Generation of capsids from unstable polyoma virions

Polyomavirus was purified from infected mouse cell lysates under mild physiological conditions. When analyzed in a sucrose gradient, a major virus peak (240S) was identified. This sucrose-isolated virus could be divided into two populations based on its stability to CsCl gradient centrifugation. Members of the unstable population were shown to eject their DNA cores when subjected to CsCl gradient centrifugation, forming empty capsids, whereas the stable population was unaffected by the same CsCl treatment. Formaldehyde fixation of the 240S virus particles stabilized the virions and prevented ejection of DNA and generation of empty capsids. When formaldehyde-fixed 240S virus was examined with the electron microscope, only full virions were observed. These results indicate that polyoma capsids are not preformed in vivo, but instead are generated when infected cell lysates are subjected to harsh CsCl purification procedures.