Interactions between saturated acyl chains confer detergent resistance on lipids and glycosylphosphatidylinositol (GPI)-anchored proteins: GPI-anchored proteins in liposomes and cells show similar behavior.
Proteins anchored by GPI are poorly solubilized from cell membranes by cold nonionic detergents because they associate with detergent-resistant membranes rich in cholesterol and sphingolipids. In this study, we demonstrated that cholesterol and sphingolipid-rich liposomes were incompletely solubilized by Triton X-100. GPI-anchored placental alkaline phosphatase incorporated in these liposomes was also not solubilized by cold Triton X-100. As sphingolipids have much higher melting temperatures (Tm) than cellular phospholipids, a property correlated with Tm might cause detergent inextractability. In support of this idea, we found that the low-Tm lipid dioleoyl phosphatidylcholine (DOPC) was efficiently extracted from detergent-resistant liposomes by Triton X-100, whereas the high-Tm lipid dipalmitoyl phosphatidylcholine (DPPC) was not. The fluorescence polarization of liposome-incorporated diphenylhexatriene was measured to determine the "fluidity" of the detergent-resistant liposomes. We found that these liposomes were about as fluid as DPPC/cholesterol liposomes, which were present in the liquid-ordered phase, and much less fluid than DOPC or DOPC/cholesterol liposomes. These findings may explain the behavior of GPI-anchored proteins, which often have saturated fatty acyl chains and should prefer a less-fluid membrane. Therefore, we propose that acyl chain interactions can influence the association of GPI-anchored proteins with detergent-resistant membrane lipids. The affinity of GPI-anchored proteins for a sphingolipid-rich membrane phase that is not in the liquid crystalline state may be important in determining their cellular localization.
Schroeder
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Department of Biochemistry and Cell Biology, State University of New York at Stony Brook 11794.
London
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Brown
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D015060:1,2-Dipalmitoylphosphatidylcholine; D000215:Acylation; D000469:Alkaline Phosphatase / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D001409:Bacillus cereus / Q000201:enzymology; D002784:Cholesterol; D003902:Detergents; D004161:Diphenylhexatriene; D004926:Escherichia coli; D005260:Female; D017261:Glycosylphosphatidylinositols; D006801:Humans; D008081:Liposomes; D010713:Phosphatidylcholines; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D010727:Phosphoric Diester Hydrolases / Q000302:isolation & purification / Q000378:metabolism; D010920:Placenta / Q000201:enzymology; D011247:Pregnancy; D011994:Recombinant Proteins / Q000378:metabolism; D012995:Solubility; D013329:Structure-Activity Relationship
D003902:Detergents; D017261:Glycosylphosphatidylinositols; D008081:Liposomes; D010713:Phosphatidylcholines; D011994:Recombinant Proteins; D004161:Diphenylhexatriene; D015060:1,2-Dipalmitoylphosphatidylcholine; D002784:Cholesterol; D000469:Alkaline Phosphatase; D010727:Phosphoric Diester Hydrolases; D043265:Phosphatidylinositol Diacylglycerol-Lyase; C017251:1,2-oleoylphosphatidylcholine