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Toxins
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Bacterial Toxins: Protein Folding and Membrane Interactions
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Bacterial Toxins: Protein Folding and Membrane Interactions

A special issue of Toxins (ISSN 2072-6651). This special issue belongs to the section "Bacterial Toxins".

Deadline for manuscript submissions: closed (31 March 2023) | Viewed by 36560

Special Issue Editor

Dr. Alexandre Chenal

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Guest Editor
Institut Pasteur Paris
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Bacterial toxins foster infection and disease by altering host tissues and by subverting the host immune response. They diffuse through compartments of various compositions, may cross several membranes to reach their target location, and finally have to properly fold into a functional state. Hence, large conformational changes occur between the sites of toxin biosynthesis and their target locations. Recent results illustrate how bacterial toxins are characterized by structural flexibility, which is essential at various steps, such as toxin secretion, folding, insertion into host membranes, transport across membranes into target cells, etc.

These structural transitions are finely tuned to the environmental conditions that bacterial toxins successively experience along their journey from bacterium cytoplasm to target location. For instance, they are able to unfold to go through narrow channels of bacterial secretion systems. Some bacterial toxins interact with host membranes due to the presence of particular lipids that confer properties modulating membrane fluidity, curvature, and charge; these properties switch these bacterial toxins from a resting to an active state. Some toxins are also sensitive to the presence of a membrane potential, a cell receptor, an electrochemical gradient, lipid asymmetry, etc. These environmental parameters trigger conformational changes of bacterial toxins required for host cell intoxication.

Finally, recent methodological advances and scientific results in bacterial toxins have opened new perspectives for basic sciences and toxin-based biotechnological applications. Taken together, the aim of this Special Issue of Toxins is to discuss these various aspects on folding and membrane interactions of bacterial toxins.

Dr. Alexandre Chenal
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a double-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Toxins is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • bacterial toxins
  • protein folding
  • structural disorder
  • membrane-induced conformational change
  • toxin membrane interactions
  • membrane insertion of toxins
  • toxin translocation across membrane
  • membrane-induced shape-shifting toxins
  • Amphitropic toxins

Published Papers (10 papers)

