The L-X-C-X-E pRB-binding motif of papillomavirus (PV) E7 proteins has been implicated in the immortalization and transformation of the host cell. However, sequencing of the complete genomes of bovine papillomavirus type 3 (BPV-3), bovine papillomavirus type 5 (BPV-5), equine papillomavirus (EQPV) and reindeer (Rangifer tarandus) papillomavirus (RPV) supports the notion that the pRB-binding motif is not ubiquitous among E7 proteins in the PV proteome. Key among the animal groups that lack the pRB-binding domain are the artiodactyl PVs, including European elk PV (EEPV), deer PV (DPV), reindeer PV (RPV), ovine PVs types 1 and 2 (OvPV-1 and -2) and bovine PVs 1, 2 and 5 (BPV-1, -2 and -5). Whereas the presence of the pRB-binding domain is normally associated with papillomas, the artiodactyl PVs are marked by the development of fibropapillomas on infection. Previous studies emphasized the role of E5 in the pathogenic mechanism of fibropapilloma development, but correlation between the lack of an E7 pRB-binding domain and the unique pathology of the artiodactyl PVs suggests a more complicated mechanism and an early evolutionary divergence from a pRB-binding ancestor.
INTRODUCTIONPapillomaviruses (PVs) are highly species-specific pathogens. They form a heterogeneous group of closed-circular, double-stranded DNA viruses measuring about 8 kb in size (Sundberg et al., 1996). Most PVs target the basal cells of dermal or mucosal epithelia (Cheville & Olson, 1964), and infection has been implicated in both benign and malignant lesions of epithelia in a broad range of animals (Sundberg & O'Banion, 1989). Though almost all known human PVs (HPVs) infect dermal or mucosal surfaces, in some animal PVs, most notably in the artiodactyl ruminant PVs, fibroblasts appear to be the primary target cells (Sundberg et al., 1985). However, regardless of species, infection generally manifests as a papilloma or fibropapilloma, and follows a cytopathic mechanism of cell proliferation (Sundberg et al., 1996). Historically, PVs had been classified according to their tissue tropism, and this grouping was supported by later phylogenetic analysis of PV sequence data (Chan et al., 1995;de Villiers, 2001;Myers, 1994). PV phylogenies typically subdivide into mucosal/genital HPVs, cutaneous EV HPVs and three main animal PV clades: the artiodactyl ruminant PVs, the distant avian PV group and a group containing canine, feline, rabbit and rodent PV types. However, sequence analysis has highlighted some significant exceptions. BPV-3, BPV-4 and BPV-6 do not group with the artiodactyl PVs, but instead form an isolated taxon (Jarrett et al., 1984), and HPV-1, HPV-41 and HPV-63 are most closely related to the canine and feline PVs, sharing little similarity to HPVs in either the mucosal or the cutaneous groups (Egawa et al., 1993).Although most work in the area has focused on HPVs, an ever-increasing number of animal PV isolates have been sequenced. These animal PVs share key clinical features of GenBank data deposited: bovine papillomavirus 3, AF486184; bo...