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Activating the intrinsic apoptosis pathway with small molecules is now a clinically validated approach to cancer therapy. In contrast, blocking apoptosis to prevent the death of healthy cells in disease settings has not been achieved. Caspases have been favored, but they act too late in apoptosis to provide long-term protection. The critical step in committing a cell to death is activation of BAK or BAX, pro-death BCL-2 proteins mediating mitochondrial damage. Apoptosis cannot proceed in their absence. Here we show that WEHI-9625, a novel tricyclic sulfone small molecule, binds to VDAC2 and promotes its ability to inhibit apoptosis driven by mouse BAK. In contrast to caspase inhibitors, WEHI-9625 blocks apoptosis before mitochondrial damage, preserving cellular function and long-term clonogenic potential. Our findings expand on the key role of VDAC2 in regulating apoptosis and demonstrate that blocking apoptosis at an early stage is both advantageous and pharmacologically tractable.

BAK and BAX are essential mediators of apoptosis that oligomerize in response to death cues, thereby causing permeabilization of the mitochondrial outer membrane. Their transition from quiescent monomers to pore-forming oligomers involves a well-characterized symmetric dimer intermediate. However, no essential secondary interface that can be disrupted by mutagenesis has been identified. Here we describe crystal structures of human BAK core domain (α2–α5) dimers that reveal preferred binding sites for membrane lipids and detergents. The phospholipid headgroup and one acyl chain (sn2) associate with one core dimer while the other acyl chain (sn1) associates with a neighboring core dimer, suggesting a mechanism by which lipids contribute to the oligomerization of BAK. Our data support a model in which, unlike for other pore-forming proteins whose monomers assemble into oligomers primarily through protein–protein interfaces, the membrane itself plays a role in BAK and BAX oligomerization. Crystal structures of BAK core domain dimers suggest a mechanism by which lipids contribute to the oligomerization of BAK, which is essential for BAK-mediated permeabilization of the mitochondrial outer membrane.

Epstein–Barr virus (EBV), which is ubiquitous in the adult population, is causally associated with human malignancies. Like many infectious agents, EBV has evolved strategies to block host cell death, including through expression of viral homologues of cellular BCL-2 pro-survival proteins (vBCL-2s), such as BHRF1. Small molecule inhibitors of the cellular pro-survival BCL-2 family proteins, termed ‘BH3-mimetics’, have entered clinical trials for blood cancers with the BCL-2 inhibitor venetoclax already approved for treatment of therapy refractory chronic lymphocytic leukaemia and acute myeloid leukaemia in the elderly. The generation of BH3-mimetics that could specifically target vBCL-2 proteins may be an attractive therapeutic option for virus-associated cancers, since these drugs would be expected to only kill virally infected cells with only minimal side effects on normal healthy tissues. To achieve this, a better understanding of the contribution of vBCL-2 proteins to tumorigenesis and insights into their biochemical functions is needed. In the context of Burkitt lymphoma (BL), BHRF1 expression conferred strong resistance to diverse apoptotic stimuli. Furthermore, BHRF1 expression in mouse haematopoietic stem and progenitor cells accelerated MYC-induced lymphoma development in a model of BL. BHRF1 interacts with the cellular pro-apoptotic BCL-2 proteins, BIM, BID, PUMA and BAK, but its capability to inhibit apoptosis could not be mapped solely to one of these interactions, suggesting plasticity is a key feature of BHRF1. Site-directed mutagenesis revealed a site in BHRF1 that was critical for its interaction with PUMA and blocking DNA-damage-induced apoptosis, identifying a potentially therapeutically targetable vulnerability in BHRF1.

A body of data supports the existence of core (α2-α5) dimers of BAK and BAX in the oligomeric, membrane-perturbing conformation of these essential apoptotic effector molecules. Molecular structures for these dimers have only been captured for truncated constructs encompassing the core domain alone. Here, we report a crystal structure of BAK α2-α8 dimers (i.e., minus its flexible N-terminal helix and membrane-anchoring C-terminal segment) that has been obtained through the activation of monomeric BAK with the detergent C12E8. Core dimers are evident, linked through the crystal by contacts via latch (α6-α8) domains. This crystal structure shows activated BAK dimers with the extended latch domain present. Our data provide direct evidence for the conformational change converting BAK from inert monomer to the functional dimer that destroys mitochondrial integrity. This dimer is the smallest functional unit for recombinant BAK or BAX described so far.

BAX and BAK are essential mediators of intrinsic apoptosis that permeabilize the mitochondrial outer membrane. BAX activation requires its translocation from cytosol to mitochondria where conformational changes cause its oligomerization. To better understand the critical step of translocation, we examined its blockade by mutation near the C terminus (P168G) or by antibody binding near the N terminus. Similarities in the crystal structures of wild-type and BAX P168G but significant other differences suggest that cytosolic BAX exists as an ensemble of conformers, and that the distribution of conformers within the ensemble determines the different functions of wild-type and mutant proteins. We also describe the structure of BAX in complex with an antibody, 3C10, that inhibits cytosolic BAX by limiting exposure of the membrane-associating helix α9, as does the P168G mutation. Our data for both means of BAX inhibition argue for an allosteric model of BAX regulation that derives from properties of the ensemble of conformers.

Apoptotic stimuli activate and oligomerize the proapoptotic proteins Bak and Bax, resulting in mitochondrial outer-membrane permeabilization and subsequent cell death. This activation can occur when certain BH3-only proteins interact directly with Bak and Bax. Recently published crystal structures reveal that Bax separates into core and latch domains in response to BH3 peptides. The distinguishing characteristics of BH3 peptides capable of directly activating Bax were also elucidated. Here we identify specific BH3 peptides capable of "unlatching" Bak and describe structural insights into Bak activation and oligomerization. Crystal structures and crosslinking experiments demonstrate that Bak undergoes a conformational change similar to that of Bax upon activation. A structure of the Bak core domain dimer provides a high-resolution image of this key intermediate in the pore-forming oligomer. Our results confirm an analogous mechanism for activation and dimerization of Bak and Bax in response to certain BH3 peptides.