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Research

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12 pages, 2855 KiB  
Article
Mutation of a Threonine Residue in αD-β4 Loop of Cyt2Aa2 Protein Influences Binding on Fluid Lipid Membranes
by Chontida Tangsongcharoen, Jose L. Toca-Herrera, Boonhiang Promdonkoy and Sudarat Tharad
Toxins 2023, 15(2), 167; https://0-doi-org.brum.beds.ac.uk/10.3390/toxins15020167 - 19 Feb 2023
Viewed by 1257
Abstract
Cyt proteins are insecticidal proteins originally from Bacillus thuringiensis. The lipid binding of the Cyt2Aa2 protein depends on the phase of the lipid bilayer. In this work, the importance of the conserved T144 residue in the αD-β4 loop for lipid binding on [...] Read more.
Cyt proteins are insecticidal proteins originally from Bacillus thuringiensis. The lipid binding of the Cyt2Aa2 protein depends on the phase of the lipid bilayer. In this work, the importance of the conserved T144 residue in the αD-β4 loop for lipid binding on fluid lipid membranes was investigated via atomic force microscopy (AFM). Lipid membrane fluidity could be monitored for the following lipid mixture systems: POPC/DPPC, POPC/SM, and DOPC/SM. AFM results revealed that the T144A mutant was unable to bind to pure POPC bilayers. Similar topography between the wildtype and T144A mutant was seen for the POPC/Chol system. Small aggregates of T144A mutant were observed in the POPC and DOPC domains of the lipid mixture systems. In addition, the T144A mutant had no cytotoxic effect against human colon cancer cells. These results suggest that alanine replacement into threonine 144 hinders the binding of Cyt2Aa2 on liquid lipid membranes. These observations provide a possibility to modify the Cyt2Aa2 protein to specific cells via lipid phase selection. Full article
(This article belongs to the Special Issue Bacterial Toxins: Protein Folding and Membrane Interactions)
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15 pages, 2361 KiB  
Article
Different Biological Activities of Histidine-Rich Peptides Are Favored by Variations in Their Design
by Morane Lointier, Candice Dussouillez, Elise Glattard, Antoine Kichler and Burkhard Bechinger
Toxins 2021, 13(5), 363; https://0-doi-org.brum.beds.ac.uk/10.3390/toxins13050363 - 20 May 2021
Cited by 5 | Viewed by 2641
Abstract
The protein transduction and antimicrobial activities of histidine-rich designer peptides were investigated as a function of their sequence and compared to gene transfection, lentivirus transduction and calcein release activities. In membrane environments, the peptides adopt helical conformations where the positioning of the histidine [...] Read more.
The protein transduction and antimicrobial activities of histidine-rich designer peptides were investigated as a function of their sequence and compared to gene transfection, lentivirus transduction and calcein release activities. In membrane environments, the peptides adopt helical conformations where the positioning of the histidine side chains defines a hydrophilic angle when viewed as helical wheel. The transfection of DNA correlates with calcein release in biophysical experiments, being best for small hydrophilic angles supporting a model where lysis of the endosomal membrane is the limiting factor. In contrast, antimicrobial activities show an inverse correlation suggesting that other interactions and mechanisms dominate within the bacterial system. Furthermore, other derivatives control the lentiviral transduction enhancement or the transport of proteins into the cells. Here, we tested the transport into human cell lines of luciferase (63 kDa) and the ribosome-inactivating toxin saporin (30 kDa). Notably, depending on the protein, different peptide sequences are required for the best results, suggesting that the interactions are manifold and complex. As such, designed LAH4 peptides assure a large panel of biological and biophysical activities whereby the optimal result can be tuned by the physico-chemical properties of the sequences. Full article
(This article belongs to the Special Issue Bacterial Toxins: Protein Folding and Membrane Interactions)
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16 pages, 2099 KiB  
Article
Pathogenic Pore Forming Proteins of Plasmodium Triggers the Necrosis of Endothelial Cells Attributed to Malaria Severity
by Abhishek Shivappagowdar, Swati Garg, Akriti Srivastava, Rahul S. Hada, Inderjeet Kalia, Agam P. Singh, Lalit C. Garg, Soumya Pati and Shailja Singh
