The main eukaryotic membrane lipids are the glycerophospholipids such as phosphatidylcholine (PtdCho; PC). Their diacylglycerol (DAG) backbone carries a phosphate (phosphatidic acid; PA) and either a choline (forming PtdCho), ethanolamine (forming phosphatidylethanolamine (PtdEtn); PE), serine (forming phosphatidylserine (PtdSer); PS), or inositol (forming phosphatidylinositol (PtdIns); PI). The prototypical phospholipid, dipalmitoyl-PtdCho, exhibits nearly cylindrical molecular geometry with a cross-sectional surface area of 64 Ų and a head-to- tail length of 19 Å (REF. 122). The phosphosphingolipid sphingomyelin (SM) and the glycosphingolipid glucosylceramide (GlcCer) have a ceramide (Cer) backbone, consisting of a sphingoid base (such as sphingosine; Sph), which is an amide linked to a fatty acid. Yeast sphingolipids carry a C26 fatty acid and have phosphoinositol-X substituents that contain additional mannoses and phosphates. Breakdown products of membrane lipids serve as lipid second messengers. The glycerolipid-derived signalling molecules include lysoPtdCho (LPC), lysoPA (LPA), PA and DAG. The sphingolipid-derived signalling molecules include sphingosylphosphorylcholine (SPC), Sph, sphingosine-1-phosphate (S1P), Cer-1-phosphate (C1P) and Cer. Arachidonic acid (AA) yields the signalling eicosanoids and endocannabinoids. The various phosphorylated PtdIns molecules (also known as the phosphoinositides) mark cellular membranes and recruit cytosolic proteins. They are interconverted by the actions of kinases and phosphatases. Figure modified with permission from REF. 5 © (2005) Macmillan Publishers Ltd.

The lipid composition of different membranes varies throughout the cell. The lipid compositional data are expressed as a percentage of the total phospholipid (PL) in mammals (blue) and yeast (light blue). As a measure of sterol content, the molar ratio of cholesterol (CHOL; in mammals) and ergosterol (ERG; in yeast) to phospholipid is also included. The main panel shows the site of synthesis of the major phospholipids (blue) and lipids that are involved in signalling and organelle recognition pathways (red). It should be appreciated that the levels of signalling and recognition lipids are significantly below 1% of the total phospholipid, except for ceramide (Cer). The major glycerophospholipids assembled in the endoplasmic reticulum (ER) are phosphatidylcholine (PtdCho; PC), phosphatidylethanolamine (PtdEtn; PE), phosphatidylinositol (PtdIns; PI), phosphatidylserine (PtdSer; PS) and phosphatidic acid (PA). In addition, the ER synthesizes Cer, galactosylceramide (GalCer), cholesterol and ergosterol. Both the ER and lipid droplets participate in steryl ester and triacylglycerol (TG) synthesis. The Golgi lumen is the site of synthesis of sphingomyelin (SM), complex glycosphingolipids (GSLs) and yeast inositol sphingolipid (ISL) synthesis. PtdCho is also synthesized in the Golgi, and may be coupled to protein secretion at the level of its diacylglycerol (DAG) precursor. Approximately 45% of the phospholipid in mitochondria (mostly PtdEtn, PA and cardiolipin (CL)) is autonomously synthesized by the organelle. BMP (bis(monoacylglycero)phosphate) is a major phospholipid in the inner membranes of late endosomes26. PI(3,5)P₂, phosphatidylinositol-(3,5)-bisphosphate; PI(4,5)P₂, phosphatidylinositol-(4,5)-bisphosphate; PI(3,4,5)P3, phosphatidylinositol-(3,4,5)-trisphosphate; PI4P, phosphatidylinositol- 4-phosphate; R, remaining lipids; S1P, sphingosine-1-phosphate.

As lipids move from the endoplasmic reticulum (ER) to the Golgi, plasma membrane and into endosomes, intrinsic lipid transporters dictate the phospholipid distribution across the bilayer. a In the ER, non-specific transbilayer equilibration of phospholipids has been demonstrated, and the membrane exhibits a nearly symmetric lipid distribution between bilayer leaflets. b In the Golgi, P4 ATPases translocate phosphatidylserine (PtdSer; PS) and phosphatidylethanolamine (PtdEtn; PE) to the cytosolic face. Sphingomyelin (SM) is produced by SM synthase from ceramide (Cer) on the lumenal side. Neither phosphatidylcholine (PtdCho; PC) nor SM molecules that are resident in the lumenal leaflet are transported to the cytosolic face. Thus, asymmetry in the Golgi is generated by the specific transport of PtdSer and PtdEtn and lack of transport of SM and PtdCho. In SM synthesis, PtdCho is converted to diacylglycerol (DAG), which freely equilibrates across bilayers. DAG can serve as a substrate for the cholinephosphotransferase isozyme, the product of which is SM. c At the plasma membrane, P4 ATPases transport PtdSer andPtdEtn to the cytosolic face, with little or no transport of PtdCho or SM to the cytosolic face under basal conditions. This homeostatic distribution can be disrupted by activation of scramblase and/or inhibition of the P4 ATPases. d Within endosomes, fluorescent PtdCho and SM and glycosphingolipids (GSLs) are restricted to the lumenal leaflet by a lack of specific transport mechanisms. P4 ATPases are recycled through endosomes123.

a Phosphatidylcholine (PtdCho (PC); red) is synthesized on the cytosolic endoplasmic reticulum (ER) surface (1) and freely flips across the ER membrane. On both sides it diffuses laterally in any membrane (2). PtdCho travels through vesicles to the Golgi (3), the plasma membrane (4) and endosomes (not shown). It is transported by transfer proteins between the cytosolic surfaces of organelles (5), maybe through contact sites (6 and 7). The ceramide (Cer) transfer protein CERT transports Cer from the ER to the Golgi (7) for lumenal sphingomyelin (SM) synthesis. SM cannot flip and travels in the vesicle lumen (5). Plasma-membrane enrichment of SM and cholesterol predicts their concentration at anterograde budding sites. Adapted from REF. 8. A model for non-vesicular lipid transport predicts carrier protein interaction with a donor membrane protein, promoting ligation of the cargo lipid (green). Cargo engagement facilitates carrier dissociation from the donor and diffusion to the acceptor membrane, which is recognized by distinct phosphoinositides. Binding between the carrier and the acceptor membrane induces cargo release, which is predicted to reduce the affinity of the carrier for the acceptor membrane and enable re-initiation of the cycle. Accessory proteins on donor and/or acceptor membranes may enhance the association and dissociation of carrier proteins. They may also facilitate the formation of contact sites, which restricts the diffusion path of the lipid carrier. The model provides the necessary elements of directionality and specificity to produce net lipid transfer.