Amaranthin-Like Proteins with Aerolysin Domains in Plants

Phylogeny, envenomation syndrome, and membrane permeabilising venom produced by Australia's electric caterpillar Comana monomorpha

Zygaenoidea is a superfamily of lepidopterans containing many venomous species, including the Limacodidae (nettle caterpillars) and Megalopygidae (asp caterpillars). Venom proteomes have been recently documented for several species from each of these families, but further data are required to understand the evolution of venom in Zygaenoidea. In this study, we examined the 'electric' caterpillar from North-Eastern Australia, a limacodid caterpillar densely covered in venomous spines. We used DNA barcoding to identify this caterpillar as the larva of the moth Comana monomorpha (Turner, 1904). We report the clinical symptoms of C. monomorpha envenomation, which include acute pain, and erythema and oedema lasting for more than a week. Combining transcriptomics of venom spines with proteomics of venom harvested from the spine tips revealed a venom markedly different in composition from previously examined limacodid venoms that are rich in peptides. In contrast, the venom of C. monomorpha is rich in aerolysin-like proteins similar to those found in venoms of asp caterpillars (Megalopygidae). Consistent with this composition, the venom potently permeabilises sensory neurons and human neuroblastoma cells. This study highlights the diversity of venom composition in Limacodidae.

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.

Mannose-specific plant and microbial lectins as antiviral agents: A review

Lectins are non-immunological carbohydrate-binding proteins classified on the basis of their structure, origin, and sugar specificity. The binding specificity of such proteins with the surface glycan moiety determines their activity and clinical applications. Thus, lectins hold great potential as diagnostic and drug discovery agents and as novel biopharmaceutical products. In recent years, significant advancements have been made in understanding plant and microbial lectins as therapeutic agents against various viral diseases. Among them, mannose-specific lectins have being proven as promising antiviral agents against a variety of viruses, such as HIV, Influenza, Herpes, Ebola, Hepatitis, Severe Acute Respiratory Syndrome Coronavirus-1 (SARS-CoV-1), Middle Eastern Respiratory Syndrome Coronavirus (MERS-CoV) and most recent Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2). The binding of mannose-binding lectins (MBLs) from plants and microbes to high-mannose containing N-glycans (which may be simple or complex) of glycoproteins found on the surface of viruses has been found to be highly specific and mainly responsible for their antiviral activity. MBLs target various steps in the viral life cycle, including viral attachment, entry and replication. The present review discusses the brief classification and structure of lectins along with antiviral activity of various mannose-specific lectins from plants and microbial sources and their diagnostic and therapeutic applications against viral diseases.

Taxonomic Distribution and Molecular Evolution of Mytilectins

R-type lectins are a widespread group of sugar-binding proteins found in nearly all domains of life, characterized by the presence of a carbohydrate-binding domain that adopts a β-trefoil fold. Mytilectins represent a recently described subgroup of β-trefoil lectins, which have been functionally characterized in a few mussel species (Mollusca, Bivalvia) and display attractive properties, which may fuel the development of artificial lectins with different biotechnological applications. The detection of different paralogous genes in mussels, together with the description of orthologous sequences in brachiopods, supports the formal description of mytilectins as a gene family. However, to date, an investigation of the taxonomic distribution of these lectins and their molecular diversification and evolution was still lacking. Here, we provide a comprehensive overview of the evolutionary history of mytilectins, revealing an ancient monophyletic evolutionary origin and a very broad but highly discontinuous taxonomic distribution, ranging from heteroscleromorphan sponges to ophiuroid and crinoid echinoderms. Moreover, the overwhelming majority of mytilectins display a chimera-like architecture, which combines the β-trefoil carbohydrate recognition domain with a C-terminal pore-forming domain, suggesting that the simpler structure of most functionally characterized mytilectins derives from a secondary domain loss.

