Recent studies have further suggested that only particular PDZ pools or isoforms within the cell are susceptible to degradation [119] and [120], and that this function of E6 may be carefully regulated during the virus life-cycle [118]. Further studies are needed to precisely define the role of these interactions in vivo. Other unique characteristics of the high-risk E6 proteins include their capacity to upregulate telomerase activity [121], [122] and [123] and to maintain telomere integrity during repeated cell divisions, and their ability to mediate the degradation of p53 within the cell. Both high- and low-risk E6 proteins inactivate aspects of p53 function,

which suggests an important life-cycle function,

but only the high-risk types stimulate its ubiquitination and proteosome-dependent degradation [124], [125] and [126]. In fact the high-risk types use degradatory pathways A-1210477 cost to target many of their substrates. For E7, this involves components of the CUL2 ubiquitin ligase complex, while for E6 it involves the cellular ubiquitin ligase E6AP [127]. With the use of more advanced proteomics technology, it is becoming clear that both E6 and E7 have a very large number of cellular substrates, and that the identity of these substrates differs between HPV types of the same high-risk clade, as well as between the high- and low-risk groupings themselves [128]. Indeed, there appears to be no single characteristic that can define high-risk types selleck compound as cancer-causing. This is exemplified by studies showing very little concordance between cancer risk, and the capacity of the E6 oncoproteins from the high-risk types to degrade p53, degrade PDZ substrates and induce keratinocyte

immortalisation. In the case of E6, recent structural studies are suggestive of a complex multimeric protein that has potential to associate with multiple protein partners at any given time [125] and [129]. While such functional differences of undoubtedly contribute to the respective abilities of the high- and low-risk HPV types to cause neoplasia and cancer, it is important to remember that a key function of the E6 and E7 proteins in most HPV types is not to promote basal cell proliferation, but rather, to stimulate cell cycle re-entry in the mid-epithelial layers in order to allow genome amplification. The expression of the E6 and E7 proteins in the upper epithelial layers allows the infected cell to re-enter S-phase, and for viral genome copy-number to rise. There is also a need for the viral replication proteins E1 and E2, which increase in abundance following the upregulation of the HPV ‘late’ or ‘differentiation dependent’ promoter [130]. In HPV16, this promoter (P670) resides within the E7 open reading frame near to nucleotide position 670.