Papillomaviruses are small icosohedral, non-enveloped (naked!) viruses that range in diameter from 52-55 nm. It is believed that the virion particle consists of 72 pentameric capsomers arranged in a surface lattice with a triangulation number of T = 7. It’s neat that capsomers can exist in both the pentavalent and hexavalent state—there are 60 hexavalent capsomers and 12 pentavalent ones. Papillomavirus shares this feature of heterogenous capsomers with its closest cousin Polyomavirus.
Image above: Computer generated image of Papillomavirus Capsid Morphology
Papillomavirus capsid is constructed of two viral proteins: L1 and L2. L1, the major capsid protein, constitutes 80% of the virion while L2— the minor protein— makes up the remaining 20%.
It’s pretty cool to note that it is these viral capsid proteins that are used to create virus-like particles (abbreviated VLPs!) for the new quadravalent Gardasil HPV vaccine. This highly efficacious recombinant, non-infectious vaccine was produced using the major capsid proteins (the L1 proteins) from the four strains of HPV that are the most common cause of cervical cancer and genital warts. Clever scientists over at Merck have manipulated yeast cells to express the L1 viral proteins, which then self-assemble into VLPs and create a robust humoral immune response in humans.