Structure of the lectin regulatory domain of the cholesterol-dependent cytolysin lectinolysin reveals the basis for its lewis antigen specificity

Identifying the molecular basis of Laminin N-terminal domain Ca2+ binding using a hybrid approach

Ca2+ is a highly abundant ion involved in numerous biological processes, particularly in multicellular eukaryotic organisms where it exerts many of these functions through interactions with Ca2+ binding proteins. The laminin N-terminal (LN) domain is found in members of the laminin and netrin protein families where it plays a critical role in the function of these proteins. The LN domain of laminins and netrins is a Ca2+ binding domain and in many cases requires Ca2+ to perform its biological function. Here, we conduct a detailed examination of the molecular basis of the LN domain Ca2+ interaction combining structural, computational, bioinformatics, and biophysical techniques. By combining computational and bioinformatic techniques with x-ray crystallography we explore the molecular basis of the LN domain Ca2+ interaction and identify a conserved sequence present in Ca2+ binding LN domains. These findings enable a sequence-based prediction of LN domain Ca2+ binding ability. We use thermal shift assays and isothermal titration calorimetry to explore the biophysical properties of the LN domain Ca2+ interaction. We show that the netrin-1 LN domain exhibits a high affinity and specificity for Ca2+, which structurally stabilizes the LN domain. This study elucidates the molecular foundation of the LN domain Ca2+ binding interaction and provides a detailed functional characterization of this essential interaction, advancing our understanding of protein-Ca2+ dynamics within the context of the LN domain.

Story of Pore-Forming Proteins from Deadly Disease-Causing Agents to Modern Applications with Evolutionary Significance

Animal venoms are a complex mixture of highly specialized toxic molecules. Among them, pore-forming proteins (PFPs) or toxins (PFTs) are one of the major disease-causing toxic elements. The ability of the PFPs in defense and toxicity through pore formation on the host cell surface makes them unique among the toxin proteins. These features made them attractive for academic and research purposes for years in the areas of microbiology as well as structural biology. All the PFPs share a common mechanism of action for the attack of host cells and pore formation in which the selected pore-forming motifs of the host cell membrane-bound protein molecules drive to the lipid bilayer of the cell membrane and eventually produces water-filled pores. But surprisingly their sequence similarity is very poor. Their existence can be seen both in a soluble state and also in transmembrane complexes in the cell membrane. PFPs are prevalent toxic factors that are predominately produced by all kingdoms of life such as virulence bacteria, nematodes, fungi, protozoan parasites, frogs, plants, and also from higher organisms. Nowadays, multiple approaches to applications of PFPs have been conducted by researchers both in basic as well as applied biological research. Although PFPs are very devastating for human health nowadays researchers have been successful in making these toxic proteins into therapeutics through the preparation of immunotoxins. We have discussed the structural, and functional mechanism of action, evolutionary significance through dendrogram, domain organization, and practical applications for various approaches. This review aims to emphasize the PFTs to summarize toxic proteins together for basic knowledge as well as to highlight the current challenges, and literature gap along with the perspective of promising biotechnological applications for their future research.

A simple method to differentiate three classes of cholesterol-dependent cytolysins

Cholesterol-dependent cytolysins (CDCs) are proteinaceous toxins widely distributed in gram-positive pathogenic bacteria. CDCs can be classified into three groups (I-III) based on the mode of receptor recognition. Group I CDCs recognize cholesterol as their receptor. Group II CDC specifically recognizes human CD59 as the primary receptor on the cell membrane. Only intermedilysin from Streptococcus intermedius has been reported as a group II CDC. Group III CDCs recognize both human CD59 and cholesterol as receptors. CD59 contains five disulfide bridges in its tertiary structure. Therefore, we treated human erythrocytes with dithiothreitol (DTT) to inactivate CD59 on membranes. Our data showed that DTT treatment caused a complete loss of recognition of intermedilysin and an anti-human CD59 monoclonal antibody. In contrast, this treatment did not affect the recognition of group I CDCs, judging from the fact that DTT-treated erythrocytes were lysed with the same efficiency as mock-treated human erythrocytes. The recognition of group III CDCs toward DTT-treated erythrocytes was partially reduced, and these results are likely due to the loss of human CD59 recognition. Therefore, the degree of human CD59 and cholesterol requirements of uncharacterized group III CDCs frequently found in Mitis group streptococci can be easily estimated by comparing the amounts of hemolysis between DTT-treated and mock-treated erythrocytes.

Purification and characterization of a hemocyanin with lectin-like activity isolated from the hemolymph of speckled shrimp, Metapenaeusmonoceros

Lectins or agglutinins are mainly proteins or glycoproteins, reported to uphold an ability to agglutinate the red blood cells (RBCs) with a known sugar specificity in a diverse group of organisms. In the present study, we purified a hemocyanin (named as MmHc) from a shrimp, Metapenaeus monoceros by size-exclusion chromatography. Further characterization revealed that the purified MmHc showed hemagglutination activity that was found to be specifically inhibited by Lewis B and Lewis Y tetrasaccharides. The MmHc displayed two oligomers of molecular weight approximately ∼78 and ∼85 kDa in SDS-PAGE. The native molecular mass of MmHc was found to be ∼457 kDa as determined by size-exclusion chromatography which indicated that the purified MmHc is an oligomeric protein. MmHc showed a maximum activity within pH 7.0-8.0, while a wide range of temperature stability was observed between 4 to 55 °C, however, it did not show any dependency on metal ions for binding. Subsequently, the analysis of the peptides by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) identified the purified MmHc as shrimp hemocyanin showing significant similarity to the hemocyanin of Penaeus vannamei. The results of multiple sequence alignment and detailed analysis of the molecular interactions predicted by AutoDock suggested that besides the oxygen carrier function, this MmHc may have multiple roles and can interact well with the Lewis Y antigen through a typical sugar binding motif containing the similar hydrophilic amino acids as the conserved residues.

