Structural insights into the multispecific recognition of dipeptides of deep-sea gram-negative bacterium Pseudoalteromonas sp. strain SM9913

Antimicrobial Peptide Recognition Motif of the Substrate Binding Protein SapA from Nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) is an opportunistic pathogen associated with respiratory diseases, including otitis media and exacerbations of chronic obstructive pulmonary disease. NTHi exhibits resistance to killing by host antimicrobial peptides (AMPs) mediated by SapA, the substrate binding protein of the sensitivity to antimicrobial peptides (Sap) transporter. However, the specific mechanisms by which SapA selectively binds various AMPs such as defensins and cathelicidin are unknown. In this study, we report mutational analyses of both defensin AMPs and the SapA binding pocket to define the specificity of AMP recognition. Bactericidal assays revealed that NTHi lacking SapA are more susceptible to human beta defensins and LL-37, while remaining highly resistant to a human alpha defensin. In contrast to homologues, our research underscores the distinct specificity of NTHi SapA, which selectively recognizes and binds to peptides containing the charged-hydrophobic motif PKE and RRY. These findings provide valuable insight into the divergence of SapA among bacterial species and NTHi SapA's ability to selectively interact with specific AMPs to mediate resistance.

Biochemical studies highlight determinants for metal selectivity in the Escherichia coli periplasmic solute binding protein NikA

Nickel is an essential micronutrient for the survival of many microbes. On account of the toxicity of nickel and its scarcity in the environment, microbes have evolved specific systems for uptaking and delivering nickel to enzymes. NikA, the solute binding protein for the ATP-binding cassette (ABC) importer NikABCDE, plays a vital role in the nickel homeostasis of Escherichia coli by selectively binding nickel over other metals in the metabolically complex periplasm. While the endogenous ligand for NikA is known to be the Ni(II)-(L-His)2 complex, the molecular basis by which NikA selectively binds Ni(II)-(L-His)2 is unclear, especially considering that NikA can bind multiple metal-based ligands with comparable affinity. Here we show that, regardless of its promiscuous binding activity, NikA preferentially interacts with Ni(II)-(L-His)2, even over other metal-amino acid ligands with an identical coordination geometry for the metal. Replacing both the Ni(II) and the L-His residues in Ni(II)-(L-His)2 compromises binding of the ligand to NikA, in part because these alterations affect the degree by which NikA closes around the ligand. Replacing H416, the only NikA residue that ligates the Ni(II), with other potential metal-coordinating amino acids decreases the binding affinity of NikA for Ni(II)-(L-His)2 and compromises uptake of Ni(II) into E. coli cells, likely due to altered metal selectivity of the NikA mutants. Together, the biochemical and in vivo studies presented here define key aspects of how NikA selects for Ni(II)-(L-His)2 over other metal complexes, and can be used as a reference for studies into the metal selectivity of other microbial solute binding proteins.

Bioinformatics analysis and biochemical characterisation of ABC transporter-associated periplasmic substrate-binding proteins ModA and MetQ from Helicobacter pylori strain SS1

The human gastric pathogen Helicobacter pylori relies on the uptake of host-provided nutrients for its proliferation and pathogenicity. ABC transporters that mediate import of small molecules into the cytoplasm of H. pylori employ their cognate periplasmic substrate-binding proteins (SBPs) for ligand capture in the periplasm. The genome of the mouse-adapted strain SS1 of H. pylori encodes eight ABC transporter-associated SBPs, but little is known about their specificity or structure. In this study, we demonstrated that the SBP annotated as ModA binds molybdate (MoO₄^2-, K_D = 3.8 nM) and tungstate (WO₄^2-, K_D = 7.8 nM). In addition, we showed that MetQ binds D-methionine (K_D = 9.5 μM), but not L-methionine, which suggests the existence of as yet unknown pathway for L-methionine uptake. Homology modelling has led to identification of the ligand-binding residues.

Oxidation of trimethylamine to trimethylamine N-oxide facilitates high hydrostatic pressure tolerance in a generalist bacterial lineage

High hydrostatic pressure (HHP) is a characteristic environmental factor of the deep ocean. However, it remains unclear how piezotolerant bacteria adapt to HHP. Here, we identify a two-step metabolic pathway to cope with HHP stress in a piezotolerant bacterium. Myroides profundi D25^T, obtained from a deep-sea sediment, can take up trimethylamine (TMA) through a previously unidentified TMA transporter, TmaT, and oxidize intracellular TMA into trimethylamine N-oxide (TMAO) by a TMA monooxygenase, MpTmm. The produced TMAO is accumulated in the cell, functioning as a piezolyte, improving both growth and survival at HHP. The function of the TmaT-MpTmm pathway was further confirmed by introducing it into Escherichia coli and Bacillus subtilis Encoded TmaT-like and MpTmm-like sequences extensively exist in marine metagenomes, and other marine Bacteroidetes bacteria containing genes encoding TmaT-like and MpTmm-like proteins also have improved HHP tolerance in the presence of TMA, implying the universality of this HHP tolerance strategy in marine Bacteroidetes.

