Final Report Summary - ARFMEMBRANESENSORS (Membrane sensors in the Arf orbit)
Cells are subdivided in organelles, which carry out functions such as energy synthesis (mitochondria), protein and lipid synthesis (the endoplasmic reticulum), or degradation (lyzosomes). Cell organelles are delimited by a membrane made of lipids and proteins, which acts as a physical barrier. The lipid composition of organelle membranes is extraordinary complex and can encompass hundreds of different lipid species. One key issue in cell biology is to understand the rationale of this lipid diversity and the mechanisms by which lipids are selectively enriched in some organelle membranes.
Our ERC project was based on the idea that, given the large difference in lipid composition of organelle membranes, the physicochemical properties of their surface could act as cues to direct the selective adsorption of proteins, notably those involved membrane deformation, membrane tethering and lipid exchange. Among the most interesting candidates are proteins that contain amphipathic motifs, which, by reversibly intercalating at the water/membrane surface, are ideally suited to sense properties such as lipid packing, electrostatic and curvature. Focusing on this mode of adsorption allowed us to uncover two principles of membrane organization: (i) how to make a cholesterol gradient; (ii) how to make membranes more flexible.
#1. PI(4)P as an energy source for cholesterol transport . We show that the yeast protein Osh4p, which transfers sterols between membranes, is supersensitive to membrane bulk lipid properties. Optimal sterol transport occurs when Osh4p lands transiently on membranes. For the mammalian cousin of Osh4p, OSBP, the mechanism is different since OSBP is capable of bridging membranes. However, the breakthrough was the discovery that, after having transferred sterol from a donor to an acceptor membrane, Osh4p and OSBP transport another lipid named PI(4)P in the reverse direction. This mode of lipid exchange not only reconciles disparate past observations, but also gives an illuminating response to a long standing issue: how to transfer cholesterol from the endoplasmic reticulum, where it is synthesized, to its final destination, the plasma membrane: PI(4)P hydrolysis makes the sterol exchange reaction irreversible. In other words, PI(4)P is burnt for the proper transport of cholesterol.
#2. Phospholipid unsaturation, lipid packing defects, and membrane flexibility. The acyl chains of membrane phospholipids vary considerably among organelles. At the endoplasmic reticulum, phospholipids generally contain acyl chains with a single cis double bond that induces a kink in the chain. In contrast, at the plasma membrane, phospholipids preferentially bear saturated acyl chains. Last, in some specialized organelles such as synaptic vesicles, phospholipids with polyunsaturated acyl chains are abundant. Using specific AHs as membrane probes, we observed that AH adsorption is favored by the combination of lipid monounsaturation and positive curvature, hence leading to a straightforward model of the formation of lipid packing defects at the ER/Golgi interface that we explicitly characterized by molecular dynamics simulations. In striking contrast, we show that polyunsaturated phospholipids do not accentuate lipid-packing defects, but rather correct them due to the exceptional flexibility of the polyunsaturated acyl chain. Consequently, polyunsaturated membranes are more prone to deformation. A striking example is given by the endocytic proteins dynamin and endophilin, which readily fission polyunsaturated membranes. Altogether, these results give a rationale for the acyl chain profiles of particular organelles and notably synaptic vesicles. Their enrichment in polyunsaturated phospholipids should favor their fast retrieval.
Our ERC project was based on the idea that, given the large difference in lipid composition of organelle membranes, the physicochemical properties of their surface could act as cues to direct the selective adsorption of proteins, notably those involved membrane deformation, membrane tethering and lipid exchange. Among the most interesting candidates are proteins that contain amphipathic motifs, which, by reversibly intercalating at the water/membrane surface, are ideally suited to sense properties such as lipid packing, electrostatic and curvature. Focusing on this mode of adsorption allowed us to uncover two principles of membrane organization: (i) how to make a cholesterol gradient; (ii) how to make membranes more flexible.
#1. PI(4)P as an energy source for cholesterol transport . We show that the yeast protein Osh4p, which transfers sterols between membranes, is supersensitive to membrane bulk lipid properties. Optimal sterol transport occurs when Osh4p lands transiently on membranes. For the mammalian cousin of Osh4p, OSBP, the mechanism is different since OSBP is capable of bridging membranes. However, the breakthrough was the discovery that, after having transferred sterol from a donor to an acceptor membrane, Osh4p and OSBP transport another lipid named PI(4)P in the reverse direction. This mode of lipid exchange not only reconciles disparate past observations, but also gives an illuminating response to a long standing issue: how to transfer cholesterol from the endoplasmic reticulum, where it is synthesized, to its final destination, the plasma membrane: PI(4)P hydrolysis makes the sterol exchange reaction irreversible. In other words, PI(4)P is burnt for the proper transport of cholesterol.
#2. Phospholipid unsaturation, lipid packing defects, and membrane flexibility. The acyl chains of membrane phospholipids vary considerably among organelles. At the endoplasmic reticulum, phospholipids generally contain acyl chains with a single cis double bond that induces a kink in the chain. In contrast, at the plasma membrane, phospholipids preferentially bear saturated acyl chains. Last, in some specialized organelles such as synaptic vesicles, phospholipids with polyunsaturated acyl chains are abundant. Using specific AHs as membrane probes, we observed that AH adsorption is favored by the combination of lipid monounsaturation and positive curvature, hence leading to a straightforward model of the formation of lipid packing defects at the ER/Golgi interface that we explicitly characterized by molecular dynamics simulations. In striking contrast, we show that polyunsaturated phospholipids do not accentuate lipid-packing defects, but rather correct them due to the exceptional flexibility of the polyunsaturated acyl chain. Consequently, polyunsaturated membranes are more prone to deformation. A striking example is given by the endocytic proteins dynamin and endophilin, which readily fission polyunsaturated membranes. Altogether, these results give a rationale for the acyl chain profiles of particular organelles and notably synaptic vesicles. Their enrichment in polyunsaturated phospholipids should favor their fast retrieval.