E1^E4-mediated keratin phosphorylation and ubiquitylation: a mechanism for keratin depletion in HPV16-infected epithelium

The keratin IF network of epidermal keratinocytes provides a protective barrier against mechanical insult, it is also a major player in absorbing stress in these cells. The human papilloma virus (HPV) type 16 E1^E4 protein accumulates in the upper layers of HPV16-infected epithelium and is known to associate with and reorganise the keratin IF network in cells in culture. Here, we show that this function is conserved amongst a number of HPV alpha-group E1^E4 proteins and that the differentiation-dependent keratins are also targeted. Using time-lapse microscopy, HPV16 E1^E4 was found to effect a dramatic cessation of keratin IF network dynamics by associating with both soluble and insoluble keratin. Network disruption was accompanied by keratin hyperphosphorylation at several sites, including K8 S73, which is typically phosphorylated in response to stress stimuli. Keratin immunoprecipitated from E1^E4-expressing cells was also found to be ubiquitylated, indicating that it is targeted for proteasomal degradation. Interestingly, the accumulation of hyperphosphorylated, ubiquitylated E1^E4-keratin structures was found to result in an impairment of proteasomal function. These observations shed new light on the mechanism of keratin IF network reorganisation mediated by HPV16 E1^E4 and provide an insight into the depletion of keratin co-incident with E1^E4 accumulation observed in HPV-infected epithelium.