Amphibian pore-forming protein βγ-CAT drives extracellular nutrient scavenging under cell nutrient deficiency

Nutrient acquisition is essential for animal cells. βγ-CAT is a pore-forming protein (PFP) and trefoil factor complex assembled under tight regulation identified in toad Bombina maxima. Here, we reported that B. maxima cells secreted βγ-CAT under glucose, glutamine, and pyruvate deficiency to scavenge extracellular proteins for their nutrient supply and survival. AMPK signaling positively regulated the expression and secretion of βγ-CAT. The PFP complex selectively bound extracellular proteins and promoted proteins uptake through endolysosomal pathways. Elevated intracellular amino acids, enhanced ATP production, and eventually prolonged cell survival were observed in the presence of βγ-CAT and extracellular proteins. Liposome assays indicated that high concentration of ATP negatively regulated the opening of βγ-CAT channels. Collectively, these results uncovered that βγ-CAT is an essential element in cell nutrient scavenging under cell nutrient deficiency by driving vesicular uptake of extracellular proteins, providing a new paradigm for PFPs in cell nutrient acquisition and metabolic flexibility.

IgG Fc-binding protein positively regulates the assembly of pore-forming protein complex βγ-CAT evolved to drive cell vesicular delivery and transport

Cell membranes form barriers for molecule exchange between the cytosol and the extracellular environments. βγ-CAT, a complex of pore-forming protein (PFP) BmALP1 (two βγ-crystallin domains with an aerolysin pore-forming domain) and the trefoil factor BmTFF3, has been identified in toad Bombina maxima. It plays pivotal roles, via inducing channel formation in various intra- or extra- cellular vesicles, as well as in nutrient acquisition, maintaining water balance, and antigen presentation. Thus, such a protein machine should be tightly regulated. Indeed, BmALP3 (a paralog of BmALP1) oxidizes BmALP1 to form a water-soluble polymer, leading to dissociation of the βγ-CAT complex and loss of biological activity. Here, we found that the B. maxima IgG Fc-binding protein (FCGBP), a well-conserved vertebrate mucin-like protein with unknown functions, acted as a positive regulator for βγ-CAT complex assembly. The interactions among FCGBP, BmALP1, and BmTFF3 were revealed by co-immunoprecipitation assays. Interestingly, FCGBP reversed the inhibitory effect of BmALP3 on the βγ-CAT complex. Furthermore, FCGBP reduced BmALP1 polymers and facilitated the assembly of βγ-CAT with the biological pore-forming activity in the presence of BmTFF3. Our findings define the role of FCGBP in mediating the assembly of a PFP machine evolved to drive cell vesicular delivery and transport.

Pore-forming protein βγ-CAT promptly responses to fasting with capacity to deliver macromolecular nutrients

During animal fasting, the nutrient supply and metabolism switch from carbohydrates to a new reliance on the catabolism of energy-dense lipid stores. Assembled under tight regulation, βγ-CAT (a complex of non-lens βγ-crystallin and trefoil factor) is a pore-forming protein and trefoil factor complex identified in toad Bombina maxima. Here, we determined that this protein complex is a constitutive component in toad blood, that actively responds to the animal fasting. The protein complex was able to promote cellular albumin and albumin-bound fatty acid (FA) uptake in a variety of epithelial and endothelial cells, and the effects were attenuated by a macropinocytosis inhibitor. Endothelial cell-derived exosomes containing largely enriched albumin and FAs, called nutrisomes, were released in the presence of βγ-CAT. These specific nutrient vesicles were readily taken up by starved myoblast cells to support their survival. The results uncovered that pore-forming protein βγ-CAT is a fasting responsive element able to drive cell vesicular import and export of macromolecular nutrients.

