TRAFFIC, POLARITY, AND DETERGENT SOLUBILITY OF A GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED PROTEIN AFTER LDL-DEPRIVATION OF MDCK CELLS

Glycosylphosphatidylinositol-anchored proteins, GPI-proteins, are sele ctively delivered to the apical surfaces of many types of morphologica lly polarized epithelial cells. It has been proposed that the unit for targeting GPI-proteins to the apical surface is a membrane lipid doma in. This sorting domain or molecular cluster has been equated to deter gent (Triton X-100)insoluble membrane fractions that are enriched in e nriched in GPI-proteins, glycosphingolipids, and cholesterol. To deter mine the role of cholesterol in the formation of sorting domains and t o examine its importance in the intracellular traffic and membrane pol arity of GPI-proteins, we studied the behavior of a model GPI-protein, gD1-DAF, in MDCK cells cultured for 3 or 14 d without their principal source of cholesterol, serum LDL. LDL deprivation affects the intrace llular traffic of gD1-DAF. Surface expression of gD1-DAF is reduced in LDL-deprived cells; this reduction is most marked after 3 d of LDL de privation. We also find a great reduction in the fraction of gD1-DAF t hat is detergent-insoluble in these cells and a change in its membrane milieu defined by susceptibility to cleavage with PI-specific phospho lipase C. Despite these changes, the surface polarity of gD1-DAF is no different in LDL-deprived cells than in control cells.