Lipid-enveloped viruses replicate and bud from the host cell where they acquire their lipid coat. Ebola virus, which buds from the plasma membrane of the host cell, causes viral hemorrhagic fever and has a high fatality rate. To date, little has been known about how budding and egress of Ebola virus are mediated at the plasma membrane. We have found that the lipid phosphatidylserine (PS) regulates the assembly of Ebola virus matrix protein VP40. VP40 binds PS-containing membranes with nanomolar affinity, and binding of PS regulates VP40 localization and oligomerization on the plasma membrane inner leaflet. Further, alteration of PS levels in mammalian cells inhibits assembly and egress of VP40. Notably, interactions of VP40 with the plasma membrane induced exposure of PS on the outer leaflet of the plasma membrane at sites of egress, whereas PS is typically found only on the inner leaflet. Taking the data together, we present a model accounting for the role of plasma membrane PS in assembly of Ebola virus-like particles. IMPORTANCEThe lipid-enveloped Ebola virus causes severe infection with a high mortality rate and currently lacks FDA-approved therapeutics or vaccines. Ebola virus harbors just seven genes in its genome, and there is a critical requirement for acquisition of its lipid envelope from the plasma membrane of the human cell that it infects during the replication process. There is, however, a dearth of information available on the required contents of this envelope for egress and subsequent attachment and entry. Here we demonstrate that plasma membrane phosphatidylserine is critical for Ebola virus budding from the host cell plasma membrane. This report, to our knowledge, is the first to highlight the role of lipids in human cell membranes in the Ebola virus replication cycle and draws a clear link between selective binding and transport of a lipid across the membrane of the human cell and use of that lipid for subsequent viral entry. Lipid-enveloped viruses harbor a lipid membrane bilayer derived from their host cell during the budding process. This envelope provides the virus stability, protection of its genetic contents, and a reservoir for its transmembrane glycoprotein, which mediates entry into cells (1, 2). The viral lipid envelope may be a viable target for drug development, as particular alterations in the lipid coat or receptor-lipid interaction can inhibit viral entry (3-6). The lipid-dependent budding and egress of some lipid-enveloped viruses have been investigated. For example, it is well established that HIV-1 binds and utilizes 1,2-dioleoyl-sn-glycero-3-phospho-(1=-myo-inositol-4=,5=-bisphosphate) [PI(4,5)P 2 ] enriched in the plasma membrane (PM) inner leaflet for assembly and egress from the cell (7,8). Enteroviruses and flaviviruses use a phosphatidylinositol-4-phosphate [PI(4)P]-enriched organelle to replicate (9), and enteroviruses are packaged into phosphatidylserine (PS)-enriched vesicles, thereby enhancing the efficiency of viral transmission (10). The budding and egres...

Background:The Ebola virus matrix protein (VP40) regulates the plasma membrane assembly and egress of the Ebola virus. Results: The plasma membrane induces membrane penetration of the VP40 C-terminal domain. Conclusion: Membrane penetration by VP40 is important for VP40 cellular localization, oligomerization, and viral budding. Significance: A better understanding of VP40-membrane interactions will help us to understand Ebola virus assembly and budding.

Ebola virus, from the Filoviridae family has a high fatality rate in humans and nonhuman primates and to date, to the best of our knowledge, has no FDA approved vaccines or therapeutics. Viral protein 40 (VP40) is the major Ebola virus matrix protein that regulates assembly and egress of infectious Ebola virus particles. It is well established that VP40 assembles on the inner leaflet of the plasma membrane; however, the mechanistic details of VP40 membrane binding that are important for viral release remain to be elucidated. In this study, we used fluorescence quenching of a tryptophan on the membrane-binding interface with brominated lipids along with mutagenesis of VP40 to understand the depth of membrane penetration into lipid bilayers. Experimental results indicate that VP40 penetrates 8.1 Å into the hydrocarbon core of the plasma membrane bilayer. VP40 also induces substantial changes to membrane curvature as it tubulates liposomes and induces vesiculation into giant unilamellar vesicles, effects that are abrogated by hydrophobic mutations. This is a critical step in viral egress as cellular assays demonstrate that hydrophobic residues that penetrate deeply into the plasma membrane are essential for plasma membrane localization and virus-like particle formation and release from cells.

Solid-state (2)H-NMR of [(2)H(31)]-N-palmitoylsphingomyelin ([(2)H(31)]16:0SM, PSM*), supplemented by differential scanning calorimetry, was used for the first time, to our knowledge, to investigate the molecular organization of the sphingolipid in 1:1:1 mol mixtures with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (16:0-18:1PE, POPE) or 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphoethanolamine (16:0-22:6PE, PDPE) and cholesterol. When compared with (2)H-NMR data for analogous mixtures of [(2)H(31)]16:0-18:1PE (POPE*) or [(2)H(31)]16:0-22:6PE (PDPE*) with egg SM and cholesterol, molecular interactions of oleic acid (OA) versus docosahexaenoic acid (DHA) are distinguished, and details of membrane architecture emerge. SM-rich, characterized by higher-order, and PE-rich, characterized by lower-order, domains <20 nm in size are formed in the absence and presence of cholesterol in both OA- and DHA-containing membranes. Although acyl chain order within both domains increases on the addition of sterol to the two systems, the resultant differential in order between SM- and PE-rich domains is almost a factor of 3 greater with DHA than with OA. Our interpretation is that the aversion that cholesterol has for DHA--but not for OA--excludes the sterol from DHA-containing, PE-rich (nonraft) domains and excludes DHA from SM-rich/cholesterol-rich (raft) domains. We attribute, in part, the diverse health benefits associated with dietary consumption of DHA to an alteration in membrane domains.

