Domain Formation and Permeabilization Induced by the Saponin α‑Hederin and Its Aglycone Hederagenin in a Cholesterol-Containing Bilayer

Saponins and triterpenic acids have been shown to be able to interact with lipid membranes and domains enriched with cholesterol (rafts). How saponins are able to modulate lipid phase separation in membranes and the role of the sugar chains for this activity is unknown. We demonstrate in a binary membrane model composed of DMPC/Chol (3:1 mol/mol) that the saponin α-hederin and its aglycone presenting no sugar chain, the triterpenic acid hederagenin, are able to induce the formation of lipid domains. We show on multilamellar vesicles (MLV), giant unilamellar vesicles (GUV), and supported planar bilayers (SPB) that the presence of sugar units on the sapogenin accelerates domain formation and increases the proportion of sterols within these domains. The domain shape is also influenced by the presence of sugars because α-hederin and hederagenin induce the formation of tubular and spherical domains, respectively. These highly curved structures should result from the induction of membrane curvature by both compounds. In addition to the formation of domains, α-hederin and hederagenin permeabilize GUV. The formation of membrane holes by α-hederin comes along with the accumulation of lipids into nonbilayer structures in SPB. This process might be responsible for the permeabilizing activity of both compounds. In LUV, permeabilization by α-hederin was sterol-dependent. The biological implications of our results and the mechanisms involved are discussed in relation to the activity of saponins and triterpenic acids on membrane rafts, cancer cells, and hemolysis

Distantly related lipocalins share two conserved clusters of hydrophobic residues: use in homology modeling-1

<p><b>Copyright information:</b></p><p>Taken from "Distantly related lipocalins share two conserved clusters of hydrophobic residues: use in homology modeling"</p><p>http://www.biomedcentral.com/1472-6807/8/1</p><p>BMC Structural Biology 2008;8():1-1.</p><p>Published online 11 Jan 2008</p><p>PMCID:PMC2254393.</p><p></p>te) and β in gray. The numbering of 1QFT refers to that of Figure 1. Positions at the extremities of the secondary structure elements are numbered to facilitate the reading. Residues interacting with histamine in the structures of 1QFT are underlined. Those belonging to the H site are bold. Boxes indicate examples of realigned regions

Domain Formation and Permeabilization Induced by the Saponin α‑Hederin and Its Aglycone Hederagenin in a Cholesterol-Containing Bilayer

Saponins and triterpenic acids have been shown to be able to interact with lipid membranes and domains enriched with cholesterol (rafts). How saponins are able to modulate lipid phase separation in membranes and the role of the sugar chains for this activity is unknown. We demonstrate in a binary membrane model composed of DMPC/Chol (3:1 mol/mol) that the saponin α-hederin and its aglycone presenting no sugar chain, the triterpenic acid hederagenin, are able to induce the formation of lipid domains. We show on multilamellar vesicles (MLV), giant unilamellar vesicles (GUV), and supported planar bilayers (SPB) that the presence of sugar units on the sapogenin accelerates domain formation and increases the proportion of sterols within these domains. The domain shape is also influenced by the presence of sugars because α-hederin and hederagenin induce the formation of tubular and spherical domains, respectively. These highly curved structures should result from the induction of membrane curvature by both compounds. In addition to the formation of domains, α-hederin and hederagenin permeabilize GUV. The formation of membrane holes by α-hederin comes along with the accumulation of lipids into nonbilayer structures in SPB. This process might be responsible for the permeabilizing activity of both compounds. In LUV, permeabilization by α-hederin was sterol-dependent. The biological implications of our results and the mechanisms involved are discussed in relation to the activity of saponins and triterpenic acids on membrane rafts, cancer cells, and hemolysis

Multi-Scale Simulation of the Simian Immunodeficiency Virus Fusion Peptide

Fusion peptides of type I fusion glycoproteins are structural elements of several enveloped viruses which enable the fusion between host and virus membranes. It is generally suggested that these peptides can promote the early fusion steps by inducing membrane curvature and that they adopt a tilted helical conformation in membranes. Although this property has been the subject of several experimental and in silico studies, an extensive sampling of the membrane peptide interaction has not yet been done. In this study, we performed coarse-grained molecular dynamic simulations in which the lipid bilayer self-assembles around the peptide. The simulations indicate that the SIV fusion peptide can adopt two different orientations in a DPPC bilayer, a major population which adopts a tilted interfacial orientation and a minor population which is perpendicular to the bilayer. The simulations also indicate that for the SIV mutant that does not induce fusion in vitro the tilt is abolished