The choanoflagellate pore-forming lectin SaroL-1 punches holes in cancer cells by targeting the tumor-related glycosphingolipid Gb3

Choanoflagellates are primitive protozoa used as models for animal evolution. They express a large variety of multi-domain proteins contributing to adhesion and cell communication, thereby providing a rich repertoire of molecules for biotechnology. Adhesion often involves proteins adopting a β-trefoil fold with carbohydrate-binding properties therefore classified as lectins. Sequence database screening with a dedicated method resulted in TrefLec, a database of 44714 β-trefoil candidate lectins across 4497 species. TrefLec was searched for original domain combinations, which led to single out SaroL-1 in the choanoflagellate Salpingoeca rosetta, that contains both β-trefoil and aerolysin-like pore-forming domains. Recombinant SaroL-1 is shown to bind galactose and derivatives, with a stronger affinity for cancer-related α-galactosylated epitopes such as the glycosphingolipid Gb3, when embedded in giant unilamellar vesicles or cell membranes. Crystal structures of complexes with Gb3 trisaccharide and GalNAc provided the basis for building a model of the oligomeric pore. Finally, recognition of the αGal epitope on glycolipids required for hemolysis of rabbit erythrocytes suggests that toxicity on cancer cells is achieved through carbohydrate-dependent pore-formation.

A curated lectin database, structural characterization, and in vitro assays show that choanoflagellate lectin SaroL-1 binds to cancer-related α-galactosylated epitopes and can be toxic to cancer cells through a carbohydrate-dependent pore-formation mechanism.

Search for host defense markers uncovers an apple agglutination factor corresponding with fire blight resistance

Pathenogenesis-related (PR) proteins are extensively used as molecular markers to dissect the signaling cascades leading to plant defense responses. However, studies focusing on the biochemical or biological properties of these proteins remain rare. Here, we identify and characterize a class of apple (Malus domestica) PR proteins, named M. domestica AGGLUTININS (MdAGGs), belonging to the amaranthin-like lectin family. By combining molecular and biochemical approaches, we show that abundant production of MdAGGs in leaf tissues corresponds with enhanced resistance to the bacterium Erwinia amylovora, the causal agent of the disease fire blight. We also show that E. amylovora represses the expression of MdAGG genes by injecting the type 3 effector DspA/E into host cells and by secreting bacterial exopolysaccharides. Using a purified recombinant MdAGG, we show that the protein agglutinates E. amylovora cells in vitro and binds bacterial lipopolysaccharides at low pH, conditions reminiscent of the intercellular pH occurring in planta upon E. amylovora infection. We finally provide evidence that negatively charged polysaccharides, such as the free exopolysaccharide amylovoran progressively released by the bacteria, act as decoys relying on charge-charge interaction with the MdAGG to inhibit agglutination. Overall, our results suggest that the production of this particular class of PR proteins may contribute to apple innate immunity mechanisms active against E. amylovora.

Functional Characterization of an Amaranth Natterin-4-Like-1 Gene in Arabidopsis thaliana

The functional characterization of an Amaranthus hypochondriacus Natterin-4-Like-1 gene (AhN4L-1) coding for an unknown function protein characterized by the presence of an aerolysin-like pore-forming domain in addition to two amaranthin-like agglutinin domains is herewith described. Natterin and nattering-like proteins have been amply described in the animal kingdom. However, the role of nattering-like proteins in plants is practically unknown. The results described in this study, obtained from gene expression data in grain amaranth and from AhN4L-1-overexpressing Arabidopsis thaliana plants indicated that this gene was strongly induced by several biotic and abiotic conditions in grain amaranth, whereas data obtained from the overexpressing Arabidopsis plants further supported the defensive function of this gene, mostly against bacterial and fungal plant pathogens. GUS and GFP AhN4L-1 localization in roots tips, leaf stomata, stamens and pistils also suggested a defensive function in these organs, although its participation in flowering processes, such as self-incompatibility and abscission, is also possible. However, contrary to expectations, the overexpression of this gene negatively affected the vegetative and reproductive growth of the transgenic plants, which also showed no increased tolerance to salinity and water-deficit stress. The latter despite the maintenance of significantly higher chlorophyll levels and photosynthetic parameters under intense salinity stress. These results are discussed in the context of the physiological roles known to be played by related lectins and AB proteins in plants.

