Identification of the regions of the HPV 5 E6 protein involved in Bak degradation and inhibition of apoptosis

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Abstract
UVB induced DNA damage is the major aetiological agent in NMSC development, but mounting evidence suggests a role for human papillomaviruses (HPV) from genus beta, including HPV 5 and HPV 8, in the development of NMSC on sun exposed body sites. We have previously shown that UVB activates Bak, an apoptogenic mitochondrial factor that, following an apoptotic stimulus, undergoes a conformational change that leads to pore formation in the mitochondrial membrane that releases apoptotic factors. The HPV E6 protein effectively inhibits UVB‐induced apoptosis and targets Bak for proteolytic degradation. We have now identified the regions of the HPV5 E6 that are required to mediate Bak proteolysis and contribute toward the antiapoptotic activity of the protein. Interestingly, while wild‐type HPV5 E6 does not bind or target p53 for proteolysis, we have isolated specific HPV5 E6 mutants that switch target specificity from Bak to p53 in a p53 codon 72 isoform‐dependent manner. Furthermore, we demonstrate that the ability of wild‐type HPV5 E6 to target Bak or specific E6 mutants to target p53 for proteolysis is not dependent on the E6‐AP ubiquitin ligase.