Membrane cholesterol and cell fusion of hen and guinea-pig erythrocytes.

1. The cholesterol content of hen erythrocytes was modified by treating the cells with phospholipid liposomes. 2. Depletion of cellular cholesterol, by using liposomes of dipalmitoylglycerophosphocholine or phosphatidylcholine from hen erythrocytes, had no effect on the susceptibility of the cells to fusion induced by oleoylglycerol, but markedly decreased fusion induced by Sendai virus. 3. By contrast, enrichment of cellular cholesterol by using liposomes of dipalmitoylglycerophosphocholine and cholesterol increased cell fusion induced by oleoylglycerol, poly(ethylene glycol) and Sendai virus. 4. Virus-induced cell fusion of guinea-pig erythrocytes, which were enriched in cholesterol by feeding a cholesterol-rich diet to the animals, was also enhanced. 5. Hen erythrocytes that were treated with liposomes prepared from egg phosphatidylcholine contained increased quantities of phospholipid phosphorus and fused readily on incubation with retinol, independently of their cholesterol content. 6. It is suggested that cholesterol may enhance cell fusion by acting to facilitate a phase separation of protein-free areas of lipid bilayer, which subsequently provide the sites for cell fusion.