Using FIBexDB for In-Depth Analysis of Flax Lectin Gene Expression in Response to Fusarium oxysporum Infection

Plant proteins with lectin domains play an essential role in plant immunity modulation, but among a plurality of lectins recruited by plants, only a few members have been functionally characterized. For the analysis of flax lectin gene expression, we used FIBexDB, which includes an efficient algorithm for flax gene expression analysis combining gene clustering and coexpression network analysis. We analyzed the lectin gene expression in various flax tissues, including root tips infected with Fusarium oxysporum. Two pools of lectin genes were revealed: downregulated and upregulated during the infection. Lectins with suppressed gene expression are associated with protein biosynthesis (Calreticulin family), cell wall biosynthesis (galactose-binding lectin family) and cytoskeleton functioning (Malectin family). Among the upregulated lectin genes were those encoding lectins from the Hevein, Nictaba, and GNA families. The main participants from each group are discussed. A list of lectin genes, the expression of which can determine the resistance of flax, is proposed, for example, the genes encoding amaranthins. We demonstrate that FIBexDB is an efficient tool both for the visualization of data, and for searching for the general patterns of lectin genes that may play an essential role in normal plant development and defense.

The Natterin Proteins Diversity: A Review on Phylogeny, Structure, and Immune Function

Since the first record of the five founder members of the group of Natterin proteins in the venom of the medically significant fish Thalassophryne nattereri, new sequences have been identified in other species. In this work, we performed a detailed screening using available genome databases across a wide range of species to identify sequence members of the Natterin group, sequence similarities, conserved domains, and evolutionary relationships. The high-throughput tools have enabled us to dramatically expand the number of members within this group of proteins, which has a remote origin (around 400 million years ago) and is spread across Eukarya organisms, even in plants and primitive Agnathans jawless fish. Overall, the survey resulted in 331 species presenting Natterin-like proteins, mainly fish, and 859 putative genes. Besides fish, the groups with more species included in our analysis were insects and birds. The number and variety of annotations increased the knowledge of the obtained sequences in detail, such as the conserved motif AGIP in the pore-forming loop involved in the transmembrane barrel insertion, allowing us to classify them as important constituents of the innate immune defense system as effector molecules activating immune cells by interacting with conserved intracellular signaling mechanisms in the hosts.

Animal secretory endolysosome channel discovery

Secretory pore-forming proteins (PFPs) have been identified in organisms from all kingdoms of life. Our studies with the toad species Bombina maxima found an interaction network among aerolysin family PFPs (af-PFPs) and trefoil factors (TFFs). As a toad af-PFP, BmALP1 can be reversibly regulated between active and inactive forms, with its paralog BmALP3 acting as a negative regulator. BmALP1 interacts with BmTFF3 to form a cellular active complex called βγ-CAT. This PFP complex is characterized by acting on endocytic pathways and forming pores on endolysosomes, including stimulating cell macropinocytosis. In addition, cell exocytosis can be induced and/or modulated in the presence of βγ-CAT. Depending on cell contexts and surroundings, these effects can facilitate the toad in material uptake and vesicular transport, while maintaining mucosal barrier function as well as immune defense. Based on experimental evidence, we hereby propose a secretory endolysosome channel (SELC) pathway conducted by a secreted PFP in cell endocytic and exocytic systems, with βγ-CAT being the first example of a SELC protein. With essential roles in cell interactions and environmental adaptations, the proposed SELC protein pathway should be conserved in other living organisms.

