Isolation and partial characterization of two forms of cytoplasmic nucleocapsids from measles virus-infected cells

Two species of measles virus nucleocapsids with distinct buoyant densities were isolated from infected AV3 cell homogenates by isopycnic CsCl gradient centrifugation. The more buoyant or 'light' form of the nucleocapsid had a density of 1.26 to 1.28 g/ml, whereas the less buoyant or 'heavy' nucleocapsid species had a density of 1.30 g/ml. Analysis of the two nucleocapsid species by SDS--polyacrylamide gel electrophoresis showed that both forms possessed two phosphorylated polypeptides with mol. wt. of 69000 and 60000. The heavy form of nucleocapsid consisted solely of these two polypeptide species, while the light form of nucleocapsid had two additional associated polypeptides (VP4, mol.wt. 52000, and 45K, mol. wt. 45000). Ultrastructural and immunofluorescent studies suggest that the two isolated capsid forms represent the two morphologically distinct capsid species observed in vivo. This paper discusses the possible relationship between the two capsid forms and assembly of the virus.