Human papillomavirus type 16 E6 induces self-ubiquitination of the E6AP ubiquitin-protein ligase

The E6 protein of the high-risk human papillomaviruses (HPVs) and the cellu lar ubiquitin-protein ligase E6AP form a complex which causes the ubiquitin ation and degradation of p53. We show here that HPV16 E6 promotes the ubiqu itination and degradation of E6AP itself. The half-life of E6AP is shorter in HPV-positive cervical cancer cells than in HPV-negative cervical cancer cells, and E6AP is stabilized in NPV-positive cancer cells when expression of the viral oncoproteins is repressed. Expression of HPV16 E6 in cells res ults in a threefold decrease in the half-life of transfected E6AP. E6-media ted degradation of E6AP requires (i) the binding of E6 to E6AP, (ii) the ca talytic activity of E6AP, and (iii) activity of the 26S proteasome, suggest ing that E6-E6AP interaction results in E6AP self-ubiquitination and degrad ation. In addition, both in vitro and in vivo experiments indicate that E6A P self-ubiquitination results primarily from an intramolecular transfer of ubiquitin from the active-site cysteine to one or more lysine residues; how ever, intermolecular transfer can also occur in the context of an E6-mediat ed E6AP multimer. Finally, we demonstrate that an E6 mutant that is able to immortalize human mammary epithelial cells but is unable to degrade p53 re tains its ability to bind and degrade E6AP, raising the possibility that E6 mediated degradation of E6AP contributes to its ability to transform mamma lian cells.