Developing potent molecules that inhibit Bcl-2 family mediated apoptosis affords opportunities to treat cancers via reactivation of the cell death machinery. We describe the hit-to-lead development of selective Bcl-XL inhibitors originating from a high-throughput screening campaign. Small structural changes to the hit compound increased binding affinity more than 300-fold (to IC50 < 20 nM). This molecular series exhibits drug-like characteristics, low molecular weights (Mw < 450), and unprecedented selectivity for Bcl-XL. Surface plasmon resonance experiments afford strong evidence of binding affinity within the hydrophobic groove of Bcl-XL. Biological experiments using engineered Mcl-1 deficient mouse embryonic fibroblasts (MEFs, reliant only on Bcl-XL for survival) and Bax/Bak deficient MEFs (insensitive to selective activation of Bcl-2-driven apoptosis) support a mechanism-based induction of apoptosis. This manuscript describes the first series of selective small-molecule inhibitors of Bcl-XL and provides promising leads for the development of efficacious therapeutics against solid tumors and chemoresistant cancer cell lines.

SummaryThe prosurvival and proapoptotic proteins of the BCL-2 family share a similar three-dimensional fold despite their opposing functions. However, many biochemical studies highlight the requirement for conformational changes for the functioning of both types of proteins, although structural data to support such changes remain elusive. Here, we describe the X-ray structure of dimeric BCL-W that reveals a major conformational change involving helices α3 and α4 hinging away from the core of the protein. Biochemical and functional studies reveal that the α4-α5 hinge region is required for dimerization of BCL-W, and functioning of both pro- and antiapoptotic BCL-2 proteins. Hence, this structure reveals a conformational flexibility not seen in previous BCL-2 protein structures and provides insights into how these regulators of apoptosis can change conformation to exert their function.

The crystal structure of a complex between the prosurvival protein Bcl-x(L) and an α/β-peptide 21-mer is described. The α/β-peptide contains six β-amino acid residues distributed periodically throughout the sequence and adopts an α-helix-like conformation that mimics the bioactive shape of the Puma BH3 domain. The α/β-peptide forms all of the noncovalent contacts that have previously been identified as necessary for recognition of the prosurvival protein by an authentic BH3 domain. Comparison of our α/β-peptide:Bcl-x(L) structure with structures of complexes between native BH3 domains and Bcl-2 family proteins reveals how subtle adjustments, including variations in helix radius and helix bowing, allow the α/β-peptide to engage Bcl-x(L) with high affinity. Geometric comparisons of the BH3-mimetic α/β-peptide with α/β-peptides in helix-bundle assemblies provide insight on the conformational plasticity of backbones that contain combinations of α- and β-amino acid residues. The BH3-mimetic α/β-peptide displays prosurvival protein-binding preferences distinct from those of Puma BH3 itself, even though these two oligomers have identical side-chain sequences. Our results suggest origins for this backbone-dependent change in selectivity.

Schistosomiasis is an infectious disease caused by parasites of the phylum platyhelminthe. Here, we describe the identification and characterization of a Bcl-2–regulated apoptosis pathway in Schistosoma japonicum and S. mansoni. Genomic, biochemical, and cell-based mechanistic studies provide evidence for a tripartite pathway, similar to that in humans including BH3-only proteins that are inhibited by prosurvival Bcl-2–like molecules, and Bax/Bak-like proteins that facilitate mitochondrial outer-membrane permeabilization. Because Bcl-2 proteins have been successfully targeted with “BH3 mimetic” drugs, particularly in the treatment of cancer, we investigated whether schistosome apoptosis pathways could provide targets for future antischistosomal drug discovery efforts. Accordingly, we showed that a schistosome prosurvival protein, sjA, binds ABT-737, a well-characterized BH3 mimetic. A crystal structure of sjA bound to a BH3 peptide provides direct evidence for the feasibility of developing BH3 mimetics to target Bcl-2 prosurvival proteins in schistosomes, suggesting an alternative application for this class of drugs beyond cancer treatment.

The Bcl-2 family regulates induction of apoptosis at the mitochondria. Essential to this regulation are the interactions between Bcl-2 family members, which are mediated by Bcl-2 homology (BH) domains. Vaccinia virus F1L is a unique inhibitor of apoptosis that lacks significant sequence similarity with the Bcl-2 family and does not contain obvious BH domains. Despite this, F1L inhibits cytochrome c release from mitochondria by preventing Bak and Bax activation. Although F1L constitutively interacts with Bak to prevent Bak activation, the precise mechanism of this interaction remains elusive. We have identified highly divergent BH domains in F1L that were verified by the recent crystal structure of F1L (Kvansakul, M., Yang, H., Fairlie, W. D., Czabotar, P. E., Fischer, S. F., Perugini, M. A., Huang, D. C., and Colman, P. M. (2008) Cell Death Differ. 15, 1564-1571). Here we show that F1L required these BH domains to interact with ectopically expressed and endogenous Bak. The interaction between F1L and Bak was conserved across species, and both F1L and the cellular antiapoptotic protein Mcl-1 required the Bak BH3 domain for interaction. Moreover, F1L replaced Mcl-1 during infection, as the Bak x Mcl-1 complex was disrupted during vaccinia virus infection. In contrast to UV irradiation, vaccinia virus infection did not result in rapid degradation of Mcl-1, consistent with our observation that vaccinia virus did not initiate a DNA damage response. Additionally, Mcl-1 expression prevented Bak activation and apoptosis during infection with a proapoptotic vaccinia virus devoid of F1L. Our data suggest that F1L replaces the antiapoptotic activity of Mcl-1 during vaccinia virus infection by interacting with Bak using highly divergent BH domains.

