Cysmotif Searcher Pipeline for Antimicrobial Peptide Identification in Plant Transcriptomes

Identification and Characterization of CC-AMP1-like and CC-AMP2-like Peptides in Capsicum spp

Antimicrobial peptides (AMPs) are compounds with a variety of bioactive properties. Especially promising are their antibacterial activities, often toward drug-resistant pathogens. Across different AMP sources, AMPs expressed within plants are relatively underexplored with a limited number of plant AMP families identified. Recently, we identified the novel AMPs CC-AMP1 and CC-AMP2 in ghost pepper plants (Capsicum chinense x frutescens), exerting promising antibacterial activity and not classifying into any known plant AMP family. Herein, AMPs related to CC-AMP1 and CC-AMP2 were identified within both Capsicum annuum and Capsicum baccatum. In silico predictions throughout plants were utilized to illustrate that CC-AMP1-like and CC-AMP2-like peptides belong to two broader AMP families, with three-dimensional structural predictions indicating that CC-AMP1-like peptides comprise a novel subfamily of α-hairpinins. The antibacterial activities of several closely related CC-AMP1-like peptides were compared with a truncated version of CC-AMP1 possessing significantly more activity than the full peptide. This truncated peptide was further characterized to possess broad-spectrum antibacterial activity against clinically relevant ESKAPE pathogens. These findings illustrate the value in continued study of plant AMPs toward characterization of novel AMP families, with CC-AMP1-like peptides possessing promising bioactivity.

The Greater Celandine: Identification and Characterization of an Antimicrobial Peptide from Chelidonium majus

Chelidonium majus, known as Greater Celandine, is a latex-bearing plant that has been leveraged for its anticancer and antimicrobial properties. Herein, C. majus aerial tissue is mined for the presence of antimicrobial peptides. A highly abundant cysteine-rich peptide with a length of 25 amino acids, deemed CM-AMP1, is characterized through multiple mass spectrometric approaches. Electron-activated dissociation is leveraged to differentiate between isoleucine and leucine residues and complement conventional collision-induced dissociation to gain full sequence coverage of the full-length peptide. CM-AMP1 shares little sequence similarity with any proteins in publicly available databases, highlighting the novelty of its cysteine landscape and core motif. The presence of three disulfide bonds in the native peptide confers proteolytic stability, and antimicrobial activity is greatly decreased upon the alkylation of the cysteine residues. Synthetic variants of CM-AMP1 are used to confirm the activity of the full-length sequence and the core motif. To assess the biological impact, E. coli was grown in a sublethal concentration of CM-AMP1 and quantitative proteomics was used to identify proteins produced by the bacteria under stress, ultimately suggesting a membrane lytic antimicrobial mechanism of action. This study integrates multiple analytical methods for molecular and biological characterization of a unique antimicrobial peptide identified from C. majus.

Evolution of antimicrobial cysteine-rich peptides in plants

KEY MESSAGE

We analyzed the evolutionary pattern of cysteine-rich peptides (CRPs) to infer the relationship between CRP copy number and plant ecotype, and the origin of bi-domains CRPs. Plants produce cysteine-rich peptides (CRPs) that have long-lasting broad-spectrum antimicrobial activity to protect themselves from various groups of pathogens. We analyzed 240 plant genomes, ranging from algae to eudicots, and discovered that CRPs are widely distributed in plants. Our comparative genomics results revealed that CRP genes have been amplified through both whole genome and local tandem duplication. The copy number of these genes varied significantly across lineages and was associated with the plant ecotype. This may be due to their resistance to changing pathogenic environments. The conserved and lineage-specific CRP families contribute to diverse antimicrobial activities. Furthermore, we investigated the unique bi-domain CRPs that result from unequal crossover events. Our findings provide a unique evolutionary perspective on CRPs and insights into their antimicrobial and symbiosis characteristics.

