Self-assembly of purified polyomavirus capsid protein VP₁

Abstract

The polyomavirus major capsid protein VP₁, purified after expression of the recombinant gene in E. coli, was isolated as oligomers resembling the dissociated capsomeres derived from viral capsids. Image analysis of low-dose electron micrographs demonstrates that these VP₁ oligomers are exclusively pentamers. The purified VP₁ pentamers associated to form capsid-like assemblies and polymorphic aggregates at high ionic strength. The capsid-like assemblies were stabilized at low ionic strength by the addition of calcium. Self-assembly of the unmodified, recombinant DNA-generated VP₁ implies that the posttranslational charge modifications of VP₁ and the minor virion protein components, VP₂ and VP₃, are not essential for capsid formation. The nonequivalently related subunits of the penta- and hexavalent capsomeres therefore must spontaneously switch their bonding specificity during assembly.