The sera genes of the malaria-causing parasite Plasmodium encode a family of unique proteins that are maximally expressed at the time of egress of parasites from infected red blood cells. These multi-domain proteins are unique, containing a central papain-like cysteine-protease fragment enclosed between the disulfide-linked N- and C-terminal domains. However, the central fragment of several members of this family, including serine repeat antigen 5 (SERA5), contains a serine (S596) in place of the active-site cysteine. Here we report the crystal structure of the central protease-like domain of Plasmodium falciparum SERA5, revealing a number of anomalies in addition to the putative nucleophilic serine: (1) the structure of the putative active site is not conducive to binding substrate in the canonical cysteine-protease manner; (2) the side chain of D594 restricts access of substrate to the putative active site; and (3) the S(2) specificity pocket is occupied by the side chain of Y735, reducing this site to a small depression on the protein surface. Attempts to determine the structure in complex with known inhibitors were not successful. Thus, despite having revealed its structure, the function of the catalytic domain of SERA5 remains an enigma.

Apoptosis is an important part of the host's defense mechanism for eliminating invading pathogens. Some viruses express proteins homologous in sequence and function to mammalian pro-survival Bcl-2 proteins. Anti-apoptotic F1L expressed by vaccinia virus is essential for survival of infected cells, but it bears no discernable sequence homology to proteins other than its immediate orthologues in related pox viruses. Here we report that the crystal structure of F1L reveals a Bcl-2-like fold with an unusual N-terminal extension. The protein forms a novel domain-swapped dimer in which the alpha1 helix is the exchanged domain. Binding studies reveal an atypical BH3-binding profile, with sub-micromolar affinity only for the BH3 peptide of pro-apoptotic Bim and low micromolar affinity for the BH3 peptides of Bak and Bax. This binding interaction is sensitive to F1L mutations within the predicted canonical BH3-binding groove, suggesting parallels between how vaccinia virus F1L and myxoma virus M11L bind BH3 domains. Structural comparison of F1L with other Bcl-2 family members reveals a novel sequence signature that redefines the BH4 domain as a structural motif present in both pro- and anti-apoptotic Bcl-2 members, including viral Bcl-2-like proteins.

The Bcl-2 relative Bak is thought to drive apoptosis by forming homo-oligomers that permeabilize mitochondria, but how it is activated and oligomerizes is unclear. To clarify these pivotal steps toward apoptosis, we have characterized multiple random loss-of-function Bak mutants and explored the mechanism of Bak conformation change during apoptosis. Single missense mutations located to the alpha helix 2-5 region of Bak, with most altering the BH3 domain or hydrophobic groove (BH1 domain). Loss of function invariably corresponded to impaired ability to oligomerize. An essential early step in Bak activation was shown to be exposure of the BH3 domain, which became reburied in dimers. We demonstrate that oligomerization involves insertion of the BH3 domain of one Bak molecule into the groove of another and may produce symmetric Bak dimers. We conclude that this BH3:groove interaction is essential to nucleate Bak oligomerization, which in turn is required for its proapoptotic function.

Like Bcl-2, Mcl-1 is an important survival factor for many cancers, its expression contributing to chemoresistance and disease relapse. However, unlike other prosurvival Bcl-2–like proteins, Mcl-1 stability is acutely regulated. For example, the Bcl-2 homology 3 (BH3)–only protein Noxa, which preferentially binds to Mcl-1, also targets it for proteasomal degradation. In this paper, we describe the discovery and characterization of a novel BH3-like ligand derived from Bim, BimS2A, which is highly selective for Mcl-1. Unlike Noxa, BimS2A is unable to trigger Mcl-1 degradation, yet, like Noxa, BimS2A promotes cell killing only when Bcl-xL is absent or neutralized. Furthermore, killing by endogenous Bim is not associated with Mcl-1 degradation. Thus, functional inactivation of Mcl-1 does not always require its elimination. Rather, it can be efficiently antagonized by a BH3-like ligand tightly engaging its binding groove, which is confirmed here with a structural study. Our data have important implications for the discovery of compounds that might kill cells whose survival depends on Mcl-1.

Summary Many viruses express antiapoptotic proteins to counter host defense mechanisms that would otherwise trigger the rapid clearance of infected cells. For example, adenoviruses and some γ-herpesviruses express homologs of prosurvival Bcl-2 to subvert the host's apoptotic machinery. Myxoma virus, a double-stranded DNA virus of the pox family, harbors antiapoptotic M11L, its virulence factor. Analysis of its three-dimensional structure reveals that despite lacking any primary sequence similarity to Bcl-2, it adopts a virtually identical protein fold. This allows it to associate with BH3 domains, especially those of Bax and Bak. We found that M11L acts primarily by sequestering Bax and Bak, thereby blocking the killing action of these essential cell-death mediators. These findings expand the family of protein sequences that act like Bcl-2 to block apoptosis and support the conclusion that the prosurvival action of these proteins critically depends on their ability to bind and antagonize Bax and/or Bak.

The pathway to cell death in Caenorhabditis elegans is well established. In cells undergoing apoptosis, the Bcl-2 homology domain 3 (BH3)-only protein EGL-1 binds to CED-9 at the mitochondrial membrane to cause the release of CED-4, which oligomerises and facilitates the activation of the caspase CED-3. However, despite many studies, the biophysical features of the CED-4/CED-9 complex have not been fully characterised. Here, we report the purification of a soluble and stable 2 : 2 heterotetrameric complex formed by recombinant CED-4 and CED-9 coexpressed in bacteria. Consistent with previous studies, synthetic peptides corresponding to the BH3 domains of worm BH3-only proteins (EGL-1, CED-13) dissociate CED-4 from CED-9, but not from the gain-of-function CED-9 (G169E) mutant. Surprisingly, the ability of worm BH3 domains to dissociate CED-4 was specific since mammalian BH3-only proteins could not do so.