Cholesterol-dependent cytolysins: The outstanding questions

The cholesterol-dependent cytolysins (CDCs) are a major family of bacterial pore-forming proteins secreted as virulence factors by Gram-positive bacterial species. CDCs are produced as soluble, monomeric proteins that bind specifically to cholesterol-rich membranes, where they oligomerize into ring-shaped pores of more than 30 monomers. Understanding the details of the steps the toxin undergoes in converting from monomer to a membrane-spanning pore is a continuing challenge. In this review we summarize what we know about CDCs and highlight the remaining outstanding questions that require answers to obtain a complete picture of how these toxins kill cells.

Structure of the Streptococcus pyogenes NADase translocation domain and its essential role in toxin binding to oropharyngeal keratinocytes

The emergence and continued dominance of a Streptococcus pyogenes (group A Streptococcus, GAS) M1T1 clonal group is temporally correlated with acquisition of genomic sequences that confer high level expression of co-toxins streptolysin O (SLO) and NAD⁺-glycohydrolase (NADase). Experimental infection models have provided evidence that both toxins are important contributors to GAS virulence. SLO is a cholesterol-dependent pore-forming toxin capable of lysing virtually all types of mammalian cells. NADase, which is composed of an N-terminal translocation domain and C-terminal glycohydrolase domain, acts as an intracellular toxin that depletes host cell energy stores. NADase is dependent on SLO for internalization into epithelial cells, but its mechanism of interaction with the cell surface and details of its translocation mechanism remain unclear. In this study we found that NADase can bind oropharyngeal epithelial cells independently of SLO. This interaction is mediated by both domains of the toxin. We determined by NMR the structure of the translocation domain to be a β-sandwich with a disordered N-terminal region. The folded region of the domain has structural homology to carbohydrate binding modules. We show that excess NADase inhibits SLO-mediated hemolysis and binding to epithelial cells in vitro, suggesting NADase and SLO have shared surface receptors. This effect is abrogated by disruption of a putative carbohydrate binding site on the NADase translocation domain. Our data are consistent with a model whereby interactions of the NADase glycohydrolase domain and translocation domain with SLO and the cell surface increase avidity of NADase binding and facilitate toxin-toxin and toxin-cell surface interactions. Importance NADase and streptolysin O (SLO) are secreted toxins important for pathogenesis of group A Streptococcus, the agent of strep throat and severe invasive infections. The two toxins interact in solution and mutually enhance cytotoxic activity. We now find that NADase is capable of binding to the surface of human cells independently of SLO. Structural analysis of the previously uncharacterized translocation domain of NADase suggests that it contains a carbohydrate binding module. The NADase translocation domain and SLO appear to recognize similar glycan structures on the cell surface, which may be one mechanism through which NADase enhances SLO pore-forming activity during infection. Our findings provide new insight into the NADase toxin and its functional interactions with SLO during streptococcal infection.

Structures and functions of the membrane-damaging pore-forming proteins

Pore-forming proteins (PFPs) of the diverse life forms have emerged as the potent cell-killing entities owing to their specialized membrane-damaging properties. PFPs have the unique ability to perforate the plasma membranes of their target cells, and they exert this functionality by creating oligomeric pores in the membrane lipid bilayer. Pathogenic bacteria employ PFPs as toxins to execute their virulence mechanisms, whereas in the higher vertebrates PFPs are deployed as the part of the immune system and to generate inflammatory responses. PFPs are the unique dimorphic proteins that are generally synthesized as water-soluble molecules, and transform into membrane-inserted oligomeric pore assemblies upon interacting with the target membranes. In spite of sharing very little sequence similarity, PFPs from diverse organisms display incredible structural similarity. Yet, at the same time, structure-function mechanisms of the PFPs document remarkable versatility. Such notions establish PFPs as the fascinating model system to explore variety of unsolved issues pertaining to the structure-function paradigm of the proteins that interact and act in the membrane environment. In this article, we discuss our current understanding regarding the structural basis of the pore-forming functions of the diverse class of PFPs. We attempt to highlight the similarities and differences in their structures, membrane pore-formation mechanisms, and their implications for the various biological processes, ranging from the bacterial virulence mechanisms to the inflammatory immune response generation in the higher animals.

