A Dog\u27s Life

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Concentration fluctuations and phase transitions in coupled modulated bilayers

We consider the formation of finite-size domains in lipid bilayers consisting of saturated and hybrid lipids. First, we describe a monolayer model that includes a coupling between a compositional scalar field and a two-dimensional vectorial order-parameter. Such a coupling yields an effective two-dimensional microemulsion free-energy for the lipid monolayer, and its characteristic length of compositional modulations can be considered as the origin of finite-size domains in biological membranes. Next, we consider a coupled bilayer composed of two modulated monolayers, and discuss the static and dynamic properties of concentration fluctuations above the transition temperature. We also investigate the micro-phase separation below the transition temperature, and compare the micro-phase separated structures with statics and dynamics of concentration fluctuations above the transition.Comment: 14 pages, 12 figures, 1 tabl

Labeling of human erythrocyte membrane proteins by photoactivatable radioiodinated phosphatidylcholine and phosphatidylserine A search for the aminophospholipid translocase

AbstractWe have synthesized radioiodinated photoactivatable phosphatidylcholine (125-N3-PC) and phosphatidylserine (125I-N3-PS). After incubation with red blood cells in the dark, the labeled PC could be extracted but not the corresponding PS molecule, indicating that the latter was transported by the aminophospholipid translocase, but not the former. When irradiated immediately after incorporation, N3-PS, but not N3-PC, partially blocked subsequent translocation of spin-labeled aminophospholipids. Analysis of probe distribution by SDSpolyacrylamide gel electrophoresis revealed that 125I-N3-PS labeled seven membrane bound components with molecular masses between 140 and 27 kDa: one (or several) of these components should correspond to the aminophospholipid translocase

Plant phosphoinositide-dependent phospholipases C: Variations around a canonical theme

International audiencePhosphoinositide-specific phospholipase C (PI-PLC) cleaves, in a Ca 2þ-dependent manner, phosphati-dylinositol-4,5-bisphosphate (PI-4,5-P 2) into diacylglycerol (DAG) and inositol triphosphate (IP 3). PI-PLCs are multidomain proteins that are structurally related to the PI-PLCzs, the simplest animal PI-PLCs. Like these animal counterparts, they are only composed of EF-hand, X/Y and C2 domains. However, plant PI-PLCs do not have a conventional EF-hand domain since they are often truncated, while some PI-PLCs have no EF-hand domain at all. Despite this simple structure, plant PI-PLCs are involved in many essential plant processes, either associated with development or in response to environmental stresses. The action of PI-PLCs relies on the mediators they produce. In plants, IP 3 does not seem to be the sole active soluble molecule. Inositol pentakisphosphate (IP 5) and inositol hexakisphosphate (IP 6) also transmit signals, thus highlighting the importance of coupling PI-PLC action with inositol-phosphate kinases and phosphatases. PI-PLCs also produce a lipid molecule, but plant PI-PLC pathways show a peculiarity in that the active lipid does not appear to be DAG but its phosphorylated form, phosphatidic acid (PA). Besides, PI-PLCs can also act by altering their substrate levels. Taken together, plant PI-PLCs show functional differences when compared to their animal counterparts. However, they act on similar general signalling pathways including calcium homeostasis and cell phosphoproteome. Several important questions remain unanswered. The cross-talk between the soluble and lipid mediators generated by plant PI-PLCs is not understood and how the coupling between PI-PLCs and inositol-kinases or DAG-kinases is carried out remains to be established

Phytosphingosine-phosphate is a signal for AtMPK6 activation and Arabidopsis response to chilling

Long-chain bases (LCBs) are pleiotropic sphingolipidic signals in eukaryotes. We investigated the source and function of phytosphingosine-1-phosphate (PHS-P), a phospho-LCB rapidly and transiently formed in Arabidopsis thaliana on chilling.PHS-P was analysed by thin-layer chromatography following in vivo metabolic radiolabelling. Pharmacological and genetic approaches were used to identify the sphingosine kinase isoforms involved in cold-responsive PHS-P synthesis. Gene expression, mitogen-activated protein kinase activation and growth phenotypes of three LCB kinase mutants (lcbk1, sphk1 and lcbk2) were studied following cold exposure. Chilling provoked the rapid and transient formation of PHS-P in Arabidopsis cultured cells and plantlets. Cold-evoked PHS-P synthesis was reduced by LCB kinase inhibitors and abolished in the LCB kinase lcbk2 mutant, but not in lcbk1 and sphk1 mutants. lcbk2 presented a constitutive AtMPK6 activation at 22°C. AtMPK6 activation was also triggered by PHS-P treatment independently of PHS/PHS-P balance. lcbk2 mutants grew comparably with wild-type plants at 22 and 4°C, but exhibited a higher root growth at 12°C, correlated with an altered expression of the cold-responsive DELLA gene RGL3. Together, our data indicate a function for LCBK2 in planta. Furthermore, they connect PHS-P formation with plant response to cold, expanding the field of LCB signalling in plants

Europium as an inhibitor of Amyloid-β(1-42) induced membrane permeation

Soluble Amyloid-beta (Aβ) oligomers are a source of cytotoxicity in Alzheimer's disease (AD). The toxicity of Aβ oligomers may arise from their ability to interact with and disrupt cellular membranes mediated by GM1 ganglioside receptors within these membranes. Therefore, inhibition of Aβ–membrane interactions could provide a means of preventing the toxicity associated with Aβ. Here, using Surface Plasmon field-enhanced Fluorescence Spectroscopy, we determine that the lanthanide, Europium III chloride (Eu3+), strongly binds to GM1 ganglioside-containing membranes and prevents the interaction with Aβ42 leading to a loss of the peptides ability to cause membrane permeation. Here we discuss the molecular mechanism by which Eu3+ inhibits Aβ42-membrane interactions and this may lead to protection of membrane integrity against Aβ42 induced toxicity

Investigation of Domain Formation in Sphingomyelin/Cholesterol/POPC Mixtures by Fluorescence Resonance Energy Transfer and Monte Carlo Simulations

We have recently proposed a phase diagram for mixtures of porcine brain sphingomyelin (BSM), cholesterol (Chol), and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) on the basis of kinetics of carboxyfluorescein efflux induced by the amphipathic peptide δ-lysin. Although that study indicated the existence of domains, phase separations in the micrometer scale have not been observed by fluorescence microscopy in BSM/Chol/POPC mixtures, though they have for some other sphingomyelins (SM). Here we examine the same BSM/Chol/POPC system by a combination of fluorescence resonance energy transfer (FRET) and Monte Carlo simulations. The results clearly demonstrate that domains are formed in this system. Comparison of the FRET experimental data with the computer simulations allows the estimate of lipid-lipid interaction Gibbs energies between SM/Chol, SM/POPC, and Chol/POPC. The latter two interactions are weakly repulsive, but the interaction between SM and Chol is favorable. Furthermore, those three unlike lipid interaction parameters between the three possible lipid pairs are sufficient for the existence of a closed loop in the ternary phase diagram, without the need to involve multibody interactions. The calculations also indicate that the largest POPC domains contain several thousand lipids, corresponding to linear sizes of the order of a few hundred nanometers

High-resolution mapping reveals topologically distinct cellular pools of phosphatidylserine

Phosphatidylserine exists lumenally in the ER, Golgi, and mitochondria but cytoplasmically in the trans-Golgi and at the plasma membrane, which suggests that functionally important flipping may occur during trafficking