Gene Expression Patterns for Proteins With Lectin Domains in Flax Stem Tissues Are Related to Deposition of Distinct Cell Wall Types

The genomes of higher plants encode a variety of proteins with lectin domains that are able to specifically recognize certain carbohydrates. Plants are enriched in a variety of potentially complementary glycans, many of which are located in the cell wall. We performed a genome-wide search for flax proteins with lectin domains and compared the expression of the encoding genes in different stem tissues that have distinct cell wall types with different sets of major polysaccharides. Over 400 genes encoding proteins with lectin domains that belong to different families were revealed in the flax genome; three quarters of these genes were expressed in stem tissues. Hierarchical clustering of the data for all expressed lectins grouped the analyzed samples according to their characteristic cell wall type. Most lectins differentially expressed in tissues with primary, secondary, and tertiary cell walls were predicted to localize at the plasma membrane or cell wall. These lectins were from different families and had various architectural types. Three out of four flax genes for proteins with jacalin-like domains were highly upregulated in bast fibers at the stage of tertiary cell wall deposition. The dynamic changes in transcript level of many genes for lectins from various families were detected in stem tissue over the course of gravitropic response induced by plant gravistimulation. The data obtained in this study indicate a large number of lectin-mediated events in plants and provide insight into the proteins that take part in tissue specialization and reaction to abiotic stress.

130 years of Plant Lectin Research

Lectins are proteins with diverse molecular structures that share the ability to recognize and bind specifically and reversibly to carbohydrate structures without changing the carbohydrate moiety. The history of lectins started with the discovery of ricin about 130 years ago but since then our understanding of lectins has dramatically changed. Over the years the research focus was shifted from 'the characterization of carbohydrate-binding proteins' to 'understanding the biological function of lectins'. Nowadays plant lectins attract a lot of attention especially because of their potential for crop improvement and biomedical research, as well as their application as tools in glycobiology. The present review aims to give an overview of plant lectins and their applications, and how the field evolved in the last decades.

A cellular endolysosome-modulating pore-forming protein from a toad is negatively regulated by its paralog under oxidizing conditions

Endolysosomes are key players in cell physiology, including molecular exchange, immunity, and environmental adaptation. They are the molecular targets of some pore-forming aerolysin-like proteins (ALPs) that are widely distributed in animals and plants and are functionally related to bacterial toxin aerolysins. βγ-CAT is a complex of an ALP (BmALP1) and a trefoil factor (BmTFF3) in the firebelly toad (Bombina maxima). It is the first example of a secreted endogenous pore-forming protein that modulates the biochemical properties of endolysosomes by inducing pore formation in these intracellular vesicles. Here, using a large array of biochemical and cell biology methods, we report the identification of BmALP3, a paralog of BmALP1 that lacks membrane pore-forming capacity. We noted that both BmALP3 and BmALP1 contain a conserved cysteine in their C-terminal regions. BmALP3 was readily oxidized to a disulfide bond-linked homodimer, and this homodimer then oxidized BmALP1 via disulfide bond exchange, resulting in the dissociation of βγ-CAT subunits and the elimination of biological activity. Consistent with its behavior in vitro, BmALP3 sensed environmental oxygen tension in vivo, leading to modulation of βγ-CAT activity. Interestingly, we found that this C-terminal cysteine site is well conserved in numerous vertebrate ALPs. These findings uncover the existence of a regulatory ALP (BmALP3) that modulates the activity of an active ALP (BmALP1) in a redox-dependent manner, a property that differs from those of bacterial toxin aerolysins.

Role of Protein Glycosylation in Host-Pathogen Interaction

Host-pathogen interactions are fundamental to our understanding of infectious diseases. Protein glycosylation is one kind of common post-translational modification, forming glycoproteins and modulating numerous important biological processes. It also occurs in host-pathogen interaction, affecting host resistance or pathogen virulence often because glycans regulate protein conformation, activity, and stability, etc. This review summarizes various roles of different glycoproteins during the interaction, which include: host glycoproteins prevent pathogens as barriers; pathogen glycoproteins promote pathogens to attack host proteins as weapons; pathogens glycosylate proteins of the host to enhance virulence; and hosts sense pathogen glycoproteins to induce resistance. In addition, this review also intends to summarize the roles of lectin (a class of protein entangled with glycoprotein) in host-pathogen interactions, including bacterial adhesins, viral lectins or host lectins. Although these studies show the importance of protein glycosylation in host-pathogen interaction, much remains to be discovered about the interaction mechanism.

