In vitro assembly of poliovirus: V. Evidence that the self-assembly activity of 14 S particles is independent of extract assembly factor(s) and host proteins

Experiments were performed to determine whether or not the self-assembly of 14 S precursor particles into empty capsids was caused by the contamination of certain 14 S isolates with assembly factor(s) present in poliovirus-infected cell extracts. The self-assembly capacity of 14 S particle preparations was found to be directly related to the amount of viral-specific protein present. Dilution of 14 S particles markedly inhibited their self-assembly activity, whereas using ultrafiltration methods to concentrate 14 S particles increased their self-assembly activity. No consistent relationship was found for the presence or absence of particular viral or host proteins and the ability of 14 S particle preparations to self-assemble. The self-assembly capacity of 14 S particles was more sensitive to uv-inactivation than their ability to assemble into empty capsids in the presence of infec cted cell extracts. On the basis of these data and a detailed reanalysis of the effect of relative initial 14 S particle concentration on the rate of formation of empty capsids in extracts, we propose that the assembly of 14 S particles into empty capsids occurs in two steps, an initiation event and subsequent polymerization, and that extracts act by promoting the initiation event.