Saponin-induced release of cell-surface-anchored Thy-1 by serum glycosylphosphatidylinositol-specific phospholipase D

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Abstract
A glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) was purified from human serum and used for studies on the release of GPI-anchored Thy-1 glycoprotein from mouse T lymphoma cells Y191. Previous studies have shown that whereas GPI-PLD is highly active against detergent-solubilized GPI-anchored proteins, it is normally unable to release GPI-containing proteins anchored in a lipid bilayer. Confirming these findings, the addition of GPI-PLD to intact Y191 cells did not result in cleavage of Thy-1. However, pretreatment of cells with saponin, a cholesterol-sequestering agent, rendered Thy-1 susceptible to hydrolysis. Very little solubilization of GPI-containing Thy-1 occurred under these conditions. From experiments with reconstituted liposomes it was inferred that the effect of saponin on cells was to aid in the presentation of Thy-1 to GPI-PLD. Furthermore, it was concluded that cholesterol-saponin complexes formed in the membrane were not alone responsible for the effect. Rather, additional molecules in the plasma membrane are possibly involved in the presentation of Thy-1 on saponin-treated cells. This finding may have implications for a physiological role of circulating GPI-PLD in the regulation of GPI-anchored proteins on cells.