Structural investigations of pneumolysin/lipid complexes

Incomplete pneumolysin oligomers form membrane pores

Pneumolysin is a member of the cholesterol-dependent cytolysin (CDC) family of pore-forming proteins that are produced as water-soluble monomers or dimers, bind to target membranes and oligomerize into large ring-shaped assemblies comprising approximately 40 subunits and approximately 30 nm across. This pre-pore assembly then refolds to punch a large hole in the lipid bilayer. However, in addition to forming large pores, pneumolysin and other CDCs form smaller lesions characterized by low electrical conductance. Owing to the observation of arc-like (rather than full-ring) oligomers by electron microscopy, it has been hypothesized that smaller oligomers explain smaller functional pores. To investigate whether this is the case, we performed cryo-electron tomography of pneumolysin oligomers on model lipid membranes. We then used sub-tomogram classification and averaging to determine representative membrane-bound low-resolution structures and identified pre-pores versus pores by the presence of membrane within the oligomeric curve. We found pre-pore and pore forms of both complete (ring) and incomplete (arc) oligomers and conclude that arc-shaped oligomeric assemblies of pneumolysin can form pores. As the CDCs are evolutionarily related to the membrane attack complex/perforin family of proteins, which also form variably sized pores, our findings are of relevance to that class of proteins as well.

Cholesterol-dependent cytolysins

The cholesterol-dependent cytolysins (CDCs) are part of a large family of pore-forming proteins that include the human proteins perforin and the complement membrane attack complex. The activity of all family members is focused on membranes, but the proteins are themselves involved in a diverse range of phenomena. An overview of some of these phenomena is provided here, along with an historical perspective of CDCs themselves and how our understanding of their mechanism of action has developed over the years. The way in which pore formation depends on specific characteristics of the membrane under attack as well as of the protein doing the attacking is emphasised. The cholesterol-dependent cytolysins (CDCs) have been the focus of a renewed keen research interest for over ten years now. Their importance has been even further enhanced by the homology now identified between them and the membrane attack complex/perforin (MACPF) family of proteins, which includes several components of the complement cascade as well as perforin itself. In this chapter I aim to provide an overview of our understanding of the interaction between CDCs and other members of what is now called the MACPF/CDC superfamily, with their target membranes. CDCs (also in the past known as thiol-activated toxins or cholesterol-binding toxins) were originally identified from four Gram-positive bacterial genera (Clostridium, Listeria, Bacillus and Streptococcus). Well-known examples include listeriolysin, perfringolysin, streptolysin and pneumoysin. Listeriolysin from L. monocytogenes is responsible for the escape of bacteria from the phagosome to colonise the cytoplasm and has been applied as a protein adjuvant in the development of vaccines against cancer and tuberculosis, for example. Perfringolysin from C. perfringens (Fig. 1A) has become perhaps the most studied CDC4 and has an important role in pathology associated with infection (gangrene). Streptolysin from S. pyogenes is another intensely studied CDC and has been applied widely in experimental permeabilisation of biological membranes. Pneumolysin is a major virulence determinant for S. pneumoniae, allowing bacterial invasion of tissues and mediating inflammation and the activation of the complement cascade. However, CDCs have now, for example, been identified in the bacteria Arcanobacterium pyogenes and Gardnerella vaginalis and there also appear to be homologues outside prokaryotes such as the sea anemone Metridium senile pore-forming toxin metridiolysin. The homology with the MACPF family was unknown until the first structures of the canonical fold of that family were solved, revealing the now characteristic MACPF/CDC fold of a twisted 3-sheet around which helices are clustered (Fig. 1A and D). Without any significant other sequence homology, the fold of this superfamily of pore-forming and membrane-binding proteins has been conserved by compensatory mutation around a handful of key conserved glycines. The glycines presumably act as critical hinges during the dramatic refolding that CDCs are known to undergo and which is presumably the selective advantage of this specific structure that has caused it to be conserved over such a vast evolutionary timescale. While not all MACPF domains are involved in pore formation-for example, C6 and C8beta--they are all apparently involved in action on membranes. The dramatic refolding undergone by CDCs is tightly coupled to their oligomerisation and results in the conversion of the helices hemming the core 3-sheet of the MACPF/CDC domain into a pair of beta-hairpins which in tandem and alongside those from other subunits within the oligomer insert into the membrane to create a pore (Fig. 1A-C). It is obviously the basic assumption that where nonCDC members of the superfamily-such as complement proteins and perforin-act on membranes they do so by a mechanism involving similar refolding.58 Even where a member of the MACPF/CDC superfamily is not known to form a pore, or has been shown not to-at least alone-the same conformational change could have other adaptive functions during activity on or at membranes. However, the bicomponent nature of some pore-forming toxins alerts us that showing an absence of activity for one pure protein does not mean that they do not contribute to pore formation quite directly, since that may require the presence of another MACPF/CDC family member or members from the same specific system. Complement acts by a combination of the C5b-8 complex of proteins preassembled on a target membrane recruiting C9 to form a lesion, which may be a complete ring of C9 associated with the C5b-8 or an arc-electron microscopy images show both possibilities.Perforin acts in concert with granzymes, to trigger apoptosis when delivered by cytotoxic cells at their targets (damaged, transformed and infected host cells). Incomplete rings are visible for perforin also and there are many unresolved issues concerning its mechanism and the dependence ofgranzymes on it for their delivery.

