Sphingolipid Asymmetry and Transmembrane Translocation in Mammalian Cells

Sphingolipids are a typical feature of eukaryotic cells, and indeed, they have been found to fulfi ll a number of intra - and intercellular functions that are specifi c for eukaryotes. Membrane sphingolipids are organized in specialized membrane domains that are involved in the sorting of membrane proteins and lipids along the cellular vesicular transport pathways. In addition, the domains have been invoked in various types of signaling events, like the formation of the T - cell receptor complex and the formation of cell – cell signaling domains. On the other hand, individual sphingolipids act as lipid second messengers, the clearest examples being sphingosine - 1 - phosphate and ceramide. Sphingolipids act at discrete locations, and they are synthesized and degraded at defi ned locations. These are not always on the same side of the membrane, which necessitates transmembrane transport. The sites of transmembrane translocation, the molecular mechanism, and its possible regulation are the topic of the present chapter (Fig. 4.1 ).