Characterization of the plasma membrane localization and orientation of HPV16 E5 for cell-cell fusion.

Human papillomavirus (HPV) is a non-enveloped DNA virus with an approximately 8000 base pair genome. Infection with certain types of HPV is associated with cervical cancer, although the molecular mechanism by which HPV induces carcinogenesis is poorly understood. Three genes encoded by HPV16 are regarded as oncogenic - E5, E6, and E7. The role of E5 has been controversial. Expression of HPV16 E5 causes cell-cell fusion, an event that can lead to increased chromosomal instability, particularly in the presence of cell cycle checkpoint inhibitors like HPV16 E6 and E7. Using biochemical and cell biological assays to better understand HPV16 E5, we find that HPV16 E5 localizes to the plasma membrane with an intracellular amino terminus and an extracellular carboxyl-terminus. Further, HPV16 E5 must be expressed on both cells for cell fusion to occur. When the extracellular epitope of HPV16 E5 is targeted with an antibody, the number of bi-nucleated cells decreases.