of the integral (Vo) and peripheral (Vi) v-ATPase sub-complexes, as well as a dramatic reduction of the steady-state levels of one mature Vo subunit and the concomitant accumulation of its major proteolytic fragment, with unchanged levels of two Vi subunits. Similar analyses of selected E5 mutants in yeast demonstrated a correlation between E5 biology and v-ATPase disruption. Our observations suggest that wild-type HPV-16 E5 acts during the assembly of the v-ATPase to inhibit, either directly or indirectly, Vo stability and complex formation.