Listeriolysin O (LLO) is the most important virulence factor of the intracellular pathogen Listeria monocytogenes. Its main task is to enable escape of bacteria from the phagosomal vacuole into the cytoplasm. LLO belongs to the cholesterol-dependent cytolysin (CDC) family but differs from other members, as it exhibits optimal activity at low pH. Its pore forming ability at higher pH values has been largely disregarded in Listeria pathogenesis. Here we show that high cholesterol concentrations in the membrane restore the low activity of LLO at high pH values. LLO binds to lipid membranes, at physiological or even slightly basic pH values, in a cholesterol-dependent fashion. Binding, insertion into lipid monolayers, and permeabilization of calcein-loaded liposomes are maximal above approximately 35 mol % cholesterol, a concentration range typically found in lipid rafts. The narrow transition region of cholesterol concentration separating low and high activity indicates that cholesterol not only allows the binding of LLO to membranes but also affects other steps in pore formation. We were able to detect some of these by surface plasmon resonance-based assays. In particular, we show that LLO recognition of cholesterol is determined by the most exposed 3 -hydroxy group of cholesterol. In addition, LLO binds and permeabilizes J774 cells and human erythrocytes in a cholesterol-dependent fashion at physiological or slightly basic pH values. The results clearly show that LLO activity at physiological pH cannot be neglected and that its action at sites distal to cell entry may have important physiological consequences for Listeria pathogenesis.

Pore formation of cellular membranes is an ancient mechanism of bacterial pathogenesis that allows efficient damaging of target cells. Several mechanisms have been described, however, relatively little is known about the assembly and properties of pores. Listeriolysin O (LLO) is a pH-regulated cholesterol-dependent cytolysin from the intracellular pathogen Listeria monocytogenes, which forms transmembrane β-barrel pores. Here we report that the assembly of LLO pores is rapid and efficient irrespective of pH. While pore diameters at the membrane surface are comparable at either pH 5.5 or 7.4, the distribution of pore conductances is significantly pH-dependent. This is directed by the unique residue H311, which is also important for the conformational stability of the LLO monomer and the rate of pore formation. The functional pores exhibit variations in height profiles and can reconfigure significantly by merging to other full pores or arcs. Our results indicate significant plasticity of large β-barrel pores, controlled by environmental cues like pH.

Listeriolysin O (LLO) is the major factor implicated in the escape of Listeria monocytogenes from the phagolysosome. It is the only representative of cholesterol-dependent cytolysins that exhibits pH-dependent activity. Despite intense studies of LLO pH-dependence, this feature of the toxin still remains incompletely explained. Here we used fluorescence and CD spectroscopy to show that the structure of LLO is not detectably affected by pH at room temperature. We observed slightly altered haemolytic and permeabilizing activities at different pH values, which we relate to reduced binding of LLO to the lipid membranes. However, alkaline pH and elevated temperatures caused rapid denaturation of LLO. Aggregates of the toxin were able to bind Congo red and Thioflavin T dyes and were visible under transmission electron microscopy as large, amorphous, micrometersized assemblies. The aggregates had the biophysical properties of amyloid. Analytical ultracentrifugation indicated dimerization of the protein in acidic conditions, which protects the protein against premature denaturation in the phagolysosome, where toxin activity takes place. We therefore suggest that LLO spontaneously aggregates at the neutral pH found in the host cell cytosol and that this is a major mechanism of LLO inactivation. Structured digital abstractl LLO and LLO bind by electron microscopy (View interaction) l LLO and LLO bind by cosedimentation in solution (View interaction) l LLO and LLO bind by fluorescence technology (View interaction) l LLO and LLO bind by light scattering (View interaction)

The kinetics of cholesterol extraction from cellular membranes is complex and not yet completely understood. In this paper we have developed an experimental approach to directly monitor the extraction of cholesterol from lipid membranes by using surface plasmon resonance and model lipid systems. Methyl-beta-cyclodextrin was used to selectively remove cholesterol from large unilamellar vesicles of various compositions. The amount of extracted cholesterol is highly dependent on the composition of lipid membrane, i.e. the presence of sphingomyelin drastically reduced and slowed down cholesterol extraction by methyl-beta-cyclodextrin. This was confirmed also in the erythrocyte ghosts system, where more cholesterol was extracted after erythrocytes were treated with sphingomyelinase. We further show that the kinetics of the extraction is mono-exponential for mixtures of 1,2-dioleoyl-sn-glycero-3-phosphocholine and cholesterol. The kinetics is complex for ternary lipid mixtures composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine, bovine brain sphingomyelin and cholesterol. Our results indicate that the complex kinetics observed in experiments with cells may be the consequence of lateral segregation of lipids in cell plasma membrane.

