Structural basis for variations in the sensitivity of human decay accelerating factor to phosphatidylinositol-specific phospholipase C cleavage

Human decay-accelerating factor (DAF) proteins expressed on E and nucleated cells differ in their susceptibility to phosphatidylinositol (PI)-specific phospholipase C (PLC) cleavage/release. To investigate the mechanism of this difference, the glycoinositol-phospholipid anchoring moieties of E DAF, and of HeLa cell, polymorphonuclear cell, and lymphocyte DAF were structurally compared. Labeling of PI-PLC-resistant E DAF with 3-(trifluoromethyl)-3-(m-[¹²⁵I]-iodophenyl)-diazirine ([¹²⁵I]TID) and TLC analysis of nitrous acid deamination anchor fragments showed a predominant phospholipid species with less polar migration than the ¹²⁵I-TID-labeled PI. Gas chromatographic analyses of methanolyzed E protein revealed 2.20 ± 0.16 mol of fatty acids [16:0, 18:0, 18:1, 20:4, 22:4, and 22:5 (0.76, 0.36, 0.25, 0.15, 0.40, 0.28 mol, respectively)] and 0.86 ± 0.05 mol of inositol per mol of N-terminal Asp. Gas chromatography-mass spectroscopy demonstrated principally myo-inositol but also variable amounts of the chiro-isomer. Nondenaturing polyacrylamide gel electrophoresis of ¹⁴C-radiomethylated E protein revealed that pretreatment with hydroxylamine, a reagent which removes ester-linked lipids, rendered it PI-PLC susceptible. In contrast, parallel analyses of ³⁵S-cys-labeled PI-PLC-sensitive HeLa DAF protein revealed only minor amounts of the hydroxylamine-sensitive phospholipid species. Similar results were obtained with ¹²⁵I-surface-labeled DAF from polymorphonuclear cells, as well as from unstimulated peripheral blood and anti-CD3-activated lymphocytes. These findings demonstrate that, rather than PI, the E DAF anchor contains an inositol alkylacylglycerol-phospholipid which is heterogeneous with respect to acyl groups and inositol isomers, that an ester-linked substitution in this inositolphospholipid underlies the resistance of E DAF protein to PI-PLC cleavage/release, and that this structural modification is cell-specific.