Pore-forming toxins (PFT) are water-soluble proteins that possess the remarkable ability to self-assemble on the membrane of target cells, where they form pores causing cell damage. Here, we elucidate the mechanism of action of the haemolytic protein fragaceatoxin C (FraC), a α-barrel PFT, by determining the crystal structures of FraC at four different stages of the lytic mechanism, namely the water-soluble state, the monomeric lipid-bound form, an assembly intermediate and the fully assembled transmembrane pore. The structure of the transmembrane pore exhibits a unique architecture composed of both protein and lipids, with some of the lipids lining the pore wall, acting as assembly cofactors. The pore also exhibits lateral fenestrations that expose the hydrophobic core of the membrane to the aqueous environment. The incorporation of lipids from the target membrane within the structure of the pore provides a membrane-specific trigger for the activation of a haemolytic toxin.

Cell-surface Fcγ receptors mediate innate and adaptive immune responses. Human Fcγ receptor I (hFcγRI) binds IgGs with high affinity and is the only Fcγ receptor that can effectively capture monomeric IgGs. However, the molecular basis of hFcγRI's interaction with Fc has not been determined, limiting our understanding of this major immune receptor. Here we report the crystal structure of a complex between hFcγRI and human Fc, at 1.80 Å resolution, revealing an unique hydrophobic pocket at the surface of hFcγRI perfectly suited for residue Leu235 of Fc, which explains the high affinity of this complex. Structural, kinetic and thermodynamic data demonstrate that the binding mechanism is governed by a combination of non-covalent interactions, bridging water molecules and the dynamic features of Fc. In addition, the hinge region of hFcγRI-bound Fc adopts a straight conformation, potentially orienting the Fab moiety. These findings will stimulate the development of novel therapeutic strategies involving hFcγRI.

Equinatoxin-II is a eukaryotic pore-forming toxin belonging to the family of actinoporins. Its interaction with model membranes is largely modulated by the presence of sphingomyelin. We have used large unilamellar vesicles and lipid monolayers to gain further information about this interaction. The coexistence of gel and liquid-crystal lipid phases in sphingomyelin/phosphatidylcholine mixtures and the coexistence of liquid-ordered and liquiddisordered lipid phases in phosphatidylcholine/cholesterol or sphingomyelin/phosphatidylcholine/cholesterol mixtures favor membrane insertion of equinatoxin-II. Phosphatidylcholine vesicles are not permeabilized by equinatoxin-II. However, the localized accumulation of phospholipase C-generated diacylglycerol creates conditions for toxin activity. By using epifluorescence microscopy of transferred monolayers, it seems that lipid packing defects arising at the interfaces between coexisting lipid phases may function as preferential binding sites for the toxin. The possible implications of such a mechanism in the assembly of a toroidal pore are discussed.Equinatoxin II (Eqt-II) 1 is a member of the actinoporins, a group of sea anemone cytolysins (1). It is a 179-amino acid residue protein with a molecular mass of 19.8 kDa and an isoelectric point of 10.5 (2). Its three-dimensional structure has been solved by x-ray crystallography and NMR (3, 4). Eqt-II forms cation-selective pores with a diameter of ϳ2 nm in cell and model membranes (5-7). The mechanism of pore formation is a multistep process consisting of (i) membrane binding of the water-soluble monomer, (ii) oligomerization on the membrane surface, and (iii) pore formation (1,(5)(6)(7)(8)(9)(10)(11). This mechanism is common to other actinoporins like sticholysin-II from Stichodactyla helianthus (12, 13). Membrane insertion of Eqt-II and sticholysins is favored by the presence of sphingomyelin within the target membrane (6, 8, 14 -16). The recent finding of a phosphocholine binding site in the three-dimensional structure of sticholysin-II (13) supports the role of sphingomyelin as a specific receptor for actinoporins, as other authors have suggested (17, 18). However, the presence of sphingomyelin is not strictly necessary for the lytic activity of these toxins, which are also active in phosphatidylcholine/cholesterol mixtures (14, 16). Therefore, other factors are likely to govern their mechanism of action.Mixtures of sphingomyelin, phosphatidylcholine, and cholesterol are characteristic of the so-called rafts, microdomains in which the concentration of membrane components (lipids or proteins) and their physicochemical properties are different from the surrounding environment. The increasing amount of information pointing to the existence of lipid domains in cell and model membranes and their implication in many crucial biological processes has been extensively reviewed (19 -26). One important characteristic of rafts is their resistance to detergent solubilization (27)(28)(29)(30). This property is associated with the fact...

Enzymes are classically proposed to accelerate reactions by binding substrates within active site environments that are structurally preorganized to optimize binding interactions with reaction transition states rather than ground states. This is a remarkably formidable task considering the limited 0.1 -1 Å scale of most substrate rearrangements. The flexibility of active site functional groups along the coordinate of substrate rearrangement, the distance scale on which enzymes can distinguish structural rearrangement, and the energetic significance of discrimination on that scale remain open questions that are fundamental to a basic physical understanding of enzyme active sites and catalysis. We bring together high resolution X-ray crystallography, 1 H and 19 F NMR spectroscopy, quantum mechanical calculations, and transition state analog binding measurements to test the distance scale on which non-covalent forces can constrain side chain and ligand relaxation or translation along a specific coordinate and the energetic consequences of such geometric constraints within the active site of bacterial ketosteroid isomerase (KSI). Our results strongly suggest that packing and binding interactions within the KSI active site can constrain local side chain reorientation and prevent hydrogen bond shortening by 0.1 Å or less. Further, this constraint has substantial energetic effects on ligand binding and stabilization of negative charge within the oxyanion hole. These results provide evidence that subtle geometric effects, indistinguishable in most X-ray crystallographic structures, can have significant energetic consequences and highlight the importance of using synergistic experimental approaches to dissect enzyme function.