Subunit interactions in polyoma virus structure

Purified polyoma virions can be progressively dissociated by treatment with increasing concentrations of SDS. Complete removal of histones occurs at a lower concentration of SDS than is required to completely remove VP3 and VP2. VP2 is the last minor protein to be dissociated. The final structure contains disulfide-crosslinked VP1 and resembles empty capsids by electron microscopy. This VP1 structure retains hemagglutinating activity, and the data presented indicate that the hemagglutinin is a polymer of VP1. Disulfide-crosslinked and uncrosslinked VP1 can be distinguished in polyoma infected cell lysates by polyacrylamide gel electrophoresis. DNA release also occurs by treatment with SDS. With very low concentrations of SDS, the minor proteins begin to dissociate before release of DNA. The resulting DNA-containing structure sediments more slowly (about 80 S) in sucrose gradients than intact virions or empty capsids. Most of these 80 S particles can be converted to empty capsids by slightly higher concentrations of SDS. A residual portion retains DNA even after treatment with high salt or with concentrations of SDS at which all minor proteins are removed.