Proton motive force involved in protein transport across the outer membrane of Aeromonas salmonicida.

Many Gram-negative bacteria export proteins to the exterior. Some of these proteins are first secreted into the periplasm and then cross the outer membrane in a separate step. The source of energy required for the translocation is unknown. Export of the extracellular protein proaerolysin from the periplasm through the outer membrane of Aeromonas salmonicida is inhibited by a proton ionophore and by low extracellular pH. One possible explanation of these results is that a proton gradient across the outer membrane is required for export.

Properties of human erythrocyte phosphatidylinositol kinase and inhibition by adenosine, ADP and related compounds.

1. An improved assay for the enzyme phosphatidylinositol kinase is described. The kinase activity is increased more than 10-fold by the addition of mercaptoethanol and exogenous phosphatidylinositol in the presence of Triton X-100. The enzyme is solubilized by non-ionic detergents. 2. Phosphatidylinositol kinase is inhibited by physiological concentrations of ADP and by similar levels of adenosine. Cyclic AMP, AMP and theophylline inhibit at higher concentrations. Caffeine is much less effective than theophylline as an inhibitor. Guanine nucleotides do not appreciably inhibit the kinase. All the inhibitors tested seemed to compete with ATP. Theophylline also reduced the rate of polyphosphoinositide synthesis in intact cells. 3. Possible roles of polyphosphoinositides in energy charge maintenance and inversion and resealing are discussed.

Molecular cloning of a phospholipid-cholesterol acyltransferase from Aeromonas hydrophila. Sequence homologies with lecithin-cholesterol acyltransferase and other lipases.

We have determined the nucleotide sequence of a gene encoding Aeromonas hydrophila phospholipid-cholesterol acyltransferase, an enzyme which shares many properties with mammalian lecithin:cholesterol acyltransferase. The derived amino acid sequence of the protein contains two regions which are homologous to the proposed active sites and binding sites of the plasma acyltransferase and to similar sequences in other interfacially acting lipolytic enzymes. The amino terminus is preceded by a typical 18 amino acid signal sequence. The protein, which is released into the culture supernatant by Aeromonas hydrophila, is confined to the periplasm of Escherichia coli.

Crystallization of a proform of aerolysin, a hole-forming toxin from Aeromonas hydrophila.

Crystals of proaerolysin, the precursor of the hole-forming toxin from Aeromonas hydrophila, have been obtained. The mature form of this protein binds to a receptor on mammalian cells, aggregates and forms 30 A holes in the membrane. The crystals are tetragonal, space group P4(3)2(1)2, a = b = 104.00 A, c = 222.0 A. They contain a dimer in the asymmetric unit and diffract to a resolution of 2.6 A.

Crossing three membranes. Channel formation by aerolysin.

Aerolysin is a channel-forming toxin responsible for the pathogenicity of Aeromonas hydrophila. It crosses the inner and outer membranes of the bacteria in separate steps and is released as a 52-kDa inactive protoxin which is activated by proteolytic removal of approximately 40 amino acids from the C terminus. The toxin binds to the erythrocyte transmembrane protein glycophorin and oligomerizes before inserting into the membrane, producing a voltage gated, anion selective channel about 1 nm in diameter. Remarkably, proaerolysin appears to be dimeric, whereas the oligomer is a heptamer. Using chemical modification and site-directed mutagenesis, we have identified some of the regions of the molecule which appear to be involved in secretion and in channel formation.

Physical and chemical characterization of the oligomerization state of the Aeromonas hydrophila lipase/acyltransferase.

Aeromonas glycerophospholipid:cholesterol acyl transferase undergoes a conformational transition upon activation by treatment with trypsin. Chemical cross-linking and sedimentation velocity analysis showed that the lipase dimerizes due to removal of a region near its C-terminus. The lipase monomer has a sedimentation coefficient s20.w = 2.83 S, whereas the dimer has s20.w = 3.65 +/- 0.22 S. Hydrodynamic analysis using these sedimentation values and the masses determined by mass spectrometry indicated that the monomers are aligned side-by-side in the dimer. An important change occurs in the apparent partial specific volume of the molecule upon activation.

Characterisation of the heptameric pore-forming complex of the Aeromonas toxin aerolysin using MALDI-TOF mass spectrometry.

