Transformation of intramembrane particles of HVJ (Sendai virus) envelopes from an invisible to visible form on aging of virions

Abstract

When HVJ virions harvested from embryonated eggs 3 days after infection are freeze-fractured, large intramembrane particles, with a mode of 150-Å diameter, are exposed on the E fracture faces but not on the P fracture faces. The intramembrane particles seem to be hydrophobic parts of viral spikes of HANA and F glycoproteins. No intramembrane particles are observed in young virions harvested 1 day after infection, although many spikes project from their envelopes. When young virions are incubated at 37° for 2 days in vitro, large intramembrane particles develop and are exposed on the E fracture face: These particles resemble those observed in old virions harvested from eggs 3 days after infection. Besides this difference in intramembrane particles, morphological differences are seen between young virions and old virions aged in ovo or in vitro: in young virions the nucleocapsid strand is regularly folded under the envelope and the virion structure is rigid; but in old virions, aged in ovo or in vitro, the nucleocapsid strand is irregularly folded and detached from the envelope, and the virions are no longer rigid but are easily distorted by external forces. From these findings, the possible correlation between the transformation of intramembrane particles from an invisible to visible form and the disruption of the regular arrangement of the nucleocapsid under the viral envelope is discussed.