Amaranthus hypochondriacus seeds as a rich source of cysteine rich bioactive peptides

Amaranthus hypochondriacus is a nutritious alternative grain native to Central and South America. Increased interest in the impact of A. hypochondriacus on the human body has driven characterization of bioactive secondary metabolites. The seeds are known to contain bioactive small molecules but little is known regarding endogenous peptides. Cysteine-rich peptides (CRPs) in foodstuffs are particularly relevant because they are stabilized by disulfide bonds enhancing resistance to digestion. Here, in silico predictions, proteomics, and simulated gastrointestinal digestions are leveraged to identify digestion resistant CRPs within A. hypochondriacus seeds. Thirteen in silico predicted CRPs were detected in a seed extract providing evidence for the translation of five CRP families. Mature forms of six CRPs were characterized via top-down proteomics revealing multiple post-translational modifications. All six peptides demonstrated resistance to simulated gastrointestinal digestion, suggesting that A. hypochondriacus CRPs may exhibit bioactivity after consumption and should be prioritized for further characterization.

In silico prediction and mass spectrometric characterization of botanical antimicrobial peptides

Antimicrobial peptides (AMPs) are promising compounds for the treatment of antibiotic-resistant bacteria and are found across all organisms, including plants. Unlike most antibiotics, AMPs tend to act on more generalized and multiple targets, making development of resistance more difficult. Conventional approaches toward AMP identification include bioactivity-guided fractionation and genome mining. Complementary methods leveraging bioactivity-guided fractionation, cysteine motif-guided in silico AMP prediction, and mass spectrometric approaches can be combined to expand botanical AMP discovery. Herein, we present an integrated workflow which serves to streamline implementation toward a robust botanical AMP discovery pipeline.

Mass Spectrometric Identification of Antimicrobial Peptides from Medicinal Seeds

Traditional medicinal plants contain a variety of bioactive natural products including cysteine-rich (Cys-rich) antimicrobial peptides (AMPs). Cys-rich AMPs are often crosslinked by multiple disulfide bonds which increase their resistance to chemical and enzymatic degradation. However, this class of molecules is relatively underexplored. Herein, in silico analysis predicted 80–100 Cys-rich AMPs per species from three edible traditional medicinal plants: Linum usitatissimum (flax), Trifolium pratense (red clover), and Sesamum indicum (sesame). Bottom-up proteomic analysis of seed peptide extracts revealed direct evidence for the translation of 3–10 Cys-rich AMPs per species, including lipid transfer proteins, defensins, α-hairpinins, and snakins. Negative activity revealed by antibacterial screening highlights the importance of employing a multi-pronged approach for AMP discovery. Further, this study demonstrates that flax, red clover, and sesame are promising sources for further AMP discovery and characterization.

dbAMP 2.0: updated resource for antimicrobial peptides with an enhanced scanning method for genomic and proteomic data

The last 18 months, or more, have seen a profound shift in our global experience, with many of us navigating a once-in-100-year pandemic. To date, COVID-19 remains a life-threatening pandemic with little to no targeted therapeutic recourse. The discovery of novel antiviral agents, such as vaccines and drugs, can provide therapeutic solutions to save human beings from severe infections; however, there is no specifically effective antiviral treatment confirmed for now. Thus, great attention has been paid to the use of natural or artificial antimicrobial peptides (AMPs) as these compounds are widely regarded as promising solutions for the treatment of harmful microorganisms. Given the biological significance of AMPs, it was obvious that there was a significant need for a single platform for identifying and engaging with AMP data. This led to the creation of the dbAMP platform that provides comprehensive information about AMPs and facilitates their investigation and analysis. To date, the dbAMP has accumulated 26 447 AMPs and 2262 antimicrobial proteins from 3044 organisms using both database integration and manual curation of >4579 articles. In addition, dbAMP facilitates the evaluation of AMP structures using I-TASSER for automated protein structure prediction and structure-based functional annotation, providing predictive structure information for clinical drug development. Next-generation sequencing (NGS) and third-generation sequencing have been applied to generate large-scale sequencing reads from various environments, enabling greatly improved analysis of genome structure. In this update, we launch an efficient online tool that can effectively identify AMPs from genome/metagenome and proteome data of all species in a short period. In conclusion, these improvements promote the dbAMP as one of the most abundant and comprehensively annotated resources for AMPs. The updated dbAMP is now freely accessible at http://awi.cuhk.edu.cn/dbAMP.

