Immunocytochemical localization of capsid-related particles in subcellular fractions of poliovirus-infected cells.

The structural proteins of poliovirus can assemble into a series of different configurations (capsid-related particles, CRP). Only some seem to be true capsid precursors and the role of most CRP in morphogenesis is unclear. We used electron microscopic immunocytochemistry with monoclonal antibodies recognizing different CRP [protomers, pentamers, 65S empty capsids (EC), 74S-EC, and virions] to locate CRP in subcellular fractions containing virus-induced vesicles associated with the viral replication complex. We found pentamer antigenic CRP to be associated with the replication complex. The same pentamer antigenicity was exhibited by novel, "capsid-like" structures attached to the surface of the virus-induced vesicles. Upon solubilization of the vesicular fraction, mainly 65S-EC and only negligible amounts of pentamers were found by sucrose gradient analysis and by immunoprecipitation. We show that the pentamer antigenic particles are converted into 65S-EC when their membranous support is dissolved. We propose that the vesicular membrane prevents the assembly of 65S-EC and keeps the pentamer antigenic CRP in the appropriate concentration and configuration for association with the nascent progeny RNA.