A morphological study on the ultrastructure and assembly of murine leukemia virus using a temperature-sensitive mutant restricted in assembly

Abstract

T, a temperature-sensitive mutant of Moloney murine leukemia virus, was observed to produce large numbers of both normal particles and multiploids, (i.e., virions with more than one ribonucleoprotein component [RNP]) in all stages of assembly at the nonpermissive temperature. In thin sections, budding particles and extracellular immature particles show the general structural components characteristic of other RNA tumor viruses. Examination of the core shell confirms that it consists of polygonal subunits. The double striated tracks seen in negatively stained virions appear to be the edges of these subunits and are not likely to be a “core membrane.” An additional substructure was observed in the core shell which extends from the center of each polygonal subunit to the outer periphery of the RNP. The presence of substructures in the form of rings or short tubular substructures in the RNP is consistent with the hypothesis that the RNP is a hollow sphere formed by supercoiling of a single- or double-stranded helix. Multiploids were also observed in t-infected cells grown at the permissive temperature and in MuLV infected cells. We have demonstrated that the individual RNPs of a multiploid remain discrete entities. The volume of a spherical duplex is about twice that of a normal particle which suggests that it contains two complete genomes. The assembly of both normal particles and multiploids are essentially similar. Multiploids are apparently produced when two or more normal virions assemble in close proximity or assemble in succession at about the same site. The final steps in the assembly of both types of particles are preceded by the resealing of the cellular membrane. The laying down of the remaining section of the viral envelope occurs as a separate event. Elongation of the cytoplasmic strands linking the virions to the cell seems to occur before virion release. The significance of multiploids in mixed infections is discussed.