The mechanism of interaction of artificially generated lipid vesicles (approximately 500 A diameter) with Chinese hamster V79 cells bathed in a simple balanced salt solution was investigated. The major pathways of exogenous lipid incorporation in vesicle-treated cells are vesicle-cell fusion and vesicle-cell lipid exchange. At 37 degrees C, the fusion process is dominant, while at 2 degrees C or with energy depleted cells, exchange of lipids between vesicles and cells is important. The fusion mechanism was demonstrated using vesicles of [14C]lecithin containing trapped [13H]inulin. Consistent with a fusion hypothesis, both components became cell associated at 37 degrees C in nearly the same proportions as they were present in the applied vesicles. Additional arguments in favor of vesicle-cell fusion and against phagocytosis or adsorption of intact vesicles are presented. At 2 degrees C or with inhibitor-treated cells, the [3H]inulin uptake was largely suppressed, while the lipid uptake was reduced to a lesser extent. Evidence for vesicle-cell lipid exchange was obtained using V79 cells grown on 3H precursors for cellular lipids. [14C]lecithin vesicles, incubated with such cells, showed no change in their elution properties when subjected to molecular sieve chromatography on Sepharose 4B. However, radioactivity and thin-layer chromatographic analyses revealed that a variety of cell lipiids had been exchanged into the uniamellar vesicles. Further evidence for the fusion and exchange processes was obtained using vesicles prepared from mixtures of [3H]lecithin and [14C]cholesterol. A two-step fusion mechanism consistent with the present findings is proposed as a working model for other fusion studies.

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