3A and 4A and B, respectively. Two classes of genes, the early (E) genes (which

are required for viral DNA replication) and late (L) genes (coding for the structural proteins) exist in both PyVs and PVs. The HPV genome contains a coding region that encompasses an E region that includes up to seven ORFs encoding non-structural proteins and the late region comprises the L1 and L2 ORFs. In HPV, a ∼1 kbp non-coding region [also known as the long control region (LCR) or the upstream regulatory PD-0332991 nmr region] separates the early and late regions. The LCR harbours the origin of replication, the transcription start sites and promoter/enhancer elements that regulate viral gene expression. In PyV, both strands of DNA code for the viral proteins. One strand of DNA encodes an overlapping set of multifunctional early regulatory proteins and the other strand encode for the capsid proteins expressed late in permissive cells. Some PyVs also encode for an agno protein that facilitates virion assembly. The control region between the early and the late transcription units contains a bidirectional enhancer, early and late promoters, the viral origin of replication, the viral packaging LY294002 mw signal and binding sites for host transcription factors Table 3. Papillomavirus particles are ∼55 nm diameter, compared to ∼45 nm diameter in PyVs. Papillomaviruses encode two structural proteins: the major capsid protein, L1 (∼510 amino acids

and ∼58 kDa), and the minor protein L2 (∼470 amino acids and ∼51 kDa). In contrast, PyVs encode for three structural proteins: the major capsid protein, VP1 (∼370 amino acids and ∼41 kDa) and two minor proteins VP2 for (∼350 amino acids and ∼38 kDa) and VP3 (∼230 amino acids and ∼26 kDa). Despite significant differences in amino acid sequences of the major capsid

proteins, both PV and PyV capsids exhibit conserved features, as the 72 capsomers are pentamers of the major capsid protein and are arranged on a T = 7 icosahedral lattice. Papillomaviridae and Polyomaviridae differ in capsomer morphology and size. Papillomavirus capsomers are star-shaped, 11–12 nm in diameter, while polyomavirus are barrel-shaped, 8 nm in diameter. Intercapsomer interactions are also slightly different between these viral families (Belnap et al., 1996). The carboxyl terminus of VP1 or L1 mediates contacts between the pentamers in the capsid. While disulphite bonds stabilize the interpentamer contacts for L1, both disulphite bonds and calcium bridges stabilize these contacts for VP1 (Sapp and Day, 2009). Also, differences in receptor binding and internalization pathway also exist between PVs and PyVs, reviewed in (Sapp and Day, 2009). Polyomaviruses generally have a narrow host range and limited cell type tropism (Gjoerup and Chang, 2010). In their natural host, they are able to infect cells giving rise to a productive life cycle causing cell lysis.