Proteome-wide prediction of bacterial carbohydrate-binding proteins as a tool for understanding commensal and pathogen colonisation of the vaginal microbiome

Bacteria use carbohydrate-binding proteins (CBPs), such as lectins and carbohydrate-binding modules (CBMs), to anchor to specific sugars on host surfaces. CBPs in the gut microbiome are well studied, but their roles in the vagina microbiome and involvement in sexually transmitted infections, cervical cancer and preterm birth are largely unknown. We established a classification system for lectins and designed Hidden Markov Model (HMM) profiles for data mining of bacterial genomes, resulting in identification of >100,000 predicted bacterial lectins available at unilectin.eu/bacteria. Genome screening of 90 isolates from 21 vaginal bacterial species shows that those associated with infection and inflammation produce a larger CBPs repertoire, thus enabling them to potentially bind a wider array of glycans in the vagina. Both the number of predicted bacterial CBPs and their specificities correlated with pathogenicity. This study provides new insights into potential mechanisms of colonisation by commensals and potential pathogens of the reproductive tract that underpin health and disease states.

F-Type Lectins: Structure, Function, and Evolution

F-type lectins (FTLs) are characterized by a fucose recognition domain (F-type lectin domain; FTLD) that displays a novel jellyroll fold ("F-type" fold) and unique carbohydrate- and calcium-binding sequence motifs. This novel lectin family comprises widely distributed proteins exhibiting single, double, or greater multiples of the FTLD, either tandemly arrayed or combined with other structurally and functionally distinct domains. Further, differences in carbohydrate specificity among tandemly arrayed FTLDs present in any FTL polypeptide subunit, together with the expression of multiple FTL isoforms in a single individual supports a striking diversity in ligand recognition. Functions of FTLs in self/nonself recognition include innate immunity, fertilization, microbial adhesion, and pathogenesis, among others, revealing an extensive structural/functional diversification. The taxonomic distribution of FTLDs is surprisingly discontinuous, suggesting that this lectin family has been subject to secondary loss, lateral transfer, and functional co-option along evolutionary lineages.

X-ray crystallography shines a light on pore-forming toxins

A common form of cellular attack by pathogenic bacteria is to secrete pore-forming toxins (PFTs). Capable of forming transmembrane pores in various biological membranes, PFTs have also been identified in a diverse range of other organisms such as sea anemones, earthworms and even mushrooms and trees. The mechanism of pore formation by PFTs is associated with substantial conformational changes in going from the water-soluble to transmembrane states of the protein. The determination of the crystal structures for numerous PFTs has shed much light on our understanding of these proteins. Other than elucidating the atomic structural details of PFTs and the conformational changes that must occur for pore formation, crystal structures have revealed structural homology that has led to the discovery of new PFTs and new PFT families. Here we review some key crystallographic results together with complimentary approaches for studying PFTs. We discuss how these studies have impacted our understanding of PFT function and guided research into biotechnical applications.

Structural Basis of the Pore-Forming Toxin/Membrane Interaction

With the rapid growth of antibiotic-resistant bacteria, it is urgent to develop alternative therapeutic strategies. Pore-forming toxins (PFTs) belong to the largest family of virulence factors of many pathogenic bacteria and constitute the most characterized classes of pore-forming proteins (PFPs). Recent studies revealed the structural basis of several PFTs, both as soluble monomers, and transmembrane oligomers. Upon interacting with host cells, the soluble monomer of bacterial PFTs assembles into transmembrane oligomeric complexes that insert into membranes and affect target cell-membrane permeability, leading to diverse cellular responses and outcomes. Herein we have reviewed the structural basis of pore formation and interaction of PFTs with the host cell membrane, which could add valuable contributions in comprehensive understanding of PFTs and searching for novel therapeutic strategies targeting PFTs and interaction with host receptors in the fight of bacterial antibiotic-resistance.

Human Serum Albumin Binds Streptolysin O (SLO) Toxin Produced by Group A Streptococcus and Inhibits Its Cytotoxic and Hemolytic Effects

The pathogenicity of group A Streptococcus (GAS) is mediated by direct bacterial invasivity and toxin-associated damage. Among the extracellular products, the exotoxin streptolysin O (SLO) is produced by almost all GAS strains. SLO is a pore forming toxin (PFT) hemolitically active and extremely toxic in vivo. Recent evidence suggests that human serum albumin (HSA), the most abundant protein in plasma, is a player in the innate immunity "orchestra." We previously demonstrated that HSA acts as a physiological buffer, partially neutralizing Clostridioides difficile toxins that reach the bloodstream after being produced in the colon. Here, we report the in vitro and ex vivo capability of HSA to neutralize the cytotoxic and hemolytic effects of SLO. HSA binds SLO with high affinity at a non-conventional site located in domain II, which was previously reported to interact also with C. difficile toxins. HSA:SLO recognition protects HEp-2 and A549 cells from cytotoxic effects and cell membrane permeabilization induced by SLO. Moreover, HSA inhibits the SLO-dependent hemolytic effect in red blood cells isolated from healthy human donors. The recognition of SLO by HSA may have a significant protective role in human serum and sustains the emerging hypothesis that HSA is an important constituent of the innate immunity system.

