The vesicular trafficking protein PRA1 domain family member 2 (PRAF2) is a novel cellular binding partner of the papillomavirus E5 proteins

Human papillomaviruses (HPVs) infect the epithelium and are responsible for a wide range of diseases, including nearly all cases of cervical cancer (CC) worldwide, and are increasingly recognised as aetiological agents for head and neck cancers. HPV encodes three oncoproteins, E5, E6 and E7. The E5 protein is known to be important in the HPV lifecycle, however, there remains a great deal to be elucidated about the E5 protein. A mass spectroscopy screen identified PRA1 Domain Family Member 2 (PRAF2) as a putative cellular binding partner of E5. Overexpression of PRAF2 is associated with a negative prognosis in hepatocellular carcinoma, glioblastoma and neuroblastoma and PRAF2 interacts with the anti-apoptotic Bcl-XL and Bcl-2 proteins.
I have used immunoprecipitation to confirm interaction between PRAF2 and E5, and the binding of PRAF2 to HPV-18E5 was mapped. PRAF2 expression was elevated in HPV-16 CC cells and HPV-16 positive CC biopsies; and loss of PRAF2 in HPV-16 CC cells resulted in a reduction in colony formation and migration, suggesting a possible role for PRAF2 as an oncogene. PRAF2 expression increased upon differentiation in normal human keratinocytes and those harbouring either wild type HPV-18 or E5 knockout genome. PRAF2 overexpression reduced the thickness of stratified epithelium in HPV-18 rafts and caused loss of HPV-18E1^E4 in E5 knockout rafts, suggesting E5-PRAF2 interaction is important to the viral lifecycle. The proapoptotic function of PRAF2 was investigated in CC cells, with conflicting results. A putative PRAF2 cellular interactome was established through a proximity dependent biotinylation for the first time, with many of the proteins identified also identified as putative HPV E5 protein interactors. These data may illuminate further studies into the function of the PRAF2 protein, and also help determine the nature of the interaction between E5 and PRAF2 and shed light on the cellular functions of E5 proteins.