The three-dimensional structure of the haemagglutinin-neuraminidase (HN) from a human parainfluenza virus is described at ca 2.0 A resolution, both in native form and in complex with three substrate analogues. In support of earlier work on the structure of the homologous protein from the avian pathogen Newcastle disease virus (NDV), we observe a dimer of β-propellers and find no evidence for spatially separated sites performing the receptor-binding and neuraminidase functions of the protein. As with the NDV HN, the active site of the HN of parainfluenza viruses is structurally flexible, suggesting that it may be able to switch between a receptor-binding state and a catalytic state. However, in contrast to the NDV structures, we observe no ligand-induced structural changes that extend beyond the active site and modify the dimer interface.

The fusion of viral membranes with target-cell membranes is an essential step in the entry of enveloped viruses into cells, and recent X-ray structures of paramyxoviral envelope proteins have provided new insights into protein-mediated plasma-membrane fusion. Here, we review our understanding of the structural transitions that are involved in this fusion pathway, compare it to our understanding of influenza virus membrane fusion, and discuss the implications for retroviral membrane fusion.

The fusion protein of respiratory syncytial virus (RSV-F) is responsible for fusion of virion with host cells and infection of neighbouring cells through the formation of syncytia. A three-dimensional model structure of RSV-F was derived by homology modelling from the structure of the equivalent protein in Newcastle disease virus (NDV). Despite very low sequence homology between the two structures, most features of the model appear to have high credibility, although a few small regions in RSV-F whose secondary structure is predicted to be different to that in NDV are likely to be poorly modelled. The organization of individual residues identified in escape mutants against monoclonal antibodies correlates well with known antigenic sites. The location of residues involved in point mutations in several drug-resistant variants is also examined.

Certain BH3-only proteins transiently bind and activate Bak and Bax, initiating their oligomerization and the permeabilization of the mitochondrial outer membrane, a pivotal step in the mitochondrial pathway to apoptosis. Here we describe the first crystal structures of an activator BH3 peptide bound to Bak and illustrate their use in the design of BH3 derivatives capable of inhibiting human Bak on mitochondria. These BH3 derivatives compete for the activation site at the canonical groove, are the first engineered inhibitors of Bak activation, and support the role of key conformational transitions associated with Bak activation.

2,3-didehydro-2-deoxy-N:-acetylneuraminic acid (DANA) is a transition state analog inhibitor of influenza virus neuraminidase (NA). Replacement of the hydroxyl at the C9 position in DANA and 4-amino-DANA with an amine group, with the intention of taking advantage of an increased electrostatic interaction with a conserved acidic group in the active site to improve inhibitor binding, significantly reduces the inhibitor activity of both compounds. The three-dimensional X-ray structure of the complexes of these ligands and NA was obtained to 1.4 A resolution and showed that both ligands bind isosterically to DANA. Analysis of the geometry of the ammonium at the C4 position indicates that Glu119 may be neutral when these ligands bind. A computational analysis of the binding energies indicates that the substitution is successful in increasing the energy of interaction; however, the gains that are made are not sufficient to overcome the energy that is required to desolvate that part of the ligand that comes in contact with the protein.

Background: Membrane fusion within the Paramyxoviridae family of viruses is mediated by a surface glycoprotein termed the "F", or fusion, protein. Membrane fusion is assumed to involve a series of structural transitions of F from a metastable (prefusion) state to a highly stable (postfusion) state. No detail is available at the atomic level regarding the metastable form of these proteins or regarding the transitions accompanying fusion. Results: The three-dimensional structure of the fusion protein of Newcastle disease virus (NDV-F) has been determined. The trimeric NDV-F molecule is organized into head, neck, and stalk regions. The head is comprised of a highly twisted β domain and an additional immunoglobulin-like β domain. The neck is formed by the C-terminal extension of the heptad repeat region HR-A, capped by a four-helical bundle. The C terminus of HR-A is encased by a further helix HR-C and a 4-stranded β sheet. The stalk is formed by the remaining visible portion of HR-A and by polypeptide immediately N-terminal to the C-terminal heptad repeat region HR-B. An axial channel extends through the head and neck and is fenestrated by three large radial channels located approximately at the head–neck interface. Conclusion: We propose that prior to fusion activation, the hydrophobic fusion peptides in NDV-F are sequestered within the radial channels within the head, with the central HR-A coiled coil being only partly formed. Fusion activation then involves, inter alia, the assembly of a complete HR-A coiled coil, with the fusion peptides and transmembrane anchors being brought into close proximity. The structure of NDV-F is fundamentally different than that of influenza virus hemagglutinin, in that the central coiled coil is in the opposite orientation with respect to the viral membrane.

We have previously reported the isolation and characterization of an influenza virus variant with decreased sensitivity to the neuraminidase-specific inhibitor zanamivir. This variant, which has a mutation in the active site, Glu 119 Gly (E119G), has the same specific activity as the wild-type neuraminidase (NA), but is inherently unstable, as measured by loss of both enzyme activity and NC10 monoclonal antibody reactivity. However, despite the instability of the NA, replication of the virus in liquid culture is not adversely affected. We demonstrate here that in addition to enhanced temperature sensitivity the mutant NA was significantly more sensitive to formaldehyde and to specimen preparation for electron microscopy. Substrate, inhibitor, or monoclonal antibodies stabilized the NA against all methods of denaturation. These results suggest that the instability of the variant is primarily at the level of polypeptide chain folding rather than at the level of association of monomers into tetramers. Furthermore the presence of high levels of substrate, either cell or virus associated, may be sufficient to stabilize the NA during virus replication.

