Use of temperature-sensitive mutants to study the morphogenesis of poliovirus

Human rhinovirus type 16: mutant V1210A requires capsid-binding drug for assembly of pentamers to form virions during morphogenesis

Our laboratory has previously reported isolation of human rhinovirus type 16 (HRV16) mutants which depend on WIN 52035 to grow. A rapid rise of progeny virus infectivity occurred when drug was added late in growth cycles, suggesting that the drug-dependence lesion was at the step of virus assembly (W. Wang et al., J. Virol. 72:1210-1218, 1998). Here, we report that capsid subunits, 5S protomers and 14S pentamers, of a drug-dependent mutant were produced normally in the absence of drug, but mutant 80S empty capsids and 150S provirions were not formed, maturation cleavage of provirions (VP0 --> VP2 + VP4) did not occur, and the unassembled mutant capsid subunits were degraded with a half-life of 15 min. Drug was not required by mutant virus for attachment, uncoating, RNA synthesis and protein synthesis, and polyprotein processing except maturation cleavage. The requirement of drug for assembly of mutant pentamers to form provirions and the rapid assembly of preformed subunits (synthesized in the absence of drug) after drug addition suggested that after native pentamers (P5) have been formed they must be converted to an assembly active state (P5(*)), possibly by a conformational change induced by the binding of drug. We propose that pocket factor plays the same role in wild-type virus. In addition, we also report the construction and the properties of a full-length cDNA clone of HRV16, pR16.11, which produces in vitro transcripts with infectivity similar to that of virion RNA. This cDNA clone is available at the American Type Culture Collection.

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.

Further characterization of a morphogenetic mutant of the foot-and-mouth disease virus

In this paper we describe further characterization of a foot-and-mouth disease virus (FMDV) temperature-sensitive mutant, ts 139. This mutant was very sensitive to heat inactivation, suggesting that its viral particles are somehow altered. The electrophoretic analysis of ts 139 structural proteins indicated that VP2 has an altered mobility. Furthermore, two known protein precursors of VP2, VP0 and p88, were shown to be altered, as was p64, which supports a VP2 precursor role for p64. The ts 139 viral particle assembly pathway was analyzed during viral replication. The empty-capsid to complete-viral-particle ratio was clearly increased compared to that found for the wt strain, indicating an alteration in the morphogenetic process. This encouraged us to search for the presence of possible viral precursors of low sedimentation coefficient which have not been described previously in aphthovirus-infected cells. Analysis of the viral morphogenetic process in ts 139-infected cells demonstrated the presence of viral complexes formed by VP0, VP1 and VP3 which sedimented slightly faster than a 12 S marker. The protein composition, the sedimentation coefficient of this complex and the pulse-chase results strongly suggest that it is a morphogenetic precursor of the mature viral particle.

Toward an in vitro system for picornavirus assembly: purification of mengovirus 14S capsid precursor particles

Mengovirus 14S subviral protein particles generated in infected L cells and in a cell-free translation system primed with mengovirus RNA were purified by sucrose gradient centrifugation and immunoaffinity chromatography. The preparations from both sources contained essentially pure proteins epsilon, alpha, and gamma, as was demonstrated in terms of virus-specific proteins (by autoradiography) and total protein content (by silver staining of sodium dodecyl sulfate-polyacrylamide electrophoresis gels). These purified proteins sedimented as discrete particles at the 14S position when recentrifuged in sucrose gradients. Although their assembly properties have not yet been studied in detail, preliminary results indicate that during incubation with virion RNA the 14S particles purified from infected cells can form a structure cosedimenting with mature mengovirus.