Rapid microtubule bundling and stabilization by the Streptococcus pneumoniae neurotoxin pneumolysin in a cholesterol-dependent, non-lytic and Src-kinase dependent manner inhibits intracellular trafficking

Streptococcus pneumoniae is the most frequent cause of bacterial meningitis, leading to permanent neurological damage in 30% and lethal outcome in 25% of patients. The cholesterol-dependent cytolysin pneumolysin is a major virulence factor of S. pneumoniae. It produces rapid cell lysis at higher concentrations or apoptosis at lower concentrations. Here, we show that sublytic amounts of pneumolysin produce rapid bundling and increased acetylation of microtubules (signs of excessive microtubule stabilization) in various types of cells--neuroblastoma cells, fibroblasts and primary astrocytes. The bundling started perinuclearly and extended peripherally towards the membrane. The effect was not connected to pneumolysin's capacity to mediate calcium influx, macropore formation, apoptosis, or RhoA and Rac1 activation. Cellular cholesterol depletion and neutralization of the toxin by pre-incubation with cholesterol completely inhibited the microtubule phenotype. Pharmacological inhibition of Src-family kinases diminished microtubule bundling, suggesting their involvement in the process. The relevance of microtubule stabilization to meningitis was confirmed in an experimental pneumococcal meningitis animal model, where increased acetylation was observed. Live imaging experiments demonstrated a decrease in organelle motility after toxin challenge in a manner comparable to the microtubule-stabilizing agent taxol, thus proposing a possible pathogenic mechanism that might contribute to the CNS damage in pneumococcal meningitis.

Inactivation and Activity of Cholesterol-Dependent Cytolysins: What Structural Studies Tell Us

The homologous bacterially expressed cholesterol-dependent cytolysins (CDCs) form pores via oligomerization; this must occur preferentially once the target membrane has been engaged. Conformational changes in CDCs then drive partition from an aqueous environment to a lipidic one. This review addresses how premature oligomerization is prevented, how conformational changes are triggered, and how cooperativity between subunits brings about new functionality absent from isolated protomers. Variations are found in the answers provided by the CDCs to these issues. Some toxins use pH as a trigger of activity, but recent results have shown that dimerization in solution is an alternative way of preventing premature oligomerization, in particular for the CDC from Clostridium perfringens, perfringolysin. More controversially, there is still no resolution to the debate as to whether incomplete (arciform) oligomers form pores: recent results again suggest that they do.

Effects of the eukaryotic pore-forming cytolysin Equinatoxin II on lipid membranes and the role of sphingomyelin

Equinatoxin II (EqtII), a protein toxin from the sea anemone Actinia equina, readily creates pores in sphingomyelin-containing lipid membranes. The perturbation by EqtII of model lipid membranes composed of dimyristoylphosphatidycholine and sphingomyelin (10 mol %) was investigated using wideline phosphorus-31 and deuterium NMR. The preferential interaction between EqtII (0.1 and 0.4 mol %) and the individual bilayer lipids was studied by (31)P magic angle spinning NMR, and toxin-induced changes in bilayer morphology were examined by freeze-fracture electron microscopy. Both NMR and EM showed the formation of an additional lipid phase in sphingomyelin-containing mixed lipid multilamellar suspensions with 0.4 mol % EqtII. The new toxin-induced phase consisted of small unilamellar vesicles 20-40 nm in diameter. Deuterium NMR showed that the new lipid phase contains both dimyristoylphosphatidycholine and sphingomyelin. Solid-state (31)P NMR showed an increase in spin-lattice and a decrease in spin-spin relaxation times in mixed-lipid model membranes in the presence of EqtII, consistent with an increase in the intensity of low frequency motions. The (2)H and (31)P spectral intensity distributions confirmed a change in lipid mobility and showed the creation of an isotropic lipid phase, which was identified as the small vesicle structures visible by electron microscopy in the EqtII-lipid suspensions. The toxin appears to enhance slow motions in the membrane lipids and destabilize the membrane. This effect was greatly enhanced in sphingomyelin-containing mixed lipid membranes compared with pure phosphatidylcholine bilayers, suggesting a preferential interaction between the toxin and bilayer sphingomyelin.