Glabralysins, Potential New β-Pore-Forming Toxin Family Members from the Schistosomiasis Vector Snail Biomphalaria glabrata

Biomphalaria glabrata is a freshwater Planorbidae snail. In its environment, this mollusk faces numerous microorganisms or pathogens, and has developed sophisticated innate immune mechanisms to survive. The mechanisms of recognition are quite well understood in Biomphalaria glabrata, but immune effectors have been seldom described. In this study, we analyzed a new family of potential immune effectors and characterized five new genes that were named Glabralysins. The five Glabralysin genes showed different genomic structures and the high degree of amino acid identity between the Glabralysins, and the presence of the conserved ETX/MTX2 domain, support the hypothesis that they are pore-forming toxins. In addition, tertiary structure prediction confirms that they are structurally related to a subset of Cry toxins from Bacillus thuringiensis, including Cry23, Cry45, and Cry51. Finally, we investigated their gene expression profiles in snail tissues and demonstrated a mosaic transcription. We highlight the specificity in Glabralysin expression following immune stimulation with bacteria, yeast or trematode parasites. Interestingly, one Glabralysin was found to be expressed in immune-specialized hemocytes, and two others were induced following parasite exposure.

Messages From the Past: New Insights in Plant Lectin Evolution

Lectins are a large and diverse class of proteins, found in all kingdoms of life. Plants are known to express different types of carbohydrate-binding proteins, each containing at least one particular lectin domain which enables them to specifically recognize and bind carbohydrate structures. The group of plant lectins is heterogeneous in terms of structure, biological activity and function. Lectins control various aspects of plant development and defense. Some lectins facilitate recognition of exogenous danger signals or play a role in endogenous signaling pathways, while others are considered as storage proteins or involved in symbiotic relationships. In this study, we revisit the origin of the different plant lectin families in view of the recently reshaped tree of life. Due to new genomic sampling of previously unknown microbial lineages, the tree of life has expanded and was reshaped multiple times. In addition, more plant genomes especially from basal Phragmoplastophyta, bryophytes, and Salviniales (e.g., Chara braunii, Marchantia polymorpha, Physcomitrella patens, Azolla filiculoides, and Salvinia cucullata) have been analyzed, and annotated genome sequences have become accessible. We searched 38 plant genome sequences including core eudicots, monocots, gymnosperms, fern, lycophytes, bryophytes, charophytes, chlorophytes, glaucophytes, and rhodophytes for lectin motifs, performed an extensive comparative analysis of lectin domain architectures, and determined the phylogenetic and evolutionary history of lectins in the plant lineage. In conclusion, we describe the conservation of particular domains in plant lectin sequences obtained from algae to higher plants. The strong conservation of several lectin motifs highlights their significance for plants.

Acibenzolar-S-Methyl Reprograms Apple Transcriptome Toward Resistance to Rosy Apple Aphid

Acibenzolar-S-methyl (ASM) is a chemical compound, which is able to induce resistance in several model and non-model plants, but the end-players of this induced defense remain ill-defined. Here, we test the hypothesis that treatment with ASM can protect apple (Malus × domestica) against the rosy apple aphid (Dysaphis plantaginea) and investigate the defense molecules potentially involved in resistance. We measured aphid life traits and performed behavioral assays to study the effect of ASM on plant resistance against the aphid, and then combined transcriptomic, bioinformatics, metabolic and biochemical analyses to identify the plant compounds involved in resistance. Plants treated with ASM negatively affected several life traits of the aphid and modified its feeding and host seeking behaviors. ASM treatment elicited up-regulation of terpene synthase genes in apple and led to the emission of (E,E)-α-farnesene, a sesquiterpene that was repellent to the aphid. Several genes encoding amaranthin-like lectins were also strongly up-regulated upon treatment and the corresponding proteins accumulated in leaves, petioles and stems. Our results link the production of specific apple proteins and metabolites to the antibiosis and antixenosis effects observed against Dysaphis plantaginea, providing insight into the mechanisms underlying ASM-induced herbivore resistance.