Interaction of Macrophages and Cholesterol-Dependent Cytolysins: The Impact on Immune Response and Cellular Survival

Cholesterol-dependent cytolysins (CDCs) are key virulence factors involved in many lethal bacterial infections, including pneumonia, necrotizing soft tissue infections, bacterial meningitis, and miscarriage. Host responses to these diseases involve myeloid cells, especially macrophages. Macrophages use several systems to detect and respond to cholesterol-dependent cytolysins, including membrane repair, mitogen-activated protein (MAP) kinase signaling, phagocytosis, cytokine production, and activation of the adaptive immune system. However, CDCs also promote immune evasion by silencing and/or destroying myeloid cells. While there are many common themes between the various CDCs, each CDC also possesses specific features to optimally benefit the pathogen producing it. This review highlights host responses to CDC pathogenesis with a focus on macrophages. Due to their robust plasticity, macrophages play key roles in the outcome of bacterial infections. Understanding the unique features and differences within the common theme of CDCs bolsters new tools for research and therapy.

All major cholesterol-dependent cytolysins use glycans as cellular receptors

Cholesterol-dependent cytolysins (CDCs) form pores in cholesterol-rich membranes, but cholesterol alone is insufficient to explain their cell and host tropism. Here, we show that all eight major CDCs have high-affinity lectin activity that identifies glycans as candidate cellular receptors. Streptolysin O, vaginolysin, and perfringolysin O bind multiple glycans, while pneumolysin, lectinolysin, and listeriolysin O recognize a single glycan class. Addition of exogenous carbohydrate receptors for each CDC inhibits toxin activity. We present a structure for suilysin domain 4 in complex with two distinct glycan receptors, P1 antigen and αGal/Galili. We report a wide range of binding affinities for cholesterol and for the cholesterol analog pregnenolone sulfate and show that CDCs bind glycans and cholesterol independently. Intermedilysin binds to the sialyl-TF O-glycan on its erythrocyte receptor, CD59. Removing sialyl-TF from CD59 reduces intermedilysin binding. Glycan-lectin interactions underpin the cellular tropism of CDCs and provide molecular targets to block their cytotoxic activity.

Membrane Binding, Cellular Cholesterol Content and Resealing Capacity Contribute to Epithelial Cell Damage Induced by Suilysin of Streptococcus suis

Streptococcus (S.) suis is a major cause of economic losses in the pig industry worldwide and is an emerging zoonotic pathogen. One important virulence-associated factor is suilysin (SLY), a toxin that belongs to the family of cholesterol-dependent pore-forming cytolysins (CDC). However, the precise role of SLY in host–pathogen interactions is still unclear. Here, we investigated the susceptibility of different respiratory epithelial cells to SLY, including immortalized cell lines (HEp-2 and NPTr cells), which are frequently used in in vitro studies on S. suis virulence mechanisms, as well as primary porcine respiratory cells, which represent the first line of barrier during S. suis infections. SLY-induced cell damage was determined by measuring the release of lactate dehydrogenase after infection with a virulent S. suis serotype 2 strain, its isogenic SLY-deficient mutant strain, or treatment with the recombinant protein. HEp-2 cells were most susceptible, whereas primary epithelial cells were hardly affected by the toxin. This prompted us to study possible explanations for these differences. We first investigated the binding capacity of SLY using flow cytometry analysis. Since binding and pore-formation of CDC is dependent on the membrane composition, we also determined the cellular cholesterol content of the different cell types using TLC and HPLC. Finally, we examined the ability of those cells to reseal SLY-induced pores using flow cytometry analysis. Our results indicated that the amount of membrane-bound SLY, the cholesterol content of the cells, as well as their resealing capacity all affect the susceptibility of the different cells regarding the effects of SLY. These findings underline the differences of in vitro pathogenicity models and may further help to dissect the biological role of SLY during S. suis infections.

Nature-inspired engineering of an F-type lectin for increased binding strength

Individual lectin-carbohydrate interactions are usually of low affinity. However, high avidity is frequently attained by the multivalent presentation of glycans on biological surfaces coupled with the occurrence of high order lectin oligomers or tandem repeats of lectin domains in the polypeptide. F-type lectins are l-fucose binding lectins with a typical sequence motif, HX(26)RXDX(4)R/K, whose residues participate in l-fucose binding. We previously reported the presence of a few eukaryotic F-type lectin domains with partial sequence duplication that results in the presence of two l-fucose-binding sequence motifs. We hypothesized that such partial sequence duplication would result in greater avidity of lectin-ligand interactions. Inspired by this example from Nature, we attempted to engineer a bacterial F-type lectin domain from Streptosporangium roseum to attain avid binding by mimicking partial duplication. The engineered lectin demonstrated 12-fold greater binding strength than the wild-type lectin to multivalent fucosylated glycoconjugates. However, the affinity to the monosaccharide l-fucose in solution was similar and partial sequence duplication did not result in an additional functional l-fucose binding site. We also cloned, expressed and purified a Branchiostoma floridae F-type lectin domain with naturally occurring partial sequence duplication and confirmed that the duplicated region with the F-type lectin sequence motif did not participate in l-fucose binding. We found that the greater binding strength of the engineered lectin from S. roseum was instead due to increased oligomerization. We believe that this Nature-inspired strategy might be useful for engineering lectins to improve binding strength in various applications.

