The Lipid Moiety of the GPI-Anchor of the Major Plasma Membrane Proteins in Paramecium primaurelia is a Ceramide: Variation of the Amide-Linked Fatty Acid Composition as a Function of Growth Temperature.

The major membrane proteins of Paramecium are anchored in the plasma membrane via a glycosylphosphatidylinositol (GPI). The expression of these GPI-proteins, the surface antigen (SAg) and the surface GPI-proteins (SGPs), is temperature-dependent, different sets are expressed at 23°C and at 32 °C. To characterize the GPI-anchor lipid moieties of these proteins, a new strategy of biosynthetic radiolabeling was developed. Cells of Paramecium primaurelia, grown at 23°C or at 32 °C, were fed with [(14)C]-labeled cyanobacteria. The paramecia metabolized the cyanobacteria lipids and synthesized fatty acids with longer and more unsaturated chains. The SAg and SGPs from [(14)C]-labeled paramecia, were purified and the lipid moieties of their GPI-anchors were cleaved by a Bacillus thuringiensis phosphatidylinositol-specific phospholipase C and identified as ceramides. The GPI-anchor ceramides, from the SAg and SGPs expressed at both temperatures, contained long-chain bases which did not display variations detectable upon thin layer chromatography analysis. In contrast, the amide-linked fatty acid component varied: palmitic acid was identified as the major amidelinked fatty acid in the GPI-protein anchors from paramecia grown at 23°C, while at 32°C a C(14) fatty acid could be the prominent fatty acid. This modulation in the fatty acid composition could playa role in the antigenic variation process.