Toxins 2021, 13(1), 62; https://0-doi-org.brum.beds.ac.uk/10.3390/toxins13010062 - 15 Jan 2021
Cited by 2 | Viewed by 2436
Abstract
Severe malaria caused by Plasmodium falciparum poses a major global health problem with high morbidity and mortality. P. falciparum harbors a family of pore-forming proteins (PFPs), known as perforin like proteins (PLPs), which are structurally equivalent to prokaryotic PFPs. These PLPs are secreted [...] Read more.
Severe malaria caused by Plasmodium falciparum poses a major global health problem with high morbidity and mortality. P. falciparum harbors a family of pore-forming proteins (PFPs), known as perforin like proteins (PLPs), which are structurally equivalent to prokaryotic PFPs. These PLPs are secreted from the parasites and, they contribute to disease pathogenesis by interacting with host cells. The severe malaria pathogenesis is associated with the dysfunction of various barrier cells, including endothelial cells (EC). Several factors, including PLPs secreted by parasites, contribute to the host cell dysfunction. Herein, we have tested the hypothesis that PLPs mediate dysfunction of barrier cells and might have a role in disease pathogenesis. We analyzed various dysfunctions in barrier cells following rPLP2 exposure and demonstrate that it causes an increase in intracellular Ca2+ levels. Additionally, rPLP2 exposed barrier cells displayed features of cell death, including Annexin/PI positivity, depolarized the mitochondrial membrane potential, and ROS generation. We have further performed the time-lapse video microscopy of barrier cells and found that the treatment of rPLP2 triggers their membrane blebbing. The cytoplasmic localization of HMGB1, a marker of necrosis, further confirmed the necrotic type of cell death. This study highlights the role of parasite factor PLP in endothelial dysfunction and provides a rationale for the design of adjunct therapies against severe malaria. Full article
(This article belongs to the Special Issue Bacterial Toxins: Protein Folding and Membrane Interactions)
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12 pages, 3872 KiB  
Article
Structure of the Diphtheria Toxin at Acidic pH: Implications for the Conformational Switching of the Translocation Domain
by Mykola V. Rodnin, Maithri M. Kashipathy, Alexander Kyrychenko, Kevin P. Battaile, Scott Lovell and Alexey S. Ladokhin
Toxins 2020, 12(11), 704; https://0-doi-org.brum.beds.ac.uk/10.3390/toxins12110704 - 07 Nov 2020
Cited by 6 | Viewed by 3642
Abstract
Diphtheria toxin, an exotoxin secreted by Corynebacterium that causes disease in humans by inhibiting protein synthesis, enters the cell via receptor-mediated endocytosis. The subsequent endosomal acidification triggers a series of conformational changes, resulting in the refolding and membrane insertion of the translocation (T-)domain [...] Read more.
Diphtheria toxin, an exotoxin secreted by Corynebacterium that causes disease in humans by inhibiting protein synthesis, enters the cell via receptor-mediated endocytosis. The subsequent endosomal acidification triggers a series of conformational changes, resulting in the refolding and membrane insertion of the translocation (T-)domain and ultimately leading to the translocation of the catalytic domain into the cytoplasm. Here, we use X-ray crystallography along with circular dichroism and fluorescence spectroscopy to gain insight into the mechanism of the early stages of pH-dependent conformational transition. For the first time, we present the high-resolution structure of the diphtheria toxin at a mildly acidic pH (5–6) and compare it to the structure at neutral pH (7). We demonstrate that neither catalytic nor receptor-binding domains change their structure upon this acidification, while the T-domain undergoes a conformational change that results in the unfolding of the TH2–3 helices. Surprisingly, the TH1 helix maintains its conformation in the crystal of the full-length toxin even at pH 5. This contrasts with the evidence from the new and previously published data, obtained by spectroscopic measurements and molecular dynamics computer simulations, which indicate the refolding of TH1 upon the acidification of the isolated T-domain. The overall results imply that the membrane interactions of the T-domain are critical in ensuring the proper conformational changes required for the preparation of the diphtheria toxin for the cellular entry. Full article