Aerolysin, a virulence factor secreted by Aeromonas hydrophila, is representative of a group of beta-sheet toxins that must form stable homooligomers in order to be able to insert into biological membranes and generate channels. Electron microscopy and image analysis of two-dimensional membrane crystals had previously revealed a structure with 7-fold symmetry suggesting that aerolysin forms heptameric oligomers [Wilmsen et al. (1992) EMBO J. 11, 2457-2463]. However, this unusual molecularity of the channel remained to be confirmed by an independent method since low-resolution electron crystallography had led to artefactual data for other pore-forming toxins. In this study, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was used to measure the mass of the aerolysin oligomer preparation. A mass of 333 850 Da was measured, fitting very well with a heptameric complex (expected mass: 332 300 Da). These results confirm the earlier evidence that the aerolysin oligomer is a heptamer and also show that MALDI-TOF mass spectrometry could be a valuable tool to study non-covalent association of proteins.

A new family of lipolytic plant enzymes with members in rice, arabidopsis and maize.

We have noted a striking similarity between the sequences of proteins in a novel family of lipases we recently reported [Upton, C. and Buckley, J. T. (1995) Trends Biol. Sci. 20, 178-9] and more than 120 sequences from the database of Expressed Sequence Tags (dbEST) which correspond to at least 30 unique genes from arabidopsis, rice and maize. A cDNA (Arab-1) corresponding to one of these sequences was isolated, sequenced and translated. There was significant similarity to sequences in the new lipase family over the entire open reading frame of Arab-1 and when expressed in E. coli, the gene product was lipolytic. Arab-1 and genes for some of the other plant proteins appear to be differentially expressed. They may play a role in the regulation of lipid metabolism during plant development.

Improved detection and characterization of paroxysmal nocturnal hemoglobinuria using fluorescent aerolysin.

Paroxysmal nocturnal hemoglobinuria (PNH) is caused by a somatic mutation in the gene PIGA, which encodes an enzyme essential for the synthesis of glycosylphosphatidylinositol (GPI) anchors. The PIGA mutation results in absence or marked deficiency of more than a dozen proteins on PNH blood cells. Current flow cytometric assays for PNH rely on the use of labeled antibodies to detect deficiencies of specific GPI anchor proteins, such as CD59. However, because no single GPI anchor protein is always expressed in all cell lineages, no one monoclonal antibody can be used with confidence to diagnose PNH. We describe a new diagnostic test for PNH, based on the ability of a fluorescently labeled inactive variant of the protein aerolysin (FLAER) to bind selectively to GPI anchors. We compared GPI anchor protein expression in 8 patients with PNH using FLAER and anti-CD59. In all cases, FLAER detected similar or higher proportions of PNH monocytes and granulocytes compared with anti-CD59. Because of the increased sensitivity of detection, FLAER could detect small abnormal granulocyte populations in patients to a level of about 0.5%; samples from healthy control subjects contained substantially fewer FLAER-negative cells. FLAER gives a more accurate assessment of the GPI anchor deficit in PNH.

The use of aerolysin toxin as an aid for visualization of low numbers of African trypanosomes in whole blood.

A method was developed for detection of low numbers of African trypanosomes in whole blood. The method is based on selective lysis of erythrocytes and leukocytes by aerolysin, a toxin produced by the bacterium Aeromonas hydrophila. African trypanosomes do not bind the toxin and their viability and motility are therefore unaffected by this treatment.

The cytolytic toxin aerolysin: from the soluble form to the transmembrane channel.

Aerolysin is a cytolytic toxin which forms channels in the plasma membranes of eucaryotic cells. The protein is secreted by Aeromonas hydrophila as an inactive protoxin. Its stability and water solubility are conferred by its ability to dimerize. Maturation of the protein occurs through proteolytic removal of a C-terminal peptide outside the secreting cell. Although the aerolysin which is so produced is still a dimer, it then has the ability to oligomerize. The oligomer is the active form of the toxin, capable of forming the transmembrane channels that disrupt cells. We review here the present knowledge about the structure of aerolysin in relation to the various steps in channel formation.

Secretion and mechanism of action of the hole-forming toxin aerolysin.

Aeromonas hydrophila exports aerolysin as a protoxin which is activated by proteolysis after release. Aerolysin binds to the eucaryotic cell receptor glycophorin and oligomerizes, forming holes in the membrane. Important regions of the molecule have been identified by site-directed mutagenesis, and channel formation has been studied in planar lipid bilayers.

Coisolation of glycophorin A and polyphosphoinositides from human erythrocyte membranes.

