Abstract
Antimicrobial peptides (AMPs) and similar compounds are potential candidates for combating antibiotic-resistant bacteria. The hypothesis of directed co-aggregation of the target protein and an amyloidogenic peptide acting as an antimicrobial peptide was successfully tested for peptides synthesized on the basis of ribosomal S1 protein in the bacterial culture of T. thermophilus. Co-aggregation of the target protein and amyloidogenic peptide was also tested for the pathogenic ribosomal S1 protein from P. aeruginosa. Almost all peptides that we selected as AMPs, prone to aggregation and formation of fibrils, based on the amino acid sequence of ribosomal S1 protein from E. coli, T. thermophilus, P. aeruginosa, formed amyloid fibrils. We have demonstrated that amyloidogenic peptides are not only toxic to their target cells, but also some of them have antimicrobial activity. Controlling the aggregation of vital bacterial proteins can become one of the new directions of research and form the basis for the search and development of targeted antibacterial drugs.
Introduction
Antibiotic resistance of bacteria is a pressing global problem. The rate of development and introduction of new antibiotics for clinical use lags behind the spread of antibiotic resistance (). It should be noted that among antimicrobial drugs, only seven peptide antibiotics were approved for use, and of more than 10,000 antimicrobial peptides (AMPs), only 61 are at the stage of preclinical and clinical trials (), which demonstrates the complexity of the development and implementation of new antimicrobial substances. One of the possible solutions to combat pathogenic microorganisms is the development and use of new antimicrobial peptides (AMP) (; ).
The general mechanism of AMPs includes permeating membranes, facilitating membrane remodeling processes such as pore formation and fusion (), but peptides with alternative mechanisms of action look promising. Working on the amyloidogenic properties of proteins and peptides, O.V. Galzitskaya suggested the possibility of directed co-aggregation of an amyloidogenic peptide and a target protein, which in vivo can manifest itself as an antimicrobial effect. Analysis of the literature confirmed this possibility (; ; ; ; ; ; ). Last and Miranker showed that amyloidogenic peptides as well as antimicrobial peptides can inhibit bacterial cell growth (). The facts of the formation of fibrils by antimicrobial peptides and, conversely, the manifestation of antimicrobial activity of amyloidogenic regions of proteins indicate the presence of a certain connection between them (; ). Despite the low similarity between AMPs and amyloidogenic peptides, the latter exhibit similar activity, leading to cytotoxic effects (). It is believed that mature amyloids do not exhibit toxicity (Gosztyla et al., 2018). On the other hand, there is growing evidence that oligomers of amyloidogenic peptides exhibit antimicrobial activity (). In any case, it remains unclear how antibacterial activity and the ability to form fibrils are related. Understanding the dependence of the properties of a peptide molecule on its structure is an important component for explaining the nature of a particular phenomenon. Artificially synthesized peptides can be an ideal model for studying the relationship between antimicrobial activity and the ability to form fibrils.
Choice of Target Protein
We chose the ribosomal S1 protein as a target because it is a unique protein for a bacterial cell. This protein has a number of important functions (participates in translation initiation, translation regulation), its knockout leads to cell death, and is present only in bacteria. We carried out a bioinformatics study of its properties (, ; , ). Unique characteristics have been found for this protein. The S1 protein consists of several repeats of the S1 domain (OB-fold), and the number of such repeats depends on the type of bacteria to which this protein belongs. The number of repeats ranges from one to six, and all Gram-negative bacteria have six domains in the ribosomal S1 protein (). It turned out that this protein is important for the bacterial cell, since mutations in this protein lead to cell death. Since the function of each domain is not fully defined, each amyloidogenic site from different domains will have a different effect. However, the coaggregation of a protein with the peptide will lead to disruption of the functions of that protein, which will be tantamount to protein knockout. Structural and functional features have been well studied so far only for the ribosomal S1 protein from E. coli (Figures 1.A,C). It was demonstrated that D1-D2 domains of the ribosomal S1 protein of E. coli have high homology (67%) and both are responsible for the interaction with 30 S ribosomal subunit (; ). Domains D3-D6 interact with RNA (; ). D3 domain is of fundamental importance in the interaction with mRNA and tmRNA, as well as in the interaction with ribonuclease regB (). Moreover, D6 domain is an autogenous repressor of its own synthesis (). These facts make the S1 protein an important target for the development of antibacterial drugs (). The spatial structure was determined for the five domains from the six (except for D3) of the S1 protein from E. coli (Figure 1.C).
FIGURE 1
Peptides Prone to Aggregation From the Ribosomal S1 Protein
Domains of the ribosomal S1 protein from E. coli (six domains — 557 amino acid residues), T. thermophilus (five domains — 536 amino acid residues), and P. aeruginosa (six domains — 559 amino acid residues) were analyzed to select regions of the amino acid sequence of the protein potentially possessing amyloidogenic and antimicrobial properties. All these bacteria are Gram-negative. Based on the theoretical analysis using four programs (FoldAmyloid (
FIGURE 2

Peptides prone to aggregation from the ribosomal S1 protein. From (A)E. coli; (B)T. thermophilus (
For membrane penetration, four modified peptides were engineered by adding cell penetrating peptide (CPP) via an additional linker (GlyGlySarGly, where Sar is sarcosine) to the peptide. A fragment of Tat-HIV-1 (49–57) was added to four sequences of the selected peptides to increase an antimicrobial activity, membrane permeability, rigidity and mechanical stability of aggregate complexes (
Identity and Amyloidogenicity of the S1 Domains in the Ribosomal S1 Proteins
As mentioned above, the ribosomal S1 protein has a limited number of domains from one to six. And, as we mentioned, all Gram-negative bacteria have six repeats of the S1 domains.