To contribute to the question of the putative role of cystatins in Alzheimer disease and in neuroprotection in general, we studied the interaction between human stefin B (cystatin B) and amyloid-␤-(1-40) peptide (A␤). Using surface plasmon resonance and electrospray mass spectrometry we were able to show a direct interaction between the two proteins. As an interesting new fact, we show that stefin B binding to A␤ is oligomer specific. The dimers and tetramers of stefin B, which bind A␤, are domain-swapped as judged from structural studies. Consistent with the binding results, the same oligomers of stefin B inhibit A␤ fibril formation. When expressed in cultured cells, stefin B co-localizes with A␤ intracellular inclusions. It also co-immunoprecipitates with the APP fragment containing the A␤ epitope. Thus, stefin B is another APP/A␤-binding protein in vitro and likely in cells.Neurodegenerative diseases present a huge burden in the developed world's aging population. They are all in one way or another connected to aberrant protein folding and aggregation of the proteins involved (1). Various protein conformational disorders of the central and peripheral nervous system are known, which often appear sporadically but also run in families. These are among others: Parkinson and Alzheimer diseases, dementia with Lewy bodies, vascular and fronto-temporal dementia, and amyotrophic lateral sclerosis.The A␤ peptide implicated in Alzheimer disease pathology is a cleavage product of the membrane A␤ precursor protein (APP).3 It is the main constituent of extracellular amyloid plaques, however, together with its oligomers, it also resides intracellularly (2). It has been shown that A␤ oligomers prepared in vitro and those extracted from living cells exert cytotoxicity and cause symptoms of reversible memory loss in animal models (3). Amyloid protein oligomers have special structural properties, which are reflected in a common antioligomer antibody (4). This antibody not only binds the oligomers against which it was raised but also binds chaperones and some other proteins involved in disaggregating protein aggregates in cells (5). A␤-binding proteins, the so called "amateur chaperones," were suggested to have a potential in Alzheimer disease therapy (6, 7).It has been shown before that human cystatin C is an A␤-binding protein (8). Cystatins are single chain proteins that inhibit cysteine cathepsins (9). Human stefin B (also known as cystatin B) is a member of subfamily A of cystatins, classified as family I25 in the MEROPS scheme (10). Stefin B, a protein of 98 amino acid residues and 1 Cys, is predominantly intracellular, whereas cystatin C, a protein of 120 residues and 2 disulfide bonds, is a secretory protein. Three-dimensional structures of stefins and cystatin C have been determined, among others, the solution structure of stefin A (11) and cystatin C (12, 13).Human cystatin C has been found as a constituent of senile plaques of Alzheimer disease patients (14) and stefins A and B have also been reported to localize to a...

To explore the variability in biosensor studies, 150 participants from 20 countries were given the same protein samples and asked to determine kinetic rate constants for the interaction. We chose a protein system that was amenable to analysis using different biosensor platforms as well as by users of different expertise levels. The two proteins (a 50-kDa Fab and a 60-kDa glutathione S-transferase [GST] antigen) form a relatively high-affinity complex, so participants needed to optimize several experimental parameters, including ligand immobilization and regeneration conditions as well as analyte concentrations and injection/dissociation times. Although most participants collected binding responses that could be fit to yield kinetic parameters, the quality of a few data sets could have been improved by optimizing the assay design. Once these outliers were removed, the average reported affinity across the remaining panel of participants was 620 pM with a standard deviation of 980 pM. These results demonstrate that when this biosensor assay was designed and executed appropriately, the reported rate constants were consistent, and independent of which protein was immobilized and which biosensor was used.

Mitochondria are double-membrane-bound organelles that constantly change shape through membrane fusion and fission. Outer mitochondrial membrane fusion is controlled by Mitofusin, whose molecular architecture consists of an N-terminal GTPase domain, a first heptad repeat domain (HR1), two transmembrane domains, and a second heptad repeat domain (HR2). The mode of action of Mitofusin and the specific roles played by each of these functional domains in mitochondrial fusion are not fully understood. Here, using a combination of and fusion assays, we show that HR1 induces membrane fusion and possesses a conserved amphipathic helix that folds upon interaction with the lipid bilayer surface. Our results strongly suggest that HR1 facilitates membrane fusion by destabilizing the lipid bilayer structure, notably in membrane regions presenting lipid packing defects. This mechanism for fusion is thus distinct from that described for the heptad repeat domains of SNARE and viral proteins, which assemble as membrane-bridging complexes, triggering close membrane apposition and fusion, and is more closely related to that of the C-terminal amphipathic tail of the Atlastin protein.

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