The lipid composition of purified erythrocyte membrane glycophorin was measured. Diphosphoinositide, triphosphoinositide, and phosphatidylserine are the major phospholipids in glycophorin preparation. Nearly all of the radioactive diphosphoinositide and triphosphoinositide extracted from erythrocyte membranes by lithium dïodosalicylate are recoverd in purified glycophorin. There appeared to be no significant enrichment of other acidic membrane phospholipids in the protein. The results do not permit a firm conclusion as to whether the polyphosphoinositides are associated specifically with the membrane protein or whether fortuitous binding has occurred during purification.

Synthesis of diphosphoinositide by a supernatant fraction from Acanthamoeba castellanii.

Homogenates of the free-living amoeba Acanthamoeba castellanii incorporate phosphate from [gamma-32P]ATP into a lipid which co-chromatographs with diphosphoinositide on one- and two dimensional chromatography. Incorporation into lipids similar in mobility to triphosphoinositide is not detected. The product co-chromatographs with diphosphoinositide whether exogenous phosphatidylinositol or total amoeba lipid is the substrate. The inositide kinase is almost entirely located in the supernatant fraction after centrifugation at 100 000 g. Incorporation of phosphate from [gamma-32P]ATP is linear for at least 15 min in the presence of 0.5 mM phosphatidylinositol. The enzyme requires Mg2+ of Mn2+ as well as ATP and it is not affected by low concentrations of Ca2+. The apparent Km for phosphatidylinositol in 2 mM. Both ADP and cAMP inhibit the reaction.

Calcium ion transport by pig erythrocyte membrane vesicles.

Preincubating pig erythrocyte membranes with ATP enhances their ability to accumulate Ca(2+) against a concentration gradient. The extent of this increase is dependent on preincubation time over the period 0-60min. As the accessibility of outside membrane markers is decreased by preincubation and as accumulated Ca(2+) is not removed by EGTA [ethanedioxybis(ethylamine)tetra-acetate], it is suggested that ATP causes the formation of sealed inside-out vesicles which can transport Ca(2+) inward. The transport system requires ATP and Mg(2+) and exhibits an apparent dissociation constant for Ca(2+) of approx. 100mum. Since the dissociation constant for Ca(2+)-sensitive ATPase (adenosine triphosphatase) in these preparations is similar, it is concluded that this ATPase is responsible for Ca(2+) transport. Polyphosphoinositide concentrations are also increased during incubation with ATP; however, there is no change in their rate of synthesis or breakdown during Ca(2+) transport.

Cone-shaped lipids increase the susceptibility of phospholipids in bilayers to the action of phospholipases.

Addition of cardiolipin or diacylglycerol to dispersions of phosphatidylcholine greatly increased hydrolysis by snake venom or pancreatic phospholipase A2, as well as by a microbial phospholipase. Monogalactosyl diglyceride which, like cardiolipin and diacylglycerol, will form nonbilayer hexagonal II structures also caused an increase in the breakdown of phosphatidylcholine. Addition of digalactosyl diglyceride, a bilayer lipid from the same source, had a much smaller effect on the three phospholipases, indicating that stimulation by the nonbilayer lipids was not due to their fatty acid compositions. Stimulation of the microbial phospholipase by cardiolipin did not require the presence of calcium, leading to the conclusion that the formation of nonbilayer structures was not necessary. The results suggest that cone-shaped lipids increase the accessibility of lipids in bilayers to phospholipases by decreasing the packing of the polar head groups.

Substrate specificity of bacterial glycerophospholipid:cholesterol acyltransferase.

The substrate specificity of a bacterial analogue of the plasma enzyme lecithin:cholesterol acyltransferase (LCAT) has been examined with small unilamellar liposomes and Triton mixed micelles. In contrast to LCAT, the microbial enzyme is capable of using all of the naturally occurring phospholipids as acyl donors. In general reaction rate depends more on the length or degree of unsaturation of the acyl chains than on the nature of the phospholipid head group. Among a series of disaturated phosphatidylcholines in liposomes, dilauroylphosphatidylcholine is the preferred acyl donor. Like LCAT, the enzyme will catalyze acyl transfer by using other alcohols in addition to cholesterol. Of saturated straight chain primary alcohols 1-decanol is the preferred acyl acceptor. Cholesterol, however, is a far better acceptor than any non-sterol alcohol tested. Other steroids with equatorial hydroxyls at position C-3 and trans-fused A:B rings will also act as acceptors whereas those steroids with axial hydroxyls at C-3 or cis-fused rings are inhibitors of acyl transfer. The ability of steroids to act as acyl acceptors may be due to the nature of their interaction with the phospholipid acyl donor.