Interaction of an amphitropic protein (factor Xa) with membrane models in a complex system

Surfaces as frameworks for intracellular organization

A large fraction of soluble protein within the interior of living cells may reversibly associate with structural elements, including proteinaceous fibers and phospholipid membranes. In this opinion, we present theoretical and experimental evidence that many of these associations are due to nonspecific attraction between the protein and the surface of the fiber or membrane, and that such associations may lead to substantial changes in the association state of the adsorbed proteins, the biological function of the adsorbed proteins, and the distribution of these proteins between the many microenvironments existing within the cell.

EGF domain of coagulation factor IX is conducive to exposure of phosphatidylserine

Lipid rafts are an initiation site for many different signals. Recently, we reported that an EGF domain in activated coagulation factor IX (EGF-F9) increases lipid raft formation and accelerates cell migration. However, the detailed mechanism is not well understood. This study aimed to evaluate the effects of EGF-F9 on the cell membrane. A431 cells (derived from human squamous cell carcinoma) were treated with recombinant EGF-F9. Cells were immunocytochemically stained with probes for lipid rafts or phosphatidylserine (PS). After 3 min of treatment with EGF-F9, cholera toxin subunit B (CTxB) binding domains emerged at the adhesive tips of filopodia. Subsequently, CTxB staining was observed on the filopodial shaft. Finally, large clusters of CTxB domains were observed at the edge of cell bodies. Markers for lipid rafts, such as caveolin-1 and a GPI anchored protein, co-localized with CTxB. Staining with annexin V and XII revealed that PS was exposed at the tips of filopodia, translocated on filopodial shafts, and co-localized with CTxB at the rafts. Immunocytochemistry showed that scramblase-1 protein was present at the filopodial tips. Our data indicates that EGF-F9 accelerates PS exposure around the filopodial adhesion complex and induces clustering of lipid rafts in the cell body. PS exposure is thought to occur on cells undergoing apoptosis. Further study of the function of the EGF-F9 motif in mediating signal transduction is necessary because it is shared by a number of proteins.

Intracellular traffic of the lysine and glutamic acid rich protein KERP1 reveals features of endomembrane organization in Entamoeba histolytica

The development of amoebiasis is influenced by the expression of the lysine and glutamic acid rich protein 1 (KERP1), a virulence factor involved in Entamoeba histolytica adherence to human cells. Up to date, it is unknown how the protein transits the parasite cytoplasm towards the plasma membrane, specially because this organism lacks a well-defined endoplasmic reticulum (ER) and Golgi apparatus. In this work we demonstrate that KERP1 is present at the cell surface and in intracellular vesicles which traffic in a pathway that is independent of the ER-Golgi anterograde transport. The intracellular displacement of vesicles enriched in KERP1 relies on the actin-rich cytoskeleton activities. KERP1 is also present in externalized vesicles deposited on the surface of human cells. We further report the interactome of KERP1 with its association to endomembrane components and lipids. The model for KERP1 traffic here proposed hints for the first time elements of the endocytic and exocytic paths of E. histolytica.

Cholesterol and Sphingomyelin-Containing Model Condensed Lipid Monolayers: Heterogeneities Involving Ordered Microdomains Assessed by Two Cholesterol Derivatives