An F-type lectin domain directs the activity of Streptosporangium roseum alpha-l-fucosidase

F-type lectins are phylogenetically widespread but selectively distributed fucose-binding lectins with L-fucose- and calcium-binding sequence motifs and an F-type lectin fold. Bacterial F-type lectin domains frequently occur in tandem with various protein domains in diverse architectures, indicating a possible role in directing enzyme activities or other biological functions to distinct fucosylated niches. Here, we report the biochemical characterization of a Streptosporangium roseum protein containing an F-type lectin domain in tandem with an NPCBM-associated domain and a family GH 29A alpha-l-fucosidase domain. We show that the F-type lectin domain of this protein recognizes fucosylated glycans in both α and β linkages but has high affinity for a Fuc-α-1,2-Gal motif and that the alpha-l-fucosidase domain displays hydrolytic activity on glycan substrates with α1-2 and α1-4 linked fucose. We also show that the F-type lectin domain does not have any effect on the activity of the cis-positioned alpha-l-fucosidase domain with the synthetic substrate, 4-Methylumbelliferyl-alpha-l-fucopyranoside or on inhibition of this activity by l-fucose or deoxyfuconojirimycin hydrochloride. However, the F-type lectin domain together with the NPCBM-associated domain enhances the activity of the cis-positioned alpha-l-fucosidase domain for soluble fucosylated oligosaccharide substrates. While there are many reports of glycoside hydrolase activity towards insoluble and soluble polysaccharides being enhanced by cis-positioned carbohydrate binding modules on the polypeptide, this is the first report, to our knowledge, of enhancement of activity towards aqueous, freely diffusible, small oligosaccharides. We propose a model involving structural stabilization and a bind-and-jump action mediated by the F-type lectin domain to rationalize our findings.

Effect of naturally occurring variations of the F-type lectin sequence motif on glycan binding: studies on F-type lectin domains with typical and atypical sequence motifs

The typical F-type lectin domain (FLD) has an L-fucose-binding motif [HX(26)RXDX(4)R/K] with conserved basic residues that mediate hydrogen bonding with alpha-L-fucose. About one-third of the nonredundant FLD sequences in the publicly available databases are "atypical"; they have motifs with substitutions of these critical residues and/or variations in motif length. We addressed the question if atypical FLDs with substitutions of the critical residues retain lectin activity by performing site-directed mutagenesis and assessing the glycan-binding functions of typical and atypical FLDs. Site directed mutagenesis of an L-fucose-binding FLD from Streptosporangium roseum indicated that the critical His residue could be replaced by Ser and the second Arg by Lys without complete loss of lectin activity. Mutagenesis of His to other naturally substituting residues and mutagenesis of the first Arg to the naturally substituting residues, Lys, Ile, Ser, or Cys, resulted in loss of lectin activity. Glycan binding analysis and site-directed mutagenesis of atypical FLDs from Actinomyces turicensis, and Saccharomonospora cyanea confirmed that Ser and Thr can assume the L-fucose-binding role of the critical His, and further suggested that the residue in this position is dispensable in certain FLDs. We identified, by sequence and structural analysis of atypical FLDs, a Glu residue in the complementarity determining region, CDR5 that compensates for a lack of the critical His or other appropriate polar residue in this position. We propose that FLDs lacking a typical FLD sequence motif might nevertheless retain lectin activity through the recruitment of other strategically positioned polar residues in the CDR loops. © 2018 IUBMB Life, 71(3):385-397, 2019.

Cholesterol-dependent cytolysins: from water-soluble state to membrane pore

The cholesterol-dependent cytolysins (CDCs) are a family of bacterial toxins that are important virulence factors for a number of pathogenic Gram-positive bacterial species. CDCs are secreted as soluble, stable monomeric proteins that bind specifically to cholesterol-rich cell membranes, where they assemble into well-defined ring-shaped complexes of around 40 monomers. The complex then undergoes a concerted structural change, driving a large pore through the membrane, potentially lysing the target cell. Understanding the details of this process as the protein transitions from a discrete monomer to a complex, membrane-spanning protein machine is an ongoing challenge. While many of the details have been revealed, there are still questions that remain unanswered. In this review, we present an overview of some of the key features of the structure and function of the CDCs, including the structure of the secreted monomers, the process of interaction with target membranes, and the transition from bound monomers to complete pores. Future directions in CDC research and the potential of CDCs as research tools will also be discussed.

Haemolytic actinoporins interact with carbohydrates using their lipid-binding module

Pore-forming toxins (PFTs) are proteins endowed with metamorphic properties that enable them to stably fold in water solutions as well as in cellular membranes. PFTs produce lytic pores on the plasma membranes of target cells conducive to lesions, playing key roles in the defensive and offensive molecular systems of living organisms. Actinoporins are a family of potent haemolytic toxins produced by sea anemones vigorously studied as a paradigm of α-helical PFTs, in the context of lipid-protein interactions, and in connection with nanopore technologies. We have recently reported that fragaceatoxin C (FraC), an actinoporin, engages biological membranes with a large adhesive motif allowing the simultaneous attachment of up to four lipid molecules prior to pore formation. Since actinoporins also interact with carbohydrates, we sought to understand the molecular and energetic basis of glycan recognition by FraC. By employing structural and biophysical methodologies, we show that FraC engages glycans with low affinity using its lipid-binding module. Contrary to other PFTs requiring separate domains for glycan and lipid recognition, the small single-domain actinoporins economize resources by achieving dual recognition with a single binding module. This mechanism could enhance the recruitment of actinoporins to the surface of target tissues in their marine environment.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

