A solute-binding protein in the closed conformation induces ATP hydrolysis in a bacterial ATP-binding cassette transporter involved in the import of alginate

Allosteric regulation of nitrate transporter NRT via the signaling protein PII

Coordinated carbon and nitrogen metabolism is crucial for bacteria living in the fluctuating environments. Intracellular carbon and nitrogen homeostasis is maintained by a sophisticated network, in which the widespread signaling protein PII acts as a major regulatory hub. In cyanobacteria, PII was proposed to regulate the nitrate uptake by an ABC (ATP-binding cassette)-type nitrate transporter NrtABCD, in which the nucleotide-binding domain of NrtC is fused with a C-terminal regulatory domain (CRD). Here, we solved three cryoelectron microscopy structures of NrtBCD, bound to nitrate, ATP, and PII, respectively. Structural and biochemical analyses enable us to identify the key residues that form a hydrophobic and a hydrophilic cavity along the substrate translocation channel. The core structure of PII, but not the canonical T-loop, binds to NrtC and stabilizes the CRD, making it visible in the complex structure, narrows the substrate translocation channel in NrtB, and ultimately locks NrtBCD at an inhibited inward-facing conformation. Based on these results and previous reports, we propose a putative transport cycle driven by NrtABCD, which is allosterically inhibited by PII in response to the cellular level of 2-oxoglutarate. Our findings provide a distinct regulatory mechanism of ABC transporter via asymmetrically binding to a signaling protein.

The role of bacterial transport systems in the removal of host antimicrobial peptides in Gram-negative bacteria

Antibiotic resistance is a global issue that threatens our progress in healthcare and life expectancy. In recent years, antimicrobial peptides (AMPs) have been considered as promising alternatives to the classic antibiotics. AMPs are potentially superior due to their lower rate of resistance development, since they primarily target the bacterial membrane ('Achilles' heel' of the bacteria). However, bacteria have developed mechanisms of AMP resistance, including the removal of AMPs to the extracellular space by efflux pumps such as the MtrCDE or AcrAB-TolC systems, and