PI-Specific Phospholipase C Cleavage of a Reconstituted GPI-Anchored Protein: Modulation by the Lipid Bilayer

Release of glycosylphosphatidylinositol- (GPI-) anchored ectoenzymes from the membrane by phosphatidylinositol- (PI-) specific phospholipases may play an important role in modulating the surface expression and function of this group of proteins. To investigate how the properties of the host membrane affect anchor cleavage, porcine lymphocyte ecto-5'-nucleotidase (5'-NTase; EC 3.1.3.5) was purified, reconstituted into lipid bilayer vesicles of various lipids, and cleaved using PI-PLC from Bacillus thuringiensis (Bt-PI-PLC). Bt-PI-PLC activity was highly dependent on the chain length and unsaturation of the constituent phospholipids. Very high rates of cleavage were observed in fluid lipids with a low phase transition temperature (Tm), in lymphocyte plasma membrane, and in a lipid mixture that formed rafts. Arrhenius plots of the rate of anchor cleavage in various lipids showed a characteristic break at the bilayer Tm, together with a discontinuity close to Tm. The activation energy for GPI anchor cleavage was substantially higher in gel phase bilayers compared to those in the liquid crystalline phase. The addition of cholesterol simultaneously abolished the phase transition and the large difference in cleavage rates observed above and below Tm. Inclusion of GM1 and GT1b (components of lipid rafts) in the bilayer reduced the overall activity, but the pattern of the Arrhenius plots remained unchanged. Both gangliosides had similar effects, suggesting that bilayer surface charge has little influence on PI-PLC activity. Taken together, these results suggest that lipid fluidity and packing are the most important modulators of Bt-PI-PLC activity on GPI anchors.