Lipid monolayers are often considered as model membranes, but they are also the physiologic lipid part of the peripheral envelope of lipoproteins and cytosolic lipid bodies. However, their structural organization is still rather elusive, in particular when both cholesterol and sphingomyelin are present. To investigate such structural organization of hemimembranes, we measured, using alternative current voltammetry, the differential capacitance of condensed phosphatidylcholine-based monolayers as a function of applied potential, which is sensitive to their lipid composition and molecular arrangement. Especially, monolayers containing both sphingomyelin and cholesterol, at 15% w/w, presented specific characteristics of the differential capacitance versus potential curves recorded, which was indicative of specific interactions between these two lipid components. We then compared the behavior of two cholesterol derivatives (at 15% w/w), 21-methylpyrenyl-cholesterol (Pyr-met-Chol) and 22-nitrobenzoxadiazole-cholesterol (NBD-Chol), with that of cholesterol when present in model monolayers. Indeed, these two probes were chosen because of previous findings reporting opposite behaviors within bilayer membranes regarding their interaction with ordered lipids, with only Pyr-met-Chol mimicking cholesterol well. Remarkably, in monolayers containing sphingomyelin or not, Pyr-met-Chol and NBD-Chol presented contrasting behaviors, and Pyr-met-Chol mimicked cholesterol only in the presence of sphingomyelin. These two observations (i.e., optimal amounts of sphingomyelin and cholesterol, and the ability to discriminate between Pyr-met-Chol and NBD-Chol) can be interpreted by the existence of heterogeneities including ordered patches in sphingomyelin- and cholesterol-containing monolayers. Since such monolayer lipid arrangement shares some properties with the raft-type lipid microdomains well-described in sphingomyelin- and cholesterol-containing bilayer membranes, our data thus strongly suggest the existence of compact and ordered microdomains in model lipid monolayers.

Protein "amyloid-like" networks at the phospholipid membrane formed by 4-hydroxy-2-nonenal-modified mitochondrial creatine kinase

4-Hydroxy-2-nonenal (4-HNE) is a reactive aldehyde and a lipid peroxidation product formed in biological tissues under physiological and pathological conditions. Its concentration increases with oxidative stress and induces deleterious modifications of proteins and membranes. Mitochondrial and cytosolic isoforms of creatine kinase were previously shown to be affected by 4-HNE. In the present study, we analyzed the effect of 4-HNE on mitochondrial creatine kinase, an abundant protein from the mitochondrial intermembrane space with a key role in mitochondrial physiology. We show that this effect is double: 4-HNE induces a step-wise loss of creatine kinase activity together with a fast protein aggregation. Protein-membrane interaction is affected and amyloid-like networks formed on the biomimetic membrane. These fibrils may disturb mitochondrial organisation both at the membrane and in the inter membrane space.

Nanoscale studies of protein-membrane interactions in blood clotting

Most of the steps in the blood clotting cascade require clotting proteins to bind to membrane surfaces with exposed phosphatidylserine. In spite of the importance of these protein-membrane interactions, we still lack a detailed understanding of how clotting proteins interact with membranes and how membranes contribute so profoundly to catalysis. Our laboratories are using multidisciplinary approaches to explore, at atomic-resolution, how blood clotting protein complexes assemble and function on membrane surfaces.

Mitochondrial creatine kinase interaction with cardiolipin-containing biomimetic membranes is a two-step process involving adsorption and insertion

Mitochondrial creatine kinase (mtCK) binding to the mitochondrial inner membrane largely determines its biological functions in cellular energy homeostasis, mitochondrial physiology, and dynamics. The membrane binding mechanism is, however, not completely understood. Recent data suggest that a hydrophobic component is involved in mtCK binding to cardiolipin at the outer face of the inner mitochondrial membrane, in addition to the well known electrostatically driven process. In this manuscript, using an electrochemical method derived from alternating current polarography for differential capacity measurements, we distinctly reveal that protein-cardiolipin interaction has a two-step mechanism. For short incubation time, protein adsorption to the phospholipid charged headgroup was the only process detected, whereas on a longer time scale evidence of protein insertion was observed.

Effect of 4-hydroxynonenal on phosphatidylethanolamine containing condensed monolayer and on its interaction with apolipoprotein A-I

4-Hydroxynonenal (4HNE), generated during polyunsaturated fatty acid oxidation, is present in atherosclerotic lesions. As 4HNE is able to react with phosphatidylethanolamine (PE), we investigated, using AC polarography, whether it may alter the physico-chemical state of a condensed PE-containing phospholipid monolayer and its interaction with apoA-I. The stability of a phospholipid monolayer relative to potential (around the potential of zero charge) is dependent on lipid composition (PE>PC>PE/PC). ApoA-I insertion into PE/PC monolayer is easier than in PC monolayer. Pre-treatment of PE/PC monolayer by 4HNE does not alter monolayer stability, but decreases apo A-I insertion into the monolayer.