ICTV Virus Taxonomy Profile: Papillomaviridae

1The non-enveloped viral capsid is ~600 Å in diameter and consists of 72 pentamers of the major capsid protein, L1, and ~12 molecules of the L2 minor capsid protein (Table 1, Fig. 1).

2The viral genome varies from 5 748 to 8 607 bp. The genome comprises three functional regions. The early region encodes proteins involved in transcription, replication, and manipulation of the cellular milieu. The late region encodes the capsid proteins L1 and L2. The upstream regulatory region, located between the L1 and E6 open reading frames, contains the origin of replication as well as binding sites for viral and cellular transcription factors (Fig. 2).

3Papillomaviruses primarily infect epithelial cells. Following a micro-abrasion, the incoming virion complexes with extracellular heparin sulfate proteoglycans on the basement membrane. This interaction results in conformational changes in the L1 and L2 capsid proteins, in turn allowing for transfer of the virion to an unknown entry receptor. Following furin cleavage of L2, the virion becomes internalised using a process that shares similarities with macropinocytosis. The L2-DNA complex traffics to the trans-Golgi network, until mitosis. By metaphase, the viral DNA can be seen to be associated with host chromosomes. The viral replication cycle consists of three distinct phases of replication. Initial limited viral DNA amplification is supported by the viral E1 and E2 replication proteins. This initial burst of replication is followed by maintenance replication, during which the viral genome is maintained at a relatively low, but constant copy number in the proliferating cells of a lesion. Finally, as an infected cell completes cellular differentiation there is a switch towards differentiation-dependent genome amplification, and eventual generation of progeny virions. During maintenance replication, the viral E6 and E7 proteins are able to usurp the cellular environment, allowing for viral replication in differentiated cells. In the top layers of the differentiated epithelia, viral DNA is amplified to a high copy number and the capsid proteins self-assemble into particles encapsidating the viral DNA. As the cells slough off into the environment, infectious virions are released, completing the viral replication cycle.

4Classification of the Papillomaviridae is based on sequence identity across the L1 open reading frame. The family includes two subfamilies, Firstpapillomavirinae, which includes >50 genera and >130 species, and Secondpapillomavirinae, with a single genus and species. Genera are named according to the Greek alphabet (e.g. Alphapapillomavirus), with the prefixes ‘Dyo-’ and ‘Treis-’ indicating additional cycles through the alphabet (i.e. Greek for ‘a second or third time’).

5Full ICTV Online (10th) Report: http://www.ictv.global/report/papillomaviridae.

6The Papillomavirus Episteme (PaVE): http://pave.niaid.nih.gov.

7The human papillomavirus reference center: http://www.hpvcenter.se.

8The animal papillomavirus reference center: http://www.animalpv.org.