The structure of the complex between a recombinant single-chain Fv construct of antibody NC10 with a five-residue peptide linker between VH and VL (termed scFv(5)), and its antigen, tetrameric neuraminidase from influenza virus (NA), has been determined and refined at 2.5 A resolution. The antibody-antigen binding interface is very similar to that of a similar NC10 scFv-NA complex in which the scFv has a 15-residue peptide linker (scFv(15)), and the NC10 Fab-NA complex. However, scFv(5) and scFv(15) have different stoichiometries in solution. While scFv(15) is predominantly monomeric in solution, scFv(5) forms dimers exclusively, because the five-residue linker is not long enough to permit VH and VL domains from the same polypeptide associating and forming an antigen-binding site. Upon forming a complex with NA, scFv(15) forms a approximately 300 kDa complex corresponding to one NA tetramer binding four scFv(15) monomers, while scFv(5) forms a approximately 590 kDa complex, corresponding to two NA tetramers crosslinked by four bivalent scFv(5) dimers. However, the dimeric scFv(5) in the scFv(5)-NA crystals does not crosslink NA tetramers, and modelling studies indicate that it is not possible to pack four dimeric and simultaneously bivalent scFvs between the NA tetramers with only a five-residue linker between VH and VL. The inability arises from the exacting requirement to orient the two antigen-binding surfaces to bind the tetrameric NA antigen while avoiding steric clashes with NC10 scFv(5) dimers bound to other sites on the NA tetramer. The utility of bivalent or bifunctional scFvs with short linkers may therefore be restricted by the steric constraints imposed by binding multivalent antigens.

Background: Inhibitors of the influenza virus neuraminidase have been shown to be effective antiviral agents in humans. Several studies have reported the selection of novel influenza strains when the virus is cultured with neuraminidase inhibitors in vitro . These resistant viruses have mutations either in the neuraminidase or in the viral haemagglutinin. Inhibitors in which the glycerol sidechain at position 6 of 2-deoxy-2,3-dehydro- N -acetylneuraminic acid (Neu5Ac2en) has been replaced by carboxamide-linked hydrophobic substituents have recently been reported and shown to select neuraminidase variants. This study seeks to clarify the structural and functional consequences of replacing the glycerol sidechain of the inhibitor with other chemical constituents. Results: The neuraminidase variant Arg292→Lys is modified in one of three arginine residues that encircle the carboxylate group of the substrate. The structure of this variant in complex with the carboxamide inhibitor used for its selection, and with other Neu5Ac2en analogues, is reported here at high resolution. The structural consequences of the mutation correlate with altered inhibitory activity of the compounds compared with wild-type neuraminidase. Conclusions: The Arg292→Lys variant of influenza neuraminidase affects the binding of substrate by modification of the interaction with the substrate carboxylate. This may be one of the structural correlates of the reduced enzyme activity of the variant. Inhibitors that have replacements for the glycerol at position 6 are further affected in the Arg292→Lys variant because of structural changes in the binding site that apparently raise the energy barrier for the conformational change in the enzyme required to accommodate such inhibitors. These results provide evidence that a general strategy for drug design when the target has a high mutation frequency is to design the inhibitor to be as closely related as possible to the natural ligands of the target.

Calculation of the electrostatic potential of protein-protein complexes has led to the general assertion that protein-protein interfaces display “charge complementarity” and “electrostatic complementarity”. In this study, quantitative measures for these two terms are developed and used to investigate protein-protein interfaces in a rigorous manner. Charge complementarity (CC) was defined using the correlation of charges on nearest neighbour atoms at the interface. All 12 protein-protein interfaces studied had insignificantly small CC values. Therefore, the term charge complementarity is not appropriate for the description of protein-protein interfaces when used in the sense measured by CC. Electrostatic complementarity (EC) was defined using the correlation of surface electrostatic potential at protein-protein interfaces. All twelve protein-protein interfaces studied had significant EC values, and thus the assertion that protein-protein association involves surfaces with complementary electrostatic potential was substantially confirmed. The term electrostatic complementarity can therefore be used to describe protein-protein interfaces when used in the sense measured by EC. Taken together, the results for CC and EC demonstrate the relevance of the long-range effects of charges, as described by the electrostatic potential at the binding interface. The EC value did not partition the complexes by type such as antigen-antibody and proteinase-inhibitor, as measures of the geometrical complementarity at protein-protein interfaces have done. The EC value was also not directly related to the number of salt bridges in the interface, and neutralisation of these salt bridges showed that other charges also contributed significantly to electrostatic complementarity and electrostatic interactions between the proteins. Electrostatic complementarity as defined by EC was extended to investigate the electrostatic similarity at the surface of influenza virus neuraminidase where the epitopes of two monoclonal antibodies, NC10 and NC41, overlap. Although NC10 and NC41 both have quite high values of EC for their interaction with neuraminidase, the similarity in electrostatic potential generated by the two on the overlapping region of the epitopes is insignificant. Thus, it is possible for two antibodies to recognise the electrostatic surface of a protein in dissimilar ways.

Activation and oligomerisation of Bax, a key pro-apoptotic Bcl-2 family protein, are key steps in the mitochondrial pathway to apoptosis. The signals for apoptosis are conveyed by the distantly related BH3-only proteins, which use their short BH3 domain, an amphipathic α-helix, to interact with other Bcl-2 family members. Here we report an NMR study of interactions between BaxΔC and BH3 domain-containing peptides in the absence and presence of CHAPS, a zwitterionic detergent. We find for the first time that CHAPS interacts weakly with BaxΔC (fast exchange on the NMR chemical shift timescale), at concentrations below micelle formation and with an estimated Kd in the tens of mM. Direct and relatively strong-interactions (slow exchange on the NMR chemical shift timescale) were also observed for BaxΔC with BaxBH3 (estimated Kd of circa 150μM) or BimBH3 in the absence of CHAPS. The interaction with either peptide alone induced widespread chemical shift perturbations to BaxΔC in solution which implies that BaxΔC might have undergone significant conformation change upon binding the BH3 peptide. However, BaxΔC remained monomeric upon binding either CHAPS or a BH3 peptide alone, but the presence of both provoked it to form a dimer.