(This article belongs to the Special Issue Bacterial Toxins: Protein Folding and Membrane Interactions)
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11 pages, 830 KiB  
Article
Phylogenetic Analysis of Filifactor alocis Strains Isolated from Several Oral Infections Identified a Novel RTX Toxin, FtxA
by Jan Oscarsson, Rolf Claesson, Kai Bao, Malin Brundin and Georgios N. Belibasakis
Toxins 2020, 12(11), 687; https://0-doi-org.brum.beds.ac.uk/10.3390/toxins12110687 - 30 Oct 2020
Cited by 11 | Viewed by 2898
Abstract
Filifactor alocis is a Gram-positive asaccharolytic, obligate anaerobic rod of the phylum Firmicutes, and is considered an emerging pathogen in various oral infections, including periodontitis. We here aimed to perform phylogenetic analysis of a genome-sequenced F. alocis type strain (ATCC 35896; CCUG 47790), [...] Read more.
Filifactor alocis is a Gram-positive asaccharolytic, obligate anaerobic rod of the phylum Firmicutes, and is considered an emerging pathogen in various oral infections, including periodontitis. We here aimed to perform phylogenetic analysis of a genome-sequenced F. alocis type strain (ATCC 35896; CCUG 47790), as well as nine clinical oral strains that we have independently isolated and sequenced, for identification and deeper characterization of novel genomic elements of virulence in this species. We identified that 60% of the strains carried a gene encoding a hitherto unrecognized member of the large repeats-in-toxins (RTX) family, which we have designated as FtxA. The clinical infection origin of the ftxA-positive isolates largely varied. However, according to MLST, a clear monophylogeny was reveled for all ftxA-positive strains, along with a high co-occurrence of lactate dehydrogenase (ldh)-positivity. Cloning and expression of ftxA in E. coli, and purification of soluble FtxA yielded a protein of the predicted molecular size of approximately 250 kDa. Additional functional and proteomics analyses using both the recombinant protein and the ftxA-positive, and -negative isolates may reveal a possible role and mechanism(s) of FtxA in the virulence properties of F.alocis, and whether the gene might be a candidate diagnostic marker for more virulent strains. Full article
(This article belongs to the Special Issue Bacterial Toxins: Protein Folding and Membrane Interactions)
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14 pages, 3118 KiB  
Article
The Influence of Calcium toward Order/Disorder Conformation of Repeat-in-Toxin (RTX) Structure of Family I.3 Lipase from Pseudomonas fluorescens AMS8
by Nur Shidaa Mohd Ali, Abu Bakar Salleh, Thean Chor Leow, Raja Noor Zaliha Raja Abd Rahman and Mohd Shukuri Mohamad Ali
Toxins 2020, 12(9), 579; https://0-doi-org.brum.beds.ac.uk/10.3390/toxins12090579 - 09 Sep 2020
Cited by 2 | Viewed by 2364
Abstract
Calcium-binding plays a decisive role in the folding and stabilization of many RTX proteins, especially for the RTX domain. Although many studies have been conducted to prove the contribution of Ca2+ ion toward the folding and stabilization of RTX proteins, its functional [...] Read more.
Calcium-binding plays a decisive role in the folding and stabilization of many RTX proteins, especially for the RTX domain. Although many studies have been conducted to prove the contribution of Ca2+ ion toward the folding and stabilization of RTX proteins, its functional dynamics and conformational structural changes remain elusive. Here, molecular docking and molecular dynamics (MD) simulations were performed to analyze the contribution of Ca2+ ion toward the folding and stabilization of the RTX lipase (AMS8 lipase) structure. AMS8 lipase contains six Ca2+ ions (Ca1–Ca6). Three Ca2+ ions (Ca3, Ca4, and Ca5) were bound to the RTX parallel β-roll motif repeat structure (RTX domain). The metal ion (Ca2+) docking analysis gives a high binding energy, especially for Ca4 and Ca5 which are tightly bound to the RTX domain. The function of each Ca2+ ion is further analyzed using the MD simulation. The removal of Ca3, Ca4, and Ca5 caused the AMS8 lipase structure to become unstable and unfolded. The results suggested that Ca3, Ca4, and Ca5 stabilized the RTX domain. In conclusion, Ca3, Ca4, and Ca5 play a crucial role in the folding and stabilization of the RTX domain, which sustain the integrity of the overall AMS8 lipase structure. Full article
(This article belongs to the Special Issue Bacterial Toxins: Protein Folding and Membrane Interactions)
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Review

