Difference in capacities for virion-to-virion fusion of young and aged HVJ (Sendai virus): a model of membrane fusion.

Young and aged HVJ virions differ structurally and morphologically due to changes that occur during aging in vitro or in ovo. Young virions soon after their budding off are rod-shaped, rigid and relatively uniform in size, whereas virions that have aged in vitro after their formation are round, nonrigid and variable in size. These changes during aging seem to be due to the variation of M protein, a "skeletal" protein that is associated with both the envelope membrane proteins and nucleocapsid strands in the virions. The capacities for virion-to-virion fusion of young and aged virions were compared to clarify the relation between the membrane fusion and membrane-associating skeletal proteins. On treatment with polyethylene glycol (PEG), aged virions readily fused, forming large virion vesicles, but young virions were resistant to fusion. Further, aged virions fused even on incubation at 37 degrees C without the fusogen. Thus the capacity for virion-to-virion fusion evidently increases during aging of virions. This result suggests that skeletal proteins associating with the biological membrane are important for preventing membrane fusion, and that virion-to-virion fusion is a good model system for use in studies on the mechanism of membrane fusion.