F-Type Lectins: A Highly Diversified Family of Fucose-Binding Proteins with a Unique Sequence Motif and Structural Fold, Involved in Self/Non-Self-Recognition

The F-type lectin (FTL) family is one of the most recent to be identified and structurally characterized. Members of the FTL family are characterized by a fucose recognition domain [F-type lectin domain (FTLD)] that displays a novel jellyroll fold ("F-type" fold) and unique carbohydrate- and calcium-binding sequence motifs. This novel lectin family comprises widely distributed proteins exhibiting single, double, or greater multiples of the FTLD, either tandemly arrayed or combined with other structurally and functionally distinct domains, yielding lectin subunits of pleiotropic properties even within a single species. Furthermore, the extraordinary variability of FTL sequences (isoforms) that are expressed in a single individual has revealed genetic mechanisms of diversification in ligand recognition that are unique to FTLs. Functions of FTLs in self/non-self-recognition include innate immunity, fertilization, microbial adhesion, and pathogenesis, among others. In addition, although the F-type fold is distinctive for FTLs, a structure-based search revealed apparently unrelated proteins with minor sequence similarity to FTLs that displayed the FTLD fold. In general, the phylogenetic analysis of FTLD sequences from viruses to mammals reveals clades that are consistent with the currently accepted taxonomy of extant species. However, the surprisingly discontinuous distribution of FTLDs within each taxonomic category suggests not only an extensive structural/functional diversification of the FTLs along evolutionary lineages but also that this intriguing lectin family has been subject to frequent gene duplication, secondary loss, lateral transfer, and functional co-option.

Giant MACPF/CDC pore forming toxins: A class of their own

Pore Forming Toxins (PFTs) represent a key mechanism for permitting the passage of proteins and small molecules across the lipid membrane. These proteins are typically produced as soluble monomers that self-assemble into ring-like oligomeric structures on the membrane surface. Following such assembly PFTs undergo a remarkable conformational change to insert into the lipid membrane. While many different protein families have independently evolved such ability, members of the Membrane Attack Complex PerForin/Cholesterol Dependent Cytolysin (MACPF/CDC) superfamily form distinctive giant β-barrel pores comprised of up to 50 monomers and up to 300Å in diameter. In this review we focus on recent advances in understanding the structure of these giant MACPF/CDC pores as well as the underlying molecular mechanisms leading to their formation. Commonalities and evolved variations of the pore forming mechanism across the superfamily are discussed. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Insights into the Evolutionary Relationships of LytA Autolysin and Ply Pneumolysin-Like Genes in Streptococcus pneumoniae and Related Streptococci

Streptococcus pneumoniae (pneumococcus) is a major human pathogen. The main pneumococcal autolysin LytA and the pneumolysin Ply are two of the bacterium's most important virulence factors. The lytA- and ply-related genes are also found in other streptococci of the Mitis group (SMG). The precise characteristics of the lytA-related-but not the ply-related-genes of SMG and their prophages have been previously described. A search of the more than 400 SMG genomic sequences available in public databases (ca. 300 for S. pneumoniae), showed Streptococcus pseudopneumoniae IS7493 to harbor four ply-related genes, two of which (plyA and plyB) have 98% identical nucleotides. The plyA homolog of S. pseudopneumoniae is conserved in all S. pneumoniae strains, and seems to be included in a pathogenicity island together with the lytA gene. However, only nonencapsulated S. pneumoniae strains possess a plyB gene, which is part of an integrative and conjugative element. Notably, the existence of a bacterial lytA-related gene in a genome is linked to the presence of plyA and vice versa. The present analysis also shows there are eight main types of plyA-lytA genomic islands. A possible stepwise scenario for the evolution of the plyA-lytA island in S. pneumoniae is proposed.

Structural biology of antibody recognition of carbohydrate epitopes and potential uses for targeted cancer immunotherapies

Monoclonal antibodies represent the most successful class of biopharmaceuticals for the treatment of cancer. Mechanisms of action of therapeutic antibodies are very diverse and reflect their ability to engage in antibody-dependent effector mechanisms, internalize to deliver cytotoxic payloads, and display direct effects on cells by lysis or by modulating the biological pathways of their target antigens. Importantly, one of the universal changes in cancer is glycosylation and carbohydrate-binding antibodies can be produced to selectively recognize tumor cells over normal tissues. A promising group of cell surface antibody targets consists of carbohydrates presented as glycolipids or glycoproteins. In this review, we outline the basic principles of antibody-based targeting of carbohydrate antigens in cancer. We also present a detailed structural view of antibody recognition and the conformational properties of a series of related tissue-blood group (Lewis) carbohydrates that are being pursued as potential targets of cancer immunotherapy.