Transcriptomic Analysis of Genes Involved in Plant Defense Response to the Cucumber Green Mottle Mosaic Virus Infection

Plants have evolved a complex multilayered defense system to counteract various invading pathogens during their life cycle. In addition to silencing, considered to be a major molecular defense response against viruses, different signaling pathways activated by phytohormones trigger the expression of secondary metabolites and proteins preventing virus entry and propagation. In this study, we explored the response of cucumber plants to one of the global pathogens, cucumber green mottle mosaic virus (CGMMV), which causes severe symptoms on leaves and fruits. The inbred line of Cucumis sativus L., which is highly susceptible to CGMMV, was chosen for inoculation. Transcriptomes of infected plants at the early and late stages of infection were analyzed in comparison with the corresponding transcriptomes of healthy plants using RNA-seq. The changes in the signaling pathways of ethylene and salicylic and jasmonic acids, as well as the differences in silencing response and expression of pathogenesis-related proteins and transcription factors, were revealed. The results show that silencing was strongly suppressed in infected plants, while the salicylic acid and ethylene signaling pathways were induced. The genes encoding pathogenesis-related proteins and the genes involved in the jasmonic acid pathway changed their expression insignificantly. It was also found that WRKY and NAC were the most sensitive to CGMMV infection among the transcription factors detected.

Too Hot to Handle: Antibacterial Peptides Identified in Ghost Pepper

Capsicum spp. (hot peppers) demonstrate a range of interesting bioactive properties spanning anti-inflammatory, antioxidant, and antimicrobial activities. While several species within the genus are known to produce antimicrobial peptides (AMPs), AMP sequence mining of genomic data indicates this space remains largely unexplored. Herein, in silico AMP predictions were paired with peptidomics to identify novel AMPs from the interspecific hybrid ghost pepper (Capsicum chinense × frutescens). AMP prediction algorithms revealed 115 putative AMPs within the Capsicum chinense genome, of which 14 were identified in the aerial tissue peptidome. PepSAVI-MS, de novo sequencing, and complementary approaches were used to fully molecularly characterize two novel AMPs, CC-AMP1 and CC-AMP2, including elucidation of a pyroglutamic acid post-translational modification of CC-AMP1 and disulfide bond connectivity of both. Both CC-AMP1 and CC-AMP2 have little homology with known AMPs and exhibited low μM antimicrobial activity against Gram-negative bacteria, including Escherichia coli. These findings demonstrate the complementary nature of peptidomics, bioactivity-guided discovery, and bioinformatics-based investigations to characterize plant AMP profiles.

Proteomic response of Escherichia coli to a membrane lytic and iron chelating truncated Amaranthus tricolor defensin

BACKGROUND

Plant defensins are a broadly distributed family of antimicrobial peptides which have been primarily studied for agriculturally relevant antifungal activity. Recent studies have probed defensins against Gram-negative bacteria revealing evidence for multiple mechanisms of action including membrane lysis and ribosomal inhibition. Herein, a truncated synthetic analog containing the γ-core motif of Amaranthus tricolor DEF2 (Atr-DEF2) reveals Gram-negative antibacterial activity and its mechanism of action is probed via proteomics, outer membrane permeability studies, and iron reduction/chelation assays.