Using bioinformatics tools, 1,453 sequences of ribosomal S1 proteins from 25 different phyla were studied (
The primary structures of ribosomal S1 proteins contain regions prone to aggregation and formation of amyloids according to four programs: FoldAmyloid (
Another interesting question is how often the selected amyloidogenic regions for synthesis are found in other bacterial and eukaryotic proteins. It turns out that we did not find them in eukaryotic proteins, which is very important in order not to initiate directed aggregation with any target protein that includes a similar amino acid sequence.
Amyloidogenicity of Peptides and Formation of Fibrils
Peptides predicted by bioinformatics tools to aggregate and form amyloid fibrils have been synthesized and tested for their ability to form amyloid fibrils. Despite the strong tendency towards the aggregation of several amyloidogenic sites in the ribosomal S1 protein family, the process of fibril formation is still poorly understood. The S1 proteins studied by us from four organisms (M. mobili, T. thermophilus, P. aeruginosa and S. aureus) did not form amyloid fibrils (
Coaggregation of peptide and S1 protein was tested for 2 S1 proteins and 5 peptides: S1 T. thermophilus with V10T, R23T, R23I (
Thus, the ribosomal S1 protein of E. coli, T. thermophilus, and P. aeruginosa contains amyloidogenic sequences that can lead to aggregation of peptide molecules with each other or with other proteins that have aggregation sites (directed coaggregation mechanism). S1-related domains are found in other bacterial proteins, which may increase the number of targets for the peptide.
Testing for Antibacterial Activity and Toxicity
Peptides predicted by bioinformatics tools as prone to aggregation and formation of amyloid fibrils were synthesized and tested for antimicrobial and cytotoxic effects.
Several peptides have been tested for antimicrobial activity. Peptides from E. coli against E. coli, and peptides from T. thermophilus against T. thermophilus cells (
Previously, we were able to evaluate the antimicrobial activity of peptides synthesized on the basis of the amino acid sequence of the ribosomal S1 protein from T. thermophilus, which suppressed the growth of T. thermophilus cell culture. We have successfully tested this approach on T. thermophilus bacterial culture. Among the peptides from S1 T. thermophilus studied by us, the most effective peptide was the R23I peptide (minimum inhibitory concentration (MIC) is 50 μg/ml), the effect of which was comparable to that of the antibiotic kanamycin (
It should be noted that if amyloidogenic peptides are toxic to cells, they will not be good templates for antibiotic development. Therefore, it is necessary to carry out a test for the survival of eukaryotic cells. In our case, cell viability was estimated by resazurin cell viability assay. Human fibroblast cell survival was 70% for the peptides from T. thermophilus (
Discussion
Currently, there are many programs for predicting the antimicrobial activity of peptides (for example, AMPA or AmPEP (
Despite the low ability of bacteria to exhibit resistance to antimicrobial peptides, in order to prevent and resist the emergence of new antibiotic-resistant mutants, it is important to assess the adaptive ability of pathogenic bacteria to peptides. Proteomic profiling of bacteria makes it possible to assess molecular responses in general, which can help in identifying molecular strategies for adapting pathogens to adverse conditions and determining the potential anger of mutant strains.
It can be noted that we were just lucky, having looked at about 20 peptides from three organisms, we were able to find two peptides with antimicrobial properties and low MIC, which is comparable to the MIC of antibiotics. Moreover, AMPs against P. aeruginosa showed no toxicity to eukaryotic cells at all.
Conclusion
We have demonstrated that amyloidogenicity may be one of the important properties of AMPs. We suggested that AMPs form aggregates with target proteins by their amyloidogenic regions, which ultimately lead to cell death. This may indicate that the ability to aggregate may be combined with antimicrobial action against bacteria.
Thus, if we are talking about two sides of the same coin, we can emphasize that, on the one hand, the formation of amyloids can be functional, and on the other hand, it can be associated with a disease, and co-aggregation also has two sides: on the one hand, it is associated with infectivity, on the other hand, direct co-aggregation may be one of the possible mechanisms of AMP action.
Statements
Data availability statement
Publicly available datasets were analyzed in this study. This data can be found here: Supplementary Material from PMID: 32707977 and https://www.mdpi.com/1422-0067/22/14/7291/pdf.
Author contributions
The author confirms being the sole contributor of this work and has approved it for publication.
Funding
The study was funded by the Russian Science Foundation (grant 18-14-00321).
Acknowledgments
We are grateful to: A. V. Glyakina, O. M. Selivanova, A. K. Surin, E. I. Deryusheva, A. V. Machulin, S. R. Kurpe, R. S. Fadeev, and S. Yu. Grishin for assistance in the preparation of the manuscript.
Conflict of interest
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmolb.2021.705069/full#supplementary-material
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Summary
Keywords
amyloid, toxicity, drug, pathogenic organism, aggregation
Citation
Galzitskaya OV (2021) Exploring Amyloidogenicity of Peptides From Ribosomal S1 Protein to Develop Novel AMPs. Front. Mol. Biosci. 8:705069. doi: 10.3389/fmolb.2021.705069
Received
04 May 2021
Accepted
09 August 2021
Published
19 August 2021
Volume
8 - 2021
Edited by
Fred Harris, University of Central Lancashire, United Kingdom
Reviewed by
Ester Boix, Universitat Autònoma de Barcelona, Spain
Ashim Paul, Tel Aviv University, Israel
Ram Nagaraj, Centre for Cellular and Molecular Biology (CCMB), India
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© 2021 Galzitskaya.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Oxana V. Galzitskaya, ogalzit@vega.protres.ru
This article was submitted to Structural Biology, a section of the journal Frontiers in Molecular Biosciences
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.