In vitro assembly of poliovirus 14 S subunits: identification of the assembly promoting activity of infected cell extracts.

Purified poliovirus 14 S subunits are assembled into empty capsids in vitro only if their concentration exceeds a 1.6 nM threshold. This also holds true for the 14 S subunits in unpurified extracts of infected cells. Such an extract may promote the assembly of extraneous 14 S subunits, but only if it contributes enough 14 S subunits to raise their total concentration over the threshold. As a result of assembly, the concentration of 14 S subunits in an infected cell extract decreases exponentially, with a half life of 15 min at 37 degrees. When purified 14 S subunits of serotype 1 are mixed with extracts of cells infected with type 2 or 3, chimeric empty capsids are formed, thus showing the pooling of endogenous and extraneous 14 S subunits. In conclusion, the assembly promoting activity of infected cell extracts amounts to nothing more than the supply of endogenous 14 S subunits.