The cholesterol-dependent cytolysins pneumolysin and streptolysin O require binding to red blood cell glycans for hemolytic activity

The cholesterol-dependent cytolysin (CDC) pneumolysin (Ply) is a key virulence factor of Streptococcus pneumoniae. Membrane cholesterol is required for the cytolytic activity of this toxin, but it is not clear whether cholesterol is the only cellular receptor. Analysis of Ply binding to a glycan microarray revealed that Ply has lectin activity and binds glycans, including the Lewis histo-blood group antigens. Surface plasmon resonance analysis showed that Ply has the highest affinity for the sialyl LewisX (sLeX) structure, with a K(d) of 1.88 × 10(-5) M. Ply hemolytic activity against human RBCs showed dose-dependent inhibition by sLeX. Flow cytometric analysis and Western blots showed that blocking binding of Ply to the sLeX glycolipid on RBCs prevents deposition of the toxin in the membrane. The lectin domain responsible for sLeX binding is in domain 4 of Ply, which contains candidate carbohydrate-binding sites. Mutagenesis of these predicted carbohydrate-binding residues of Ply resulted in a decrease in hemolytic activity and a reduced affinity for sLeX. This study reveals that this archetypal CDC requires interaction with the sLeX glycolipid cellular receptor as an essential step before membrane insertion. A similar analysis conducted on streptolysin O from Streptococcus pyogenes revealed that this CDC also has glycan-binding properties and that hemolytic activity against RBCs can be blocked with the glycan lacto-N-neotetraose by inhibiting binding to the cell surface. Together, these data support the emerging paradigm shift that pore-forming toxins, including CDCs, have cellular receptors other than cholesterol that define target cell tropism.

A novel cholesterol-insensitive mode of membrane binding promotes cytolysin-mediated translocation by Streptolysin O

Cytolysin-mediated translocation (CMT), performed by Streptococcus pyogenes, utilizes the cholesterol-dependent cytolysin Streptolysin O (SLO) to translocate the NAD(+) -glycohydrolase (SPN) into the host cell during infection. SLO is required for CMT and can accomplish this activity without pore formation, but the details of SLO's interaction with the membrane preceding SPN translocation are unknown. Analysis of binding domain mutants of SLO and binding domain swaps between SLO and homologous cholesterol-dependent cytolysins revealed that membrane binding by SLO is necessary but not sufficient for CMT, demonstrating a specific requirement for SLO in this process. Despite being the only known receptor for SLO, this membrane interaction does not require cholesterol. Depletion of cholesterol from host membranes and mutation of SLO's cholesterol recognition motif abolished pore formation but did not inhibit membrane binding or CMT. Surprisingly, SLO requires the coexpression and membrane localization of SPN to achieve cholesterol-insensitive membrane binding; in the absence of SPN, SLO's binding is characteristically cholesterol-dependent. SPN's membrane localization also requires SLO, suggesting a co-dependent, cholesterol-insensitive mechanism of membrane binding occurs, resulting in SPN translocation.

Secretome analysis reveals effector candidates associated with broad host range necrotrophy in the fungal plant pathogen Sclerotinia sclerotiorum

BACKGROUND

The white mold fungus Sclerotinia sclerotiorum is a devastating necrotrophic plant pathogen with a remarkably broad host range. The interaction of necrotrophs with their hosts is more complex than initially thought, and still poorly understood.

RESULTS

We combined bioinformatics approaches to determine the repertoire of S. sclerotiorum effector candidates and conducted detailed sequence and expression analyses on selected candidates. We identified 486 S. sclerotiorum secreted protein genes expressed in planta, many of which have no predicted enzymatic activity and may be involved in the interaction between the fungus and its hosts. We focused on those showing (i) protein domains and motifs found in known fungal effectors, (ii) signatures of positive selection, (iii) recent gene duplication, or (iv) being S. sclerotiorum-specific. We identified 78 effector candidates based on these properties. We analyzed the expression pattern of 16 representative effector candidate genes on four host plants and revealed diverse expression patterns.

CONCLUSIONS

These results reveal diverse predicted functions and expression patterns in the repertoire of S. sclerotiorum effector candidates. They will facilitate the functional analysis of fungal pathogenicity determinants and should prove useful in the search for plant quantitative disease resistance components active against the white mold.

Small-angle X-ray scattering to obtain models of multivalent lectin-glycan complexes

Recent advances in small-angle X-ray scattering (SAXS) have led to the ability to model the glycans on glycoproteins and to obtain the low-resolution solution structures of complexes of lectins bound to multivalent glycan-presenting scaffolds. This progress in SAXS can respond to the increasing interest in the biological action of glycoproteins and lectins and in the design of multivalent glycan-based antagonists. Carbohydrates make up a significant part of the X-ray scattering content in SAXS and should be included in the model together with the protein, whose structure is most often based on a crystal structure or NMR ensemble, to give a far-improved fit with the experimental data. The modeling of the spatial positioning of glycans on proteins or in the architecture of lectin-glycan complexes delivers low-resolution structural information hitherto unmatched by any other method. SAXS data on the bacterial lectin FimH, strongly bound to heptyl α-D-mannose on a sevenfold derivatized β-cyclodextrin, permitted determination of the stoichiometry of the complex and the geometry of the lectin deposition on the multivalent β-cyclodextrin. The SAXS methods can be applied to larger complexes as the technique imposes no limit on the size of the macromolecular assembly in solution.

