Bovine papilloma virus type 1 (BPV-1) is a member of the papillomavirus genus, one of two in the Papovaviridae family. Papovaviruses are characterized by similarities in their capsid structure, genetics, biochemical composition and role in the formation of benign and cancerous tumors. The discoveries that the capsid structures of polyoma and simian virus 40 (SV40) (members of the genus polyomavirus) consist of 72 pentameric capsomeres raises the fundamental question of whether or not the capsids of the larger and more complex papilloma viruses have a similar, unexpected arrangement of the capsid subunits. The recent development of cryo-electron microscopy techniques, which facilitate direct visualization of the “native” morphology of biological specimens at moderate resolution (1-4 nm), provides an opportunity to critically examine the structure of BPV-1.The BPV-1 used in this study was originally isolated from a calf in 1965, passaged again in 1986, and stored in 50% glycerin and phosphate buffered saline. Virus was extracted and purified following the protocol of Cowsert et al. (1987): epithelial tissue rich in BPV-1 was mixed with an equal volume of buffer (1M NaCl, 20mM Tris, pH 7.5) and disrupted in a Waring blender. After high speed clarification, the supernatant fraction was mixed with an equal volume of Freon 113 and centrifuged at low speed. Virus from the aqueous phase was pelleted, resuspended in CsCl (ρ=l.33 g/cm3), density banded, pelleted and finally resuspended in 30 mM KCl and 6 mM Tris (pH 7.5) to a protein concentration of ˜3 mg/ml.
The capsid of small papova viruses contains 72 pentameric capsomeres: direct evidence from cryo-electron-microscopy of simian virus 40
