Conserved Features in Papillomavirus and Polyomavirus Capsids

Abstract

Capsids of papilloma and polyoma viruses (papovavirus family) are composed of 72 pentameric capsomeres arranged on a skewed icosahedral lattice (triangulation number of seven, T=7). Cottontail rabbit papillo mavirus (CRPV) was reported previously to be a T=7 laevo(left-handed) structure, whereas human wart virus, simian virus 40, and murine polyomavirus were shown to be T=7 dextro(right-handed). The CRPV structure determined by cryoelectron microscopy and image reconstruction was similar to previously determined structures of bovine papillomavirus type 1 (BPV-1) and human papillomavirus type 1 (HPV-1). CRPV capsids were observed in closed (compact) and open (swollen) forms. Both forms have star-shaped capsomeres, as do BPV-1 and HPV-1, but the open CRPV capsids are ≈2 nm larger in radius. The lattice hands of all papillomaviruses examined in this study were found to be T=7 dextro. In the region of maximum contact, papillomavirus capsomeres interact in a manner similar to that found in polyomaviruses. Although papilloma and polyoma viruses have differences in capsid size (≈60 versus≈50 nm), capsomere morphology (11 to 12 nm star-shaped versus8 nm barrel-shaped), and intercapsomere interactions (slightly different contacts between capsomeres), papovavirus capsids have a conserved, 72-pentamer, T=7 dextrostructure. These features are conserved despite significant differences in amino acid sequences of the major capsid proteins. The conserved features may be a consequence of stable contacts that occur within capsomeres and flexible links that form among capsomeres.