Resistance to UV-induced apoptosis by β-HPV5 E6 involves targeting of activated BAK for proteolysis by recruitment of the HERC1 ubiquitin ligase

Abstract

UV exposure is the main etiological agent in the development of non-melanoma skin cancer (NMSC), but mounting evidence suggests a co-factorial role for β-genus HPV types early in tumor initiation or progression. UV damage initiates an apoptotic response, driven at the mitochondrial level by BCL-2 family proteins, that eliminates damaged cells that may accumulate deleterious mutations and acquire tumorigenic properties. BAK is a pro-apoptotic BCL-2 protein that functions ultimately to form pores that permeabilize the mitochondrial outer membrane, thereby committing a cell to death, a process involving changes in BAK phosphorylation and conformation. The E6 protein of β-type HPV5 signals BAK for proteasomal degradation, a function that confers protection from UV-induced apoptosis. We find that HPV5 E6 does not constitutively target BAK for proteolysis, but targets the latter stages of BAK activation, following changes in phosphorylation and conformation. A mutational analysis identified the lysine residue on BAK required for proteolysis, and a functional siRNA screen identified the HECT domain E3 ubiquitin ligase HERC1 as being required for E6-mediated BAK degradation. We show that HERC1 interacts with BAK in E6-expressing cells that have been damaged by UV, and provide evidence that the interaction of HERC1 with BAK requires access to a hydrophobic surface on BAK that binds BH3 domains of BCL-2 proteins. We also show that HERC1 contains a putative BH3 domain that can bind to BAK. These findings reveal a specific and unique mechanism used by the HPV5 E6 protein to target BAK. What's new? Cutaneous human papillomavirus (HPV) infection may act in concert with UV radiation exposure to fuel the early progression of non-melanoma skin cancer. The molecular mechanism underlying that interaction has been elusive but may involve the E3 ubiquitin ligase HERC1, according to the present study. HERC1 was discovered to associate with the E6 protein of cutaneous HPV5. E6 prevents UV-induced apoptosis by signaling for degradation of the pro-apoptotic protein BAK. The authors show that E6 targets only activated BAK for proteolysis and requires HERC1 to carry out degradation signaling.