A Long QT Mutation Substitutes Cholesterol for Phosphatidylinositol-4,5-Bisphosphate in KCNQ1 Channel Regulation

<div><p>Introduction</p><p>Phosphatidylinositol-4,5-bisphosphate (PIP₂) is a cofactor necessary for the activity of KCNQ1 channels. Some Long QT mutations of KCNQ1, including R243H, R539W and R555C have been shown to decrease KCNQ1 interaction with PIP₂. A previous study suggested that R539W is paradoxically less sensitive to intracellular magnesium inhibition than the WT channel, despite a decreased interaction with PIP₂. In the present study, we confirm this peculiar behavior of R539W and suggest a molecular mechanism underlying it.</p><p>Methods and Results</p><p>COS-7 cells were transfected with WT or mutated KCNE1-KCNQ1 channel, and patch-clamp recordings were performed in giant-patch, permeabilized-patch or ruptured-patch configuration. Similar to other channels with a decreased PIP₂ affinity, we observed that the R243H and R555C mutations lead to an accelerated current rundown when membrane PIP₂ levels are decreasing. As opposed to R243H and R555C mutants, R539W is not more but rather less sensitive to PIP₂ decrease than the WT channel. A molecular model of a fragment of the KCNQ1 C-terminus and the membrane bilayer suggested that a potential novel interaction of R539W with cholesterol stabilizes the channel opening and hence prevents rundown upon PIP₂ depletion. We then carried out the same rundown experiments under cholesterol depletion and observed an accelerated R539W rundown that is consistent with this model.</p><p>Conclusions</p><p>We show for the first time that a mutation may shift the channel interaction with PIP₂ to a preference for cholesterol. This <i>de novo</i> interaction wanes the sensitivity to PIP₂ variations, showing that a mutated channel with a decreased affinity to PIP₂ could paradoxically present a slowed current rundown compared to the WT channel. This suggests that caution is required when using measurements of current rundown as an indicator to compare WT and mutant channel PIP₂ sensitivity.</p></div

R539W is insensitive to Ci-VSP.

<p>A, representative ruptured-patch current recordings of WT or mutant channels coexpressed with the voltage-dependent membrane phosphatase, Ci-VSP, at the first (t = 0 s) and 9^th (t = 64 s) step of depolarization. These currents were measured during the voltage-clamp protocol shown. Start-to-start interval = 8 s. The +80-mV depolarization also allows Ci-VSP activation. B, relative tail-current amplitude (measured at −40 mV) of WT or mutant channels after a depolarization to +80 mV, plotted against time. Current values are normalized to the current amplitude measured before Ci-VSP activation (time 0). C, mean relative current amplitude of WT or mutant channels measured after a 64-s Ci-VSP activation (n = 6–11). *p<0.05, ***p<0.001 versus WT. ^§R555C already ran down before Ci-VSP activation, due to basal Ci-VSP activity, it is thus assimilated to 0 (n = 18). WT condition without Ci-VSP is shown in (B) and (C).</p

R539W is insensitive to wortmannin.

<p>A, representative permeabilized-patch current recordings of WT or mutant channels measured before or after a 63-s wortmannin application (10 µmol/L), and during the voltage-clamp protocol shown. Start-to-start interval = 7 s. B, relative current amplitude of WT or mutant channels measured at the end of the depolarizing step (+80 mV), plotted against time. Current values are normalized to the current level measured before wortmannin application (time 0). These experiments were performed at 35°C in the permeabilized-patch configuration. In this configuration, it has been shown that there is no spontaneous current rundown <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0093255#pone.0093255-Loussouarn2" target="_blank">[24]</a>. C, mean relative current amplitude of WT or mutant channels measured after a 63-s wortmannin application (n = 5–7). *p<0.05, <i>versus</i> WT.</p

R539W is insensitive to osmolarity.

<p>A, B, superimposed representative permeabilized-patch recordings of WT (A) and R539W (B) KCNE1-KCNQ1 concatemer currents, respectively, measured in hyper-, iso- and hypoosmotic conditions using the voltage protocol shown in the insert. C, D, averaged tail-current density (in pA/pF), V_0.5 (in mV) and τ_deact (in ms) measured at −40 mV after a depolarization step to +80 mV for WT (C) and +120 mV for R539W (D) channel, in hyperosmotic (hyper), control (iso), and hypoosmotic solutions (hypo) (n = 10); same voltage protocol as in A. *p<0.05, ***p<0.001 (one-way ANOVA for repeated measures). Protocol and experimental conditions are similar to those used in our previous study analyzing the osmoregulation of KCNE1-KCNQ1 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0093255#pone.0093255-Piron1" target="_blank">[14]</a>.</p