Volume enlargement and recovery of Na+-dependent amino acid transport in proteoliposomes derived from Ehrlich ascites cell membranes

Modulation of the potassium channel KcsA by anionic phospholipids: Role of arginines at the non-annular lipid binding sites

The role of arginines R64 and R89 at non-annular lipid binding sites of KcsA, on the modulation of channel activity by anionic lipids has been investigated. In wild-type (WT) KcsA reconstituted into asolectin lipid membranes, addition of phosphatidic acid (PA) drastically reduces inactivation in macroscopic current recordings. Consistent to this, PA increases current amplitude, mean open time and open probability at the single channel level. Moreover, kinetic analysis reveals that addition of PA causes longer open channel lifetimes and decreased closing rate constants. Effects akin to those of PA on WT-KcsA are observed when R64 and/or R89 are mutated to alanine, regardless of the added anionic lipids. We interpret these results as a consequence of interactions between the arginines and the anionic PA bound to the non-annular sites. NMR data shows indeed that at least R64 is involved in binding PA. Moreover, molecular dynamics (MD) simulations predict that R64, R89 and surrounding residues such as T61, mediate persistent binding of PA to the non-annular sites. Channel inactivation depends on interactions within the inactivation triad (E71-D80-W67) behind the selectivity filter. Therefore, it is expected that such interactions are affected when PA binds the arginines at the non-annular sites. In support of this, MD simulations reveal that PA binding prevents interaction between R89 and D80, which seems critical to the effectiveness of the inactivation triad. This mechanism depends on the stability of the bound lipid, favoring anionic headgroups such as that of PA, which thrive on the positive charge of the arginines.

Competing Lipid-Protein and Protein-Protein Interactions Determine Clustering and Gating Patterns in the Potassium Channel from Streptomyces lividans (KcsA)

There is increasing evidence to support the notion that membrane proteins, instead of being isolated components floating in a fluid lipid environment, can be assembled into supramolecular complexes that take part in a variety of cooperative cellular functions. The interplay between lipid-protein and protein-protein interactions is expected to be a determinant factor in the assembly and dynamics of such membrane complexes. Here we report on a role of anionic phospholipids in determining the extent of clustering of KcsA, a model potassium channel. Assembly/disassembly of channel clusters occurs, at least partly, as a consequence of competing lipid-protein and protein-protein interactions at nonannular lipid binding sites on the channel surface and brings about profound changes in the gating properties of the channel. Our results suggest that these latter effects of anionic lipids are mediated via the Trp(67)-Glu(71)-Asp(80) inactivation triad within the channel structure and its bearing on the selectivity filter.

Aggregatibacter actinomycetemcomitans leukotoxin cytotoxicity occurs through bilayer destabilization

The Gram-negative bacterium, Aggregatibacter actinomycetemcomitans, is a common inhabitant of the human upper aerodigestive tract. The organism produces an RTX (Repeats in ToXin) toxin (LtxA) that kills human white blood cells. LtxA is believed to be a membrane-damaging toxin, but details of the cell surface interaction for this and several other RTX toxins have yet to be elucidated. Initial morphological studies suggested that LtxA was bending the target cell membrane. Because the ability of a membrane to bend is a function of its lipid composition, we assessed the proficiency of LtxA to release of a fluorescent dye from a panel of liposomes composed of various lipids. Liposomes composed of lipids that form nonlamellar phases were susceptible to LtxA-induced damage while liposomes composed of lipids that do not form non-bilayer structures were not. Differential scanning calorimetry demonstrated that the toxin decreased the temperature at which the lipid transitions from a bilayer to a nonlamellar phase, while (31) P nuclear magnetic resonance studies showed that the LtxA-induced transition from a bilayer to an inverted hexagonal phase occurs through the formation of an isotropic intermediate phase. These results indicate that LtxA cytotoxicity occurs through a process of membrane destabilization.

Is gamma-actin a regulator of amino acid transport?

A mutated yeast cell line incapable of growth in minimal medium with proline as the sole nitrogen source was restored to normal growth by transfection with a cDNA from mouse Ehrlich cells. The cloned cDNA (E51) was found to be 90% homologous to gamma-actin. Immediately after transfection with E51 cDNA, both alpha-aminoisobutyric acid (AIB) and proline uptake in the mutated yeast were increased, particularly at pH 5. The expression of the same E51 cDNA also enhanced amino acid uptake in Xenopus laevis oocytes after injection into the Xenopus nuclei. A mutated mammalian lymphocyte cell line (GF-17), deficient in system A transport, also showed increased Na(+)-dependent transport after transfection with E51 cDNA. Whereas the mock transfected GF-17 cells failed to grow in the selection medium, the transfectants with E51 cDNA grew better than the untransfected cells. The data are consistent with the conclusion that expression of E51 cDNA can modify inactive, endogenous amino acid transporters, permitting substantial amino acid uptake in cells deficient in amino acid transporter(s) and permitting rapid cell growth. The data suggest that the gamma-actin-like protein coded for by E51 cDNA may play a significant regulatory role in amino acid transport.

Preparation of a protein-free total brain white matter lipid fraction: characterization of liposomes

A method of preparing a total lipid extract (TLE), free of protein, by extracting brain white matter with tetrahydrofuran is presented. The optimal conditions of extraction were found to be 50 ml of THF per gram of lyophilized tissue, though fresh tissue can also be used if larger volumes of solvent are employed. The method allowed, in a short time and in a single step, a yield of TLE of 50% on a dry weight basis. Its analytical characterization revealed a qualitative and quantitative composition very similar to the lipid composition of CNS myelin, including all the phospholipid and galactolipid species, cholesterol and gangliosides, but it contained only traces (0.1%) of protein. TLE has been used to prepare liposomes, either multilamellar (MLVs) or unilamellar (LUVs, SUVs), characterized by freeze-fracture electron microscopy. A multilayered, heterogeneous population of liposomes is observed in the MLVs preparation. When these samples were submitted to a freezing and thawing procedure the resulting liposomes were single-walled, and their intravesicular volume was increased. They were quite impermeable to the monovalent cation 86Rb+ and, by contrast, rather permeable to 45Ca+ +. Their complex lipid composition, together with their permeability properties and their response to ionophores, make them very useful to study protein-lipid interactions occurring within the myelin membrane as well as the functional properties of myelin proteins in reconstitution experiments.

L-glutamine transport in native vesicles isolated from Ehrlich ascites tumor cell membranes

Native vesicles isolated from Ehrlich ascites tumor cells accumulate glutamine by means of Na(+)-dependent transport systems; thiocyanate seems to be the more effective anion. The apparent affinity constant for the process was 0.38 mM. The Arrhenius plot gave an apparent activation energy of 12.3 kJ/mol. The structural analogs of glutamine, acivicin (2.5 mM) and azaserine (2.5 mM), inhibited the net uptake by 67 and 70%, respectively. The sulfhydryl reagents mersalyl, PCMBS, NEM, and DTNB also inhibited net uptake, suggesting that sulfhydryl groups may be involved in the activity of the carrier protein. A strong inhibition was detected when the vesicles were incubated in the presence of alanine, cysteine, or serine; in addition, histidine, but not glutamate or leucine, had a negative effect on glutamine transport.