A novel method for obtaining poliovirus 14 S pentamers from procapsids and their self-assembly into virus-like shells

Poliovirus procapsids, isolated and partially purified from infected HeLa cells, were found to dissociate into 14 S pentamers following freezing at -20 degrees overnight. Unlike pentameric subunits isolated directly from virus-infected cells or those obtained by alkaline treatment of procapsids, the "freeze-dissociated" subunits self-assembled at room temperature into two kinds of empty capsids, one of which was similar to the shell of poliovirions as shown by its neutral pl, resistance to alkaline dissociation, and the presence of neutralization-related (N) epitopes. The other empty shell resembled that which was previously described as a self-assembled empty capsid (SAEC), as indicated by its acidic pl and absence of N antigenic determinants. At 37 degrees, the assembly reaction was significantly faster but only SAEC were formed. By following the assembly reaction by electron microscopy, intermediate structures were visualized; their relatively low amount at any given time was consistent with the previous hypothesis (J.R. Putnak and B.A. Phillips (1981), J. Virol. 40, 173-183) that initiation is the rate-limiting step in the morphogenesis of empty shells. The relevance of these findings to the morphogenesis of picornaviruses, and polioviruses in particular, is discussed.