The structure-activity relationships of a series of 4-amino and guanidino-4H-pyran-2-carboxylic acid 6-carboxamides are described. These compounds represent a new class of inhibitor of influenza sialidases and are particularly active against influenza A sialidase. The binding of the N-phenethyl-N-propylamide 41 to influenza A and B sialidases has been investigated using X-ray crystallography and molecular dynamics simulations. Our results suggest that formation of a hitherto unobserved intramolecular salt bridge within the enzymes may account for the observed activity and selectivity of the series.

Our understanding of carbohydrate-protein interactions has significantly advanced over the past two years. In particular, a healthy amount of literature has appeared on selectins and their relevant ligands. A significant number of carbohydrate-metabolizing enzyme crystal structures have been solved which provide useful starting points for computer-assisted drug design. Some of these proteins have been implicated either directly or indirectly in playing roles in human-disease states, for example, in inflammation, in diabetes and its complications, and in microorganism-induced diseases such as influenza and cholera.

The three-dimensional X-ray structure of a complex of the potent neuraminidase inhibitor 4-guanidino-Neu5Ac2en and influenza virus neuraminidase (Subtype N9) has been obtained utilizing diffraction data to 1.8 A resolution. The interactions of the inhibitor, solvent water molecules, and the active site residues have been accurately determined. Six water molecules bound in the native structure have been displaced by the inhibitor, and the active site residues show no significant conformational changes on binding. Sialic acid, the natural substrate, binds in a half-chair conformation that is isosteric to the inhibitor. The conformation of the inhibitor in the active site of the X-ray structure concurs with that obtained by theoretical calculations and validates the structure-based design of the inhibitor. Comparison of known high-resolution structures of neuraminidase subtypes N2, N9, and B shows good structural conservation of the active site protein atoms, but the location of the water molecules in the respective active sites is less conserved. In particular, the environment of the 4-guanidino group of the inhibitor is strongly conserved and is the basis for the antiviral action of the inhibitor across all presently known influenza strains. Differences in the solvent structure in the active site may be related to variation in the affinities of inhibitors to different subtypes of neuraminidase.

The determination of the 3-dimensional structure of the influenza virus neuraminidase in 1983 has served as a platform for understanding interactions between antibodies and protein antigens, for investigating antigenic variation in influenza viruses, and for devising new inhibitors of the enzyme. That work is reviewed here, together with more recent developments that have resulted in one of the inhibitors entering clinical trials as an anti-influenza virus drug.

Background While it is well known that different antibodies can be produced against a particular antigen, and even against a particular site on an antigen, up until now there have been no structural studies of cross-reacting antibodies of this type. One antibody– antigen complex whose structure is known is that of the influenza virus antigen, neuraminidase, in complex with the NC41 antibody. Another anti-neuraminidase antibody, NC10, binds to an overlapping site on the antigen. The structure of the complex formed by this antibody with neuraminidase is described here and compared with the NC41-containing complex. Results The crystal structure of the NC10 Fab– neuraminidase complex has been refined to a nominal resolution of 2.5a. Approximately 80% of the binding site of the NC10 antibody on neuraminidase overlaps with that of the NC41 antibody. The epitope residues of neuraminidase are often engaged in quite different interactions with the two antibodies. Although the NC10 and NC41 antibodies have identical amino acid sequences within the first complementarity determining region of their heavy chains, this is not the basis of the cross-reaction. Conclusions The capacity of two different proteins to bind to the same target structure on a third protein need not be based on the existence of identical or homologous amino acid sequences within those proteins. As we have demonstrated, amino acid residues on the common target structure may be in quite different chemical environments, and may also adopt different conformations within two protein– protein complexes.

Background N -acetylneuraminate lyase catalyzes the cleavage of N -acetylneuraminic acid (sialic acid) to form pyruvate and N - acetyl- d -mannosamine. The enzyme plays an important role in the regulation of sialic acid metabolism in bacteria. The reverse reaction can be exploited for the synthesis of sialic acid and some of its derivatives. Results The structure of the enzyme from Escherichia coli has been determined to 2.2 a resolution by X-ray crystallography. The enzyme is shown to be a tetramer, in which each subunit consists of an α / β -barrel domain followed by a carboxy-terminal extension of three α -helices. Conclusions The active site of the enzyme is tentatively identified as a pocket at the carboxy-terminal end of the eight- stranded β -barrel. Lys165 lies within this pocket and is probably the reactive residue which forms a Schiff base intermediate with the substrate. The sequence of N - acetylneuraminate lyase has similarities to those of dihydrodipicolinate synthase and MosA (an enzyme implicated in rhizopine synthesis) suggesting that these last two enzymes share a similar structure to N -acetylneuraminate lyase.

There are now many successful examples of the design of new ligands based on knowledge of target protein structures. In most cases those ligands are unsuitable as drugs because of problems of toxicity, stability or bioavailability. The past twelve months have also seen the description of the structures of many proteins which are either known to be targets for existing drugs or have clear potential to be utilized in therapy.

The structure of the trypanosomal trans-sialidase reveals a canonical sialidase catalytic site elaborated with a conformational switch that creates an adjacent binding pocket for lactose.

Pro-survival members of the Bcl-2 family of proteins restrain the pro-apoptotic activity of Bax, either directly through interactions with Bax or indirectly by sequestration of activator BH3-only proteins, or both. Mutations in Bax that promote apoptosis can provide insight into how Bax is regulated. Here, we describe crystal structures of the pro-survival proteins Mcl-1 and Bcl-x(L) in complex with a 34-mer peptide from Bax that encompasses its BH3 domain. These structures reveal canonical interactions between four signature hydrophobic amino acids from the BaxBH3 domain and the BH3-binding groove of the pro-survival proteins. In both structures, Met-74 from the Bax peptide engages with the BH3-binding groove in a fifth hydrophobic interaction. Various Bax Met-74 mutants disrupt interactions between Bax and all pro-survival proteins, but these Bax mutants retain pro-apoptotic activity. Bax/Bak-deficient mouse embryonic fibroblast cells reconstituted with several Bax Met-74 mutants are more sensitive to the BH3 mimetic compound ABT-737 as compared with cells expressing wild-type Bax. Furthermore, the cells expressing Bax Met-74 mutants are less viable in colony assays even in the absence of an external apoptotic stimulus. These results support a model in which direct restraint of Bax by pro-survival Bcl-2 proteins is a barrier to apoptosis.

