Expression of HPV L1 and L2 Capsid Proteins in Insect Cells and Assembly of HPV Capsids
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The human papillomavirus (HPV), which causes both common (plantar) and genital warts, is also the most prevalent sexually transmitted virus in the United States. Human papillomaviruses are small, non-enveloped viruses that infect the basal layer of epithelial tissue. The HPV genome consists of circular, double stranded DNA 8,000 base pairs in size packaged into a capsid composed of L1 and L2 proteins. The objective of this project was to study the interaction of L1 and L2 using recombinant baculoviruses to express these HPV capsid proteins. Epitope-tagged versions of the L2 protein (either with a 6 histidine or FLAG epitope added to the N- or C-tenninal ends of the protein) were constructed and to facilitate detection of L2 using commercially available monoclonal antibodies. Sucrose gradients and cesium chloride gradients were used to separate the larger capsid sized particles from other cellular debris. Bands and pellets were found in the sucrose gradients, while thick white bands were seen in the cesium chloride gradients at a density of 1.29 g/cm3. However, whole HPV capsids were not seen by electron microscopy. The Western blots did, nonetheless, show that the L1 and L2 proteins were made, though often cleaved into smaller pieces. The difficulties forming capsids mirrored the problems scene using HPV -16. These and other cancer causing, "high risk", HPV s may have additional requirements in order to form capsids.