Table 1: A Summary of the Molecular Activity of HPV Early Proteins

Protein	Function/s	Molecular Activity
E1	Maintains viral genome. Contributes to viral genome replication.	E1 interacts with cellular DNA polymerase and replication proteins to initiate viral DNA replication.
E2	Maintains viral genome. Anchors episomes to mitotic chromosomes in basal cells. Controls transcription.	E2 engages E1 to viral origin and binds E1 to cellular proteins essential for DNA replication. Low levels of E2 activate transcription and high levels of E2 inhibit transcription through interaction with viral early promoters. E2 represses hTERT promoter activity.
E4	Function unclear. E4 has potential roles in the late stage of viral lifecycle, apoptosis and cell cycle arrest.	E4 may bind to keratins and lead to breakdown of the cytokeratin network. E4 may bind to cdk/cyclin complexes leading to arrest of the cell cycle in G2 phase. E4 may interact with E2 and mediate relocation of E2 from nucleus to cytoplasm.
E5	Transformation	HPV16 E5 interacts with E6 and E7 to stimulate cell proliferation. E5 may inhibit apoptosis and cell to cell communication.
E6	Transformation	HR-HPV E6 mediates ubiquitination and degradation of p53. HR-HPV E6 activates expression of hTERT.
E7	Transformation	E7 disrupts association of pRb with E2F transcription factors, activates cellular proteins involved in viral DNA replication and pushes the cell from G1 phase to S phase. Stimulates cell proliferation through interaction with histone deacetylases, AP1 transcription complex, p21 and p27 cyclin-dependant kinase inhibitors.