Control of Bacterial Virulence through the Peptide Signature of the Habitat

To optimize fitness, pathogens selectively activate their virulence program upon host entry. Here, we report that the facultative intracellular bacterium Listeria monocytogenes exploits exogenous oligopeptides, a ubiquitous organic N source, to sense the environment and control the activity of its virulence transcriptional activator, PrfA. Using a genetic screen in adsorbent-treated (PrfA-inducing) medium, we found that PrfA is functionally regulated by the balance between activating and inhibitory nutritional peptides scavenged via the Opp transport system. Activating peptides provide essential cysteine precursor for the PrfA-inducing cofactor glutathione (GSH). Non-cysteine-containing peptides cause promiscuous PrfA inhibition. Biophysical and co-crystallization studies reveal that peptides inhibit PrfA through steric blockade of the GSH binding site, a regulation mechanism directly linking bacterial virulence and metabolism. L. monocytogenes mutant analysis in macrophages and our functional data support a model in which changes in the balance of antagonistic Opp-imported oligopeptides promote PrfA induction intracellularly and PrfA repression outside the host.

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Listeria PrfA virulence regulation is controlled by antagonistic nutritional peptides

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Opp-imported peptides regulate PrfA upstream of the activating cofactor GSH

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PrfA is activated by peptides that provide essential cysteine for GSH biosynthesis

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Blockade of PrfA’s GSH binding site by peptides inhibits virulence gene activation

Molecular Basis of Unexpected Specificity of ABC Transporter-Associated Substrate-Binding Protein DppA from Helicobacter pylori

The gastric pathogen Helicobacter pylori has limited ability to use carbohydrates as a carbon source, relying instead on exogenous amino acids and peptides. Uptake of certain peptides by H. pylori requires an ATP binding cassette (ABC) transporter annotated dipeptide permease (Dpp). The transporter specificity is determined by its cognate substrate-binding protein DppA, which captures ligands in the periplasm and delivers them to the permease. Here, we show that, unlike previously characterized DppA proteins, H. pylori DppA binds, with micromolar affinity, peptides of diverse amino acid sequences ranging between two and eight residues in length. We present analysis of the 1.45-Å-resolution crystal structure of its complex with the tetrapeptide STSA, which provides a structural rationale for the observed broad specificity. Analysis of the molecular surface revealed a ligand-binding pocket that is large enough to accommodate peptides of up to nine residues in length. The structure suggests that H. pylori DppA is able to recognize a wide range of peptide sequences by forming interactions primarily with the peptide main chain atoms. The loop that terminates the peptide-binding pocket in DppAs from other bacteria is significantly shorter in the H. pylori protein, providing an explanation for its ability to bind longer peptides. The subsites accommodating the two N-terminal residues of the peptide ligand make the greatest contribution to the protein-ligand binding energy, in agreement with the observation that dipeptides bind with affinity close to that of longer peptides.IMPORTANCE The World Health Organization listed Helicobacter pylori as a high-priority pathogen for antibiotic development. The potential of using peptide transporters in drug design is well recognized. We discovered that the substrate-binding protein of the ABC transporter for peptides, termed dipeptide permease, is an unusual member of its family in that it directly binds peptides of diverse amino acid sequences, ranging between two and eight residues in length. We also provided a structural rationale for the observed broad specificity. Since the ability to import peptides as a source of carbon is critical for H. pylori, our findings will inform drug design strategies based on inhibition or fusion of membrane-impermeant antimicrobials with peptides.

Illicit Transport via Dipeptide Transporter Dpp is Irrelevant to the Efficacy of Negamycin in Mouse Thigh Models of Escherichia coli Infection

Negamycin is a hydrophilic antimicrobial translation inhibitor that crosses the lipophilic inner membrane of Escherichia coli via at least two transport routes to reach its intracellular target. In a minimal salts medium, negamycin's peptidic nature allows illicit entry via a high-affinity route by hijacking the Dpp dipeptide transporter. Transport via a second, low-affinity route is energetically driven by the membrane potential, seemingly without the direct involvement of a transport protein. In mouse thigh models of E. coli infection, no evidence for Dpp-mediated transport of negamycin was found. The implication is that for the design of new negamycin-based analogs, the physicochemical properties required for cell entry via the low-affinity route need to be retained to achieve clinical success in the treatment of infectious diseases. Furthermore, clinical resistance to such analogs due to mutations affecting their ribosomal target or transport is expected to be rare and similar to that of aminoglycosides.