Complex Sphingolipid Profiling and Identification of an Inositol Phosphorylceramide Synthase in Dictyostelium discoideum

Abstract

Dictyostelium discoideum is a professional phagocyte frequently used as experimental model to study cellular processes underlying the recognition, engulfment and infection course of microbial pathogens. Sphingolipids are abundant components of the plasma membrane that bind cholesterol, control vital membrane properties, participate in signal transmission and serve as adhesion molecules in recognition processes relevant to immunity and infection. While the pathway of sphingolipid biosynthesis has been well characterized in plants, animals and fungi, the identity of sphingolipids produced in D. discoideum, an organism at the crossroads between uni- and multicellular life, is not known. Combining lipidomics with a bioinformatics-based cloning strategy for key sphingolipid biosynthetic enzymes, we show here that D. discoideum produces phosphoinositol-containing sphingolipids with predominantly phytoceramide backbones. Cell-free expression of candidate inositol-phosphorylceramide (IPC) synthases from D. discoideum in defined lipid environments enabled identification of an enzyme that selectively catalyses the transfer of phosphoinositol from phosphatidylinositol onto ceramide. The corresponding IPC synthase, DdIPCS1, is non-homologous to but shares multiple sequence motifs with yeast IPC and human sphingomyelin synthases and localizes to the Golgi apparatus as well as the contractile vacuole of D. discoideum. Collectively, these findings open up important opportunities for exploring a role of sphingolipids in phagocytosis and infection across major evolutionary boundaries.