We have studied the differences between erythrocytes and erythrocyte ghosts as target membranes for the study of Sendai virus fusion activity. Fusion was monitored continuously by fluorescence dequenching of R18-labeled virus. Experiments were carried out either with or without virus/target membrane prebinding. When Sendai virus was added directly to a erythrocyte/erythrocyte ghost suspension, fusion was always lower than that obtained when experiments were carried out with virus already bound to the erythrocyte/erythrocyte ghost in the cold, since with virus prebinding fusion can be triggered more rapidly. Although virus binding to both erythrocytes and erythrocyte ghosts was similar, fusion activity was much more pronounced when erythrocyte ghosts were used as target membranes. These observations indicate that intact erythrocytes and erythrocyte ghosts are not equivalent as target membranes for the study of Sendai virus fusion activity. Fusion of Sendai virus with both target membranes was inhibited when erythrocytes or erythrocyte ghosts were pretreated with proteinase K, suggesting a role of target membrane proteins in this process. Treatment of both target membranes with neuraminidase, which removes sialic acid residues (the biological receptors for Sendai virus) greatly reduced viral binding. Interestingly, this treatment had no significant effect on the fusion reaction itself.

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