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.
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