Functional dissection of Toxoplasma gondii perforin-like protein 1 reveals a dual domain mode of membrane binding for cytolysis and parasite egress

The recently discovered role of a perforin-like protein (PLP1) for rapid host cell egress by the protozoan parasite Toxoplasma gondii expanded the functional diversity of pore-forming proteins. Whereas PLP1 was found to be necessary for rapid egress and pathogenesis, the sufficiency for and mechanism of membrane attack were yet unknown. Here we further dissected the PLP1 knock-out phenotype, the mechanism of PLP1 pore formation, and the role of each domain by genetic complementation. We found that PLP1 is sufficient for membrane disruption and has a conserved mechanism of pore formation through target membrane binding and oligomerization to form large, multimeric membrane-embedded complexes. The highly conserved, central MACPF domain and the β-sheet-rich C-terminal domain were required for activity. Loss of the unique N-terminal extension reduced lytic activity and led to a delay in rapid egress, but did not significantly decrease virulence, suggesting that small amounts of lytic activity are sufficient for pathogenesis. We found that both N- and C-terminal domains have membrane binding activity, with the C-terminal domain being critical for function. This dual mode of membrane association may promote PLP1 activity and parasite egress in the diverse cell types in which this parasite replicates.

Manipulating the Lewis antigen specificity of the cholesterol-dependent cytolysin lectinolysin

The cholesterol-dependent cytolysins (CDCs) attack cells by punching large holes in their membranes. Lectinolysin from Streptococcus mitis is unique among CDCs due to the presence of an N-terminal lectin domain that enhances the pore-forming activity of the toxin. We recently determined the crystal structures of the lectin domain in complex with various glycans. These structures revealed the molecular basis for the Lewis antigen specificity of the toxin. Based on this information we have used in silico molecular modeling to design a mutant toxin, which we predicted would increase its specificity for Lewis y, an antigen found on the surface of cancer cells. Surprisingly, we found by surface plasmon resonance binding experiments that the resultant mutant lectin domain exhibited higher specificity for Lewis b antigens instead. We then undertook comparative crystallographic and molecular dynamics simulation studies of the wild-type and mutant lectin domains to understand the molecular basis for the disparity between the theoretical and experimental results. The crystallographic results revealed that the net number of interactions between Lewis y and wild-type versus mutant was unchanged whereas there was a loss of a hydrogen bond between mutant and Lewis b compared to wild-type. In contrast, the molecular dynamics studies revealed that the Lewis b antigen spent more time in the binding pocket of the mutant compared to wild-type and the reverse was true for Lewis y. The results of these simulation studies are consistent with the conclusions drawn from the surface plasmon resonance studies. This work is part of a program to engineer lectinolysin so that it will target and kill specific cells in human diseases.

Insights into the recognition of the human glycome by microbial carbohydrate-binding modules

Mammalian glycans are often very complex and consequently both commensal bacteria and bacterial pathogens have developed specialized and often elaborate carbohydrate-active enzyme (CAZyme) systems to interact with these sugars. These enzymes are frequently multimodular, with modular functions most often conferring catalysis (glycoside hydrolase catalytic modules) or carbohydrate-binding (carbohydrate-binding modules or CBMs). Structure-function studies of five CBM families are revealing specificities for complex mammalian carbohydrates. Three of these CBM families (32, 47, and 51) show significant structural identity between their β-sandwich folds, suggesting a shared evolutionary precursor, but have divergent binding specificities. The family 40 and 41 CBMs recognize sialic acid and glycogen, respectively, through different modes of sugar binding, though they also adopt all β-structure folds. A structural view of new models generated for complete CAZymes suggests three distinct modes of CBM deployment: (i) formation of the catalytic site, (ii) coordinated catalysis and binding, and (iii) general substrate adherence.

Host glycan recognition by a pore forming toxin

An exposed F-type lectin domain fused to the N-terminus of a cholesterol-dependent cytolysin scaffold allows Streptococcus mitis lectinolysin to cluster at fucose-rich sites on target cell membranes, thereby leading to increased pore-forming toxin activity. In this issue of Structure, Feil and coworkers define the structural basis for lectinolysin glycan-binding specificity.

Lewis histo-blood group α1,3/α1,4 fucose residues may both mediate binding to GII.4 noroviruses

Human noroviruses cause recurrent epidemics of gastroenteritis known to be dominated by the clinically important GII.4 genotype which recognizes human Secretor gene-dependent ABH histo-blood group antigens (HBGAs) as attachment factors. There is increasing evidence that GII.4 noroviruses have undergone evolutionary changes to recognize Lewis antigens and non-Secretor saliva. In this study, we have investigated the possibilities of the Lewis α1,3/α1,4 fucoses as mediators of binding of GII.4 noroviruses to Lewis antigens. The study was carried out using molecular dynamics simulations of Lewis type-1 and type-2 chain HBGAs in complex with VA387 P domain dimers in explicit water. Based on the computer simulations, we suggest the possibility of two receptor binding modes for Lewis HBGAs: the "Secretor pose" with the Secretor Fucα1,2 in the binding site and the "Lewis pose" with the Lewis Fucα1,3/α1,4 residues in the binding site. This was further supported by an extensive GlyVicinity analysis of the Protein Data Bank with respect to the occurrence of the Lewis and Secretor poses in complexes of Lewis antigens with lectins and antibodies as well as GII norovirus strains. The Lewis pose can also explain the interactions of GII.4 norovirus strains with Le(x) and SLe(x) structures. Moreover, the present model suggests binding of complex branched polysaccharides, with the Lewis antigens at the nonreducing end, to P domain dimers of GII.4 strains. Our results are relevant for understanding the evolution of norovirus binding specificities and for in silico design of future antiviral therapeutics.