Epstein-Barr virus (EBV) is associated with human malignancies, especially those affecting the B cell compartment such as Burkitt lymphoma. The virally encoded homolog of the mammalian pro-survival protein Bcl-2, BHRF1 contributes to viral infectivity and lymphomagenesis. In addition to the pro-apoptotic BH3-only protein Bim, its key target in lymphoid cells, BHRF1 also binds a selective sub-set of pro-apoptotic proteins (Bid, Puma, Bak) expressed by host cells. A consequence of BHRF1 expression is marked resistance to a range of cytotoxic agents and in particular, we show that its expression renders a mouse model of Burkitt lymphoma untreatable. As current small organic antagonists of Bcl-2 do not target BHRF1, the structures of it in complex with Bim or Bak shown here will be useful to guide efforts to target BHRF1 in EBV-associated malignancies, which are usually associated with poor clinical outcomes., Author Summary Altruistic suicide of infected host cells is a key defense mechanism to combat viral infection. To ensure their own survival and proliferation, certain viruses, including Epstein-Barr virus (EBV), have mechanisms to subvert apoptosis, including the expression of homologs of the mammalian pro-survival protein Bcl-2. EBV was first identified in association with Burkitt lymphoma and it is also linked to certain Hodgkin's lymphomas and nasopharyngeal carcinoma. Whereas increased expression of Bcl-2 promotes malignancies such as human follicular lymphoma, the precise role of the EBV encoded Bcl-2 homolog BHRF1 in EBV-associated malignancies is less well defined. BHRF1 is known to bind the pro-apoptotic BH3-only protein Bim, and here we demonstrate that it also binds other pro-apoptotic proteins (Bid, Puma, Bak) expressed by host cells. Crystal structures of BHRF1 with the BH3 regions of Bim and Bak illustrate these interactions in atomic detail. A consequence of BHRF1 expression is marked resistance to a range of cytotoxic agents, and we show that its expression renders a mouse model of Burkitt lymphoma untreatable. As current antagonists of Bcl-2 do not target BHRF1, our crystal structures will be useful to guide efforts to target BHRF1 in EBV-associated malignancies, which are usually associated with poor clinical outcomes.

Crystallographic studies of neuraminidase-sialic acid complexes indicate that sialic acid is distorted on binding the enzyme. Three arginine residues on the enzyme interact with the carboxylate group of the sugar which is observed to be equatorial to the saccharide ring as a consequence of its distorted geometry. The glycosidic oxygen is positioned within hydrogen-bonding distance of Asp-151, implicating this residue in catalysis.

The crystal structure of the complex between neuraminidase from influenza virus (subtype N9 and isolated from an avian source) and the antigen-binding fragment (Fab) of monoclonal antibody NC41 has been refined by both least-squares and simulated annealing methods to an R-factor of 0.191 using 31,846 diffraction data in the resolution range 8.0 to 2.5 A. The resulting model has a root-mean-square deviation from ideal bond-length of 0.016 A. One fourth of the tetrameric complex comprises the crystallographic model, which has 6577 non-hydrogen atoms and consists of 389 protein residues and eight carbohydrate residues in the neuraminidase, 214 residues in the Fab light chain, and 221 residues in the heavy chain. One putative Ca ion buried in the neuraminidase, and 73 water molecules, are also included. A remarkable shape complementarity exists between the interacting surfaces of the antigen and the antibody, although the packing density of atoms at the interface is somewhat looser than in the interior of a protein. Similarly, there is a high degree of chemical complementarity between the antigen and antibody, mediated by one buried salt-link, two solvated salt-links and 12 hydrogen bonds. The antibody-binding site on neuraminidase is discontinuous and comprises five chain segments and 19 residues in contact, whilst 33 neuraminidase residues in eight segments have 899 A2 of surface area buried by the interaction (to a 1.7 A probe), including two hexose units. Seventeen residues in NC41 Fab lying in five of the six complementarity determining regions (CDRs) make contact with the neuraminidase and 36 antibody residues in seven segments have 916 A2 of buried surface area. The interface is more extensive than those of the three lysozyme-Fab complexes whose crystal structures have been determined, as judged by buried surface area and numbers of contact residues. There are only small differences (less than 1.5 A) between the complexed and uncomplexed neuraminidase structures and, at this resolution and accuracy, those differences are not unequivocal. The main-chain conformations of five of the CDRs follow the predicted canonical structures. The interface between the variable domains of the light and heavy chains is not as extensive as in other Fabs, due to less CDR-CDR interaction in NC41. The first CDR on the NC41 Fab light chain is positioned so that it could sterically hinder the approach of small as well as large substrates to the neuraminidase active-site pocket, suggesting a possible mechanism for the observed inhibition of enzyme activity by the antibody.(ABSTRACT TRUNCATED AT 400 WORDS)

Interactions between Bcl-2 homology-3 (BH3)-only proteins and their pro-survival Bcl-2 family binding partners initiate the intrinsic apoptosis pathway. These interactions are mediated by a short helical motif, the BH3 domain, on the BH3-only protein, which inserts into a hydrophobic groove on the pro-survival molecule. To identify novel peptidic ligands that bind Mcl-1, a pro-survival protein relative of Bcl-2, both human and mouse Mcl-1 were screened against large randomized phage-displayed peptide libraries. We identified a number of 16-mer peptides with sub-micromolar affinity that were highly selective for Mcl-1, as well as being somewhat selective for the species of Mcl-1 (human or mouse) against which the library was panned. Interestingly, these sequences all strongly resembled natural BH3 domain sequences. By switching residues within the best of the human Mcl-1-binding sequences, or extending beyond the core sequence identified, we were able to alter the pro-survival protein interaction profile of this peptide such that it now bound all members tightly and was a potent killer when introduced into cells. Introduction of an amide lock constraint within this sequence also increased its helicity and binding to pro-survival proteins. These data provide new insights into the determinants of BH3 domain:pro-survival protein affinity and selectivity.