RESULTS

Atr-DEF2(G39-C54) demonstrated activity against two Gram-negative human bacterial pathogens, Escherichia coli and Klebsiella pneumoniae. Quantitative proteomics revealed changes in the E. coli proteome in response to treatment of sub-lethal concentrations of the truncated defensin, including bacterial outer membrane (OM) and iron acquisition/processing related proteins. Modification of OM charge is a common response of Gram-negative bacteria to membrane lytic antimicrobial peptides (AMPs) to reduce electrostatic interactions, and this mechanism of action was confirmed for Atr-DEF2(G39-C54) via an N-phenylnaphthalen-1-amine uptake assay. Additionally, in vitro assays confirmed the capacity of Atr-DEF2(G39-C54) to reduce Fe3+ and chelate Fe2+ at cell culture relevant concentrations, thus limiting the availability of essential enzymatic cofactors.

CONCLUSIONS

This study highlights the utility of plant defensin γ-core motif synthetic analogs for characterization of novel defensin activity. Proteomic changes in E. coli after treatment with Atr-DEF2(G39-C54) supported the hypothesis that membrane lysis is an important component of γ-core motif mediated antibacterial activity but also emphasized that other properties, such as metal sequestration, may contribute to a multifaceted mechanism of action.

Multiple Classes of Antimicrobial Peptides in Amaranthus tricolor Revealed by Prediction, Proteomics, and Mass Spectrometric Characterization

Traditional medicinal plants are rich reservoirs of antimicrobial agents, including antimicrobial peptides (AMPs). Advances in genomic sequencing, in silico AMP predictions, and mass spectrometry-based peptidomics facilitate increasingly high-throughput bioactive peptide discovery. Herein, Amaranthus tricolor aerial tissue was profiled via MS-based proteomics/peptidomics, identifying AMPs predicted in silico. Bottom-up proteomics identified seven novel peptides spanning three AMP classes including lipid transfer proteins, snakins, and a defensin. Characterization via top-down peptidomic analysis of Atr-SN1, Atr-DEF1, and Atr-LTP1 revealed unexpected proteolytic processing and enumerated disulfide bonds. Bioactivity screening of isolated Atr-LTP1 showed activity against the high-risk ESKAPE bacterial pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, and Enterobacter cloacae). These results highlight the potential for integrating AMP prediction algorithms with complementary -omics approaches to accelerate characterization of biologically relevant AMP peptidoforms.

Defense Peptides From the α-Hairpinin Family Are Components of Plant Innate Immunity

Plant immunity represents a sophisticated system, including both basal and inducible mechanisms, to prevent pathogen infection. Antimicrobial peptides (AMPs) are among the innate immunity components playing a key role in effective and rapid response against various pathogens. This review is devoted to a small family of defense peptides called α-hairpinins. The general characters of the family, as well as the individual features of each member, including biological activities, structures of precursor proteins, and spatial structures, are described. Possible applications of α-hairpinin peptides in drug design are discussed.

Predicting Antimicrobial and Other Cysteine-Rich Peptides in 1267 Plant Transcriptomes

Antimicrobial peptides (AMPs) are a key component of innate immunity in various organisms including bacteria, insects, mammals, and plants. Their mode of action decreases the probability of developing resistance in pathogenic organisms, which makes them a promising object of study. However, molecular biology methods for searching for AMPs are laborious and expensive, especially for plants. Earlier, we developed a computational pipeline for identifying potential AMPs based on the cysteine motifs they usually possess. Since most motifs are too species-specific, a wide-scale screening of novel data is required to maintain the accuracy of searching algorithms. We have performed a search for potential AMPs in 1267 plant transcriptomes using our pipeline. On average, 50–150 peptides were revealed in each transcriptome. The data was verified by a BLASTp search in nr database to confirm peptide functions and by using random nucleotide sequences to estimate the fraction of erroneous predictions. The datasets obtained will be useful both for molecular biologists investigating AMPs in various organisms and for bioinformaticians developing novel algorithms of motif searching in transcriptomic and genomic sequences. The results obtained will represent a good reference point for future investigations in the fields mentioned above.