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20 pages, 7755 KiB  
Review
Bacterial Type I Toxins: Folding and Membrane Interactions
by Sylvie Nonin-Lecomte, Laurence Fermon, Brice Felden and Marie-Laure Pinel-Marie
Toxins 2021, 13(7), 490; https://0-doi-org.brum.beds.ac.uk/10.3390/toxins13070490 - 14 Jul 2021
Cited by 11 | Viewed by 3447 | Correction
Abstract
Bacterial type I toxin-antitoxin systems are two-component genetic modules that encode a stable toxic protein whose ectopic overexpression can lead to growth arrest or cell death, and an unstable RNA antitoxin that inhibits toxin translation during growth. These systems are widely spread among [...] Read more.
Bacterial type I toxin-antitoxin systems are two-component genetic modules that encode a stable toxic protein whose ectopic overexpression can lead to growth arrest or cell death, and an unstable RNA antitoxin that inhibits toxin translation during growth. These systems are widely spread among bacterial species. Type I antitoxins are cis- or trans-encoded antisense small RNAs that interact with toxin-encoding mRNAs by pairing, thereby inhibiting toxin mRNA translation and/or inducing its degradation. Under environmental stress conditions, the up-regulation of the toxin and/or the antitoxin degradation by specific RNases promote toxin translation. Most type I toxins are small hydrophobic peptides with a predicted α-helical transmembrane domain that induces membrane depolarization and/or permeabilization followed by a decrease of intracellular ATP, leading to plasmid maintenance, growth adaptation to environmental stresses, or persister cell formation. In this review, we describe the current state of the art on the folding and the membrane interactions of these membrane-associated type I toxins from either Gram-negative or Gram-positive bacteria and establish a chronology of their toxic effects on the bacterial cell. This review also includes novel structural results obtained by NMR concerning the sprG1-encoded membrane peptides that belong to the sprG1/SprF1 type I TA system expressed in Staphylococcus aureus and discusses the putative membrane interactions allowing the lysis of competing bacteria and host cells. Full article
(This article belongs to the Special Issue Bacterial Toxins: Protein Folding and Membrane Interactions)
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21 pages, 1720 KiB  
Review
Folding Control in the Path of Type 5 Secretion
by Nathalie Dautin
Toxins 2021, 13(5), 341; https://0-doi-org.brum.beds.ac.uk/10.3390/toxins13050341 - 11 May 2021
Cited by 6 | Viewed by 4246
Abstract
The type 5 secretion system (T5SS) is one of the more widespread secretion systems in Gram-negative bacteria. Proteins secreted by the T5SS are functionally diverse (toxins, adhesins, enzymes) and include numerous virulence factors. Mechanistically, the T5SS has long been considered the simplest of [...] Read more.
The type 5 secretion system (T5SS) is one of the more widespread secretion systems in Gram-negative bacteria. Proteins secreted by the T5SS are functionally diverse (toxins, adhesins, enzymes) and include numerous virulence factors. Mechanistically, the T5SS has long been considered the simplest of secretion systems, due to the paucity of proteins required for its functioning. Still, despite more than two decades of study, the exact process by which T5SS substrates attain their final destination and correct conformation is not totally deciphered. Moreover, the recent addition of new sub-families to the T5SS raises additional questions about this secretion mechanism. Central to the understanding of type 5 secretion is the question of protein folding, which needs to be carefully controlled in each of the bacterial cell compartments these proteins cross. Here, the biogenesis of proteins secreted by the Type 5 secretion system is discussed, with a focus on the various factors preventing or promoting protein folding during biogenesis. Full article
(This article belongs to the Special Issue Bacterial Toxins: Protein Folding and Membrane Interactions)
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20 pages, 1801 KiB  
Review
Structural Basis of the Pore-Forming Toxin/Membrane Interaction
by Yajuan Li, Yuelong Li, Hylemariam Mihiretie Mengist, Cuixiao Shi, Caiying Zhang, Bo Wang, Tingting Li, Ying Huang, Yuanhong Xu and Tengchuan Jin
Toxins 2021, 13(2), 128; https://0-doi-org.brum.beds.ac.uk/10.3390/toxins13020128 - 09 Feb 2021
Cited by 20 | Viewed by 6051
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Bacterial Toxins: Protein Folding and Membrane Interactions)
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17 pages, 1369 KiB  
Review
Harnessing the Membrane Translocation Properties of AB Toxins for Therapeutic Applications
by Numa Piot, F. Gisou van der Goot and Oksana A. Sergeeva
Toxins 2021, 13(1), 36; https://0-doi-org.brum.beds.ac.uk/10.3390/toxins13010036 - 06 Jan 2021
Cited by 8 | Viewed by 6043
Abstract
Over the last few decades, proteins and peptides have become increasingly more common as FDA-approved drugs, despite their inefficient delivery due to their inability to cross the plasma membrane. In this context, bacterial two-component systems, termed AB toxins, use various protein-based membrane translocation [...] Read more.
Over the last few decades, proteins and peptides have become increasingly more common as FDA-approved drugs, despite their inefficient delivery due to their inability to cross the plasma membrane. In this context, bacterial two-component systems, termed AB toxins, use various protein-based membrane translocation mechanisms to deliver toxins into cells, and these mechanisms could provide new insights into the development of bio-based drug delivery systems. These toxins have great potential as therapies both because of their intrinsic properties as well as the modular characteristics of both subunits, which make them highly amenable to conjugation with various drug classes. This review focuses on the therapeutical approaches involving the internalization mechanisms of three representative AB toxins: botulinum toxin type A, anthrax toxin, and cholera toxin. We showcase several specific examples of the use of these toxins to develop new therapeutic strategies for numerous diseases and explain what makes these toxins promising tools in the development of drugs and drug delivery systems. Full article
(This article belongs to the Special Issue Bacterial Toxins: Protein Folding and Membrane Interactions)
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