Mechanisms and impact of ceramide phosphoethanolamine biosynthesis

Abstract

Sphingolipids represent an essential class of membrane molecules in eukaryotic cells. They are primarily found in the outer leaflet of the plasma membrane where they help create a rigid and impermeable barrier to the extracellular environment. While sphingomyelin (SM) is the most abundant sphingolipid in mammals and nematodes, insects produce the SM analogue ceramide phosphoethanolamine (CPE) as a major plasma membrane constituent. Little is known about the biological role of CPE or about the enzymes responsible for CPE biosynthesis. SM production is mediated by a SM synthase (SMS) in the lumen of the Golgi. We identified an SMS-related enzyme, SMSr, that catalyses CPE production in the endoplasmic reticulum (ER) of both insect and mammalian cells. Curiously, while SMSr synthesizes only trace amounts of CPE, we discovered that the enzyme is a critical mediator of ceramide homeostasis in the ER with a key role in protecting cells against ceramide-induced suicide. We also found that bulk production of CPE in insects is independent of SMSr but requires a different enzyme (CPES) that shares mechanistic features with the enzymes responsible for phosphatidylethanolamine production via the Kennedy pathway. Using a bioinformatics-based cloning strategy, we identified CPES and found that this enzyme is unique for insects, sea anemones and Hydra. We also show that CPES utilizes CDP-ethanolamine as the donor molecule of the CPE head group. Contrary to SM production in mammals and nematodes, CPE production by CPES seems to occur on the cytosolic surface of the Golgi. Besides uncovering two unique enzymes of sphingolipid biosynthesis, our findings challenge prevailing concepts of membrane lipid organization in animal cells and reveal a novel mechanism of sphingolipid homeostasis.