In vitro assembly of polioviruses. IV. Evidence for the existence of two assembly steps in the formation of empty capsids from 14 S particles.

Evidence obtained from kinetic studies indicated that the assaembly reaction mediated by cytoplasmic extract prepared from poliovirus-infected HeLa cells was conposed of at least two distinct activities. One activity (assembly activity) converted mature 14 S particles into 73 S empty capsids. The other activity (activation) seemed to act on “immature” 14 S particles, enabling them to be assembled. Unlike the assembly activity, the activation reaction was depleted rapidly during the reaction in vitro. The activation factor(s) was not associated with isolated 14 S particles themselves but, like the assembly activity, appeared to be associated with the rough membrane fraction obtained from infected cells.