The major mammalian plasma membrane lipids are phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), and cholesterol. Whereas PC-cholesterol interactions are well studied, far less is known about those between PE and cholesterol. Here, we investigated the molecular organization of cholesterol in PEs that vary in their degree of acyl chain unsaturation. For heteroacid sn-1 saturated (palmitoyl), sn-2 unsaturated (various acyl chain) PEs, cholesterol solubility determined by X-ray diffraction was essentially identical with 1 (oleoyl, 51 +/- 3 mol %) and 2 (linoleoyl, 49 +/- 2 mol %) double bonds before decreasing progressively with 4 (arachidonyl, 41 +/- 3 mol %) and 6 (docosahexaenoyl, 31 +/- 3 mol %) double bonds. With 6 double bonds in each chain, cholesterol solubility was further reduced to 8.5 +/- 1 mol %. However, (2)H NMR experiments established that the orientation of cholesterol in the same heteroacid PE membranes was unaffected by the degree of acyl chain unsaturation. A tilt angle of 15 +/- 1 degrees was measured when equimolar [3alpha-(2)H(1)]cholesterol was added, regardless of the number of double bonds in the sn-2 chain. The finding that solubility of cholesterol in sn-1 saturated PEs depends on the amount of polyunsaturation in the sn-2 chain of PE differs from the equivalent PCs that universally incorporate approximately 50 mol % sterol. Unlike PCs, a differential in affinity for cholesterol and tendency to drive lateral segregation is inferred between polyunsaturated PEs. This distinction may have biological implications reflected by the health benefits of dietary polyunsaturated fatty acids that are often taken up into PE > PC.

Ebola virus (EBOV) causes viral hemorrhagic fever in humans and can have clinical fatality rates of ~60%. The EBOV genome consists of negative sense RNA that encodes seven proteins including viral protein 40 (VP40). VP40 is the major Ebola virus matrix protein and regulates assembly and egress of infectious Ebola virus particles. It is well established that VP40 assembles on the inner leaflet of the plasma membrane of human cells to regulate viral budding where VP40 can produce virus like particles (VLPs) without other Ebola virus proteins present. The mechanistic details, however, of VP40 lipid-interactions and protein-protein interactions that are important for viral release remain to be elucidated. Here, we mutated a loop region in the N-terminal domain of VP40 (Lys127, Thr129, and Asn130) and find that mutations (K127A, T129A, and N130A) in this loop region reduce plasma membrane localization of VP40. Additionally, using total internal reflection fluorescence microscopy and number and brightness analysis we demonstrate these mutations greatly reduce VP40 oligomerization. Lastly, VLP assays demonstrate these mutations significantly reduce VLP release from cells. Taken together, these studies identify an important loop region in VP40 that may be essential to viral egress.

Background: Ebola virus VP40 matrix protein is able to produce virus-like particles from cells. Results: VP40 induces negative membrane curvature and scission from in vitro membranes. Conclusion: Phosphatidylserine plays an important role in VP40-induced vesiculation. Significance: Elucidation of VP40 interactions with lipids could lead to a better understanding of the Ebola virus replication cycle.

We employed solid state (2)H NMR, complemented by computer simulations, to compare molecular organization in model membranes composed of 1-elaidoyl-2-stearoylphosphatidylcholine (t18:1-18:0PC), 1-oleoyl-2-stearoylphosphatidylcholine (c18:1-18:0PC), and 1,2-distearoylphosphatidylcholine (18:0-18:0PC). These phospholipids have elaidic acid (EA) containing a trans double bond, oleic acid (OA) containing a cis double bond, and saturated stearic acid (SA), respectively, at the sn-1 position and were synthesized with perdeuterated SA at the sn-2 position. The temperature of the chain melting transition is depressed less for t18:1-18:0PC (31.5 degrees C) than c18:1-18:0PC (7 degrees C) relative to 18:0-18:0PC (53 degrees C), reflecting the smaller deviation from the linear conformation produced by a trans as opposed to cis double bond. Acyl chain order in t18:1-18:0PC (S(CD) = 0.135) in the liquid crystalline state is much closer to that of c18:1-18:0PC (S(CD) = 0.128) than that of the substantially more ordered 18:0-18:0PC (S(CD) > 0.156), which is attributed to the reduced energy barrier to rotation about the C-C single bonds next to either a trans or cis carbon double bond. A conformation that somewhat resembles a saturated chain and an intrinsic disorder approaching that of a cis unsaturated chain characterize EA and, we speculate, may play a role in the adverse impact dietary trans fatty acids (TFA) have on biological function.