Antagonists of anti-apoptotic Bcl-2 family members hold promise as cancer therapeutics. Apoptosis is triggered when a peptide containing a BH3 motif or a small molecule BH3 peptidomimetic, such as ABT 737, binds to the relevant Bcl-2 family members. ABT-737 is an antagonist of Bcl-2, Bcl-x(L), and Bcl-w but not of Mcl-1. Here we describe new structures of mutant BH3 peptides bound to Bcl-x(L) and Mcl-1. These structures suggested a rationale for the failure of ABT-737 to bind Mcl-1, but a designed variant of ABT-737 failed to acquire binding affinity for Mcl-1. Rather, it was selective for Bcl-x(L), a result attributable in part to significant backbone refolding and movements of helical segments in its ligand binding site. To date there are few reported crystal structures of organic ligands in complex with their pro-survival protein targets. Our structure of this new organic ligand provided insights into the structural transitions that occur within the BH3 binding groove, highlighting significant differences in the structural properties of members of the Bcl-2 pro-survival protein family. Such differences are likely to influence and be important in the quest for compounds capable of selectively antagonizing the different family members.

The mechanism by which the cell death mediator Bax becomes activated to cause mitochondrial damage, a key step for the intrinsic pathway to apoptosis, remain highly contentious. Although some data support a role for certain BH3-only proteins, such as Bim or tBid, to directly activate Bax, others have led to the conclusion that BH3-only proteins act indirectly by antagonizing the prosurvival Bcl-2 proteins, thereby allowing Bax activation to proceed. A recent paper in Nature by Gavathiotis et al. provides the first biophysical evidence for a direct interaction between a BH3 domain, that of Bim, with Bax. Here, we review these intriguing observations and discuss their implications for our understanding of how the BH3-only proteins initiate apoptosis.

An atomic model of the tetrameric surface glycoprotein neuraminidase of influenza virus A/Tokyo/3/67 has been built and refined based on X-ray diffraction data at 2.2 A resolution. The crystallographic residual is 0.21 for data between 6 and 2.2 A resolution and the r.m.s. deviations from ideal geometry are 0.02 A for bond lengths and 3.9 degrees for bond angles. The model includes amino acid residues 83 to 469, four oligosaccharide structures N-linked at asparagine residues 86, 146, 200 and 234, a single putative Ca2+ ion site, and 85 water molecules. One of the oligosaccharides participates in a novel crystal contact. The folding pattern is a beta-sheet propeller as described earlier and details of the intramolecular interactions between the six beta-sheets are presented. Strain-invariant residues are clustered around the propeller axis on the upper surface of the molecule where they line the wall of a cavity into which sialic has been observed to bind. Strain-variable residues implicated in binding to antibodies surround this site.

The crystal structure of the N9 subtype neuraminidase of influenza virus was refined by simulated annealing and conventional techniques to an R-factor of 0.172 for data in the resolution range 6.0 to 2.2 A. The r.m.s. deviation from ideal values of bond lengths is 0.014 A. The structure is similar to that of N2 subtype neuraminidase both in secondary structure elements and in their connections. The three-dimensional structures of several escape mutants of neuraminidase, selected with antineuraminidase monoclonal antibodies, are also reported. In every case, structural changes associated with the point mutation are confined to the mutation site or to residues that are spatially immediately adjacent to it. The failure of antisera to cross-react between N2 and N9 subtypes may be correlated with the absence of conserved, contiguous surface structures of area 700 A2 or more.

The genetic basis of antigen receptor diversity is understood for both B and T cell systems. Structural aspects of five specific antibody-antigen interactions have also been described. During the past year, new data comparing the structures of liganded and unliganded antibodies have given support to the hypothesis that there exists a structural dimension to diversity. Other advances include the determination of the structure of an antibody-peptide complex and the establishment of an experimental system that could supply data on the conformation of many complementarity-determining regions of antigen receptors.

Studies of the cell death pathway in the nematode Caenorhabditis elegans provided the first evidence of the evolutionary conservation of apoptosis signalling. Here we show that the worm Bcl-2 homology domain-3 (BH3)-only protein EGL-1 binds mammalian pro-survival proteins very poorly, but can be converted into a high-affinity ligand for Bcl-2 and Bcl-x(L) by subtle mutation of the cysteine residue at position 62 within the BH3 domain. A 100-fold increase in affinity was observed following a single atom change (cysteine to serine substitution), and a further 10-fold increase by replacement with glycine. The low affinity of wild-type EGL-1 for mammalian pro-survival proteins and its poor expression correlates with its weak killing activity in mammalian cells whereas the high-affinity C62G mutant is a very potent killer of cells lacking Mcl-1. Cell killing by the C62S mutant with intermediate affinity only occurs when this EGL-1 BH3 domain is placed in a more stable context, namely that of Bim(S), which allows higher expression, though the kinetics of cell death now vary depending on whether Mcl-1 is neutralized by Noxa or genetically deleted. These results demonstrate how levels of BH3-only proteins, target affinity and the spectrum of neutralization of pro-survival proteins all contribute to killing activity.

Crystal structure of ABT-737 complexed with Bcl-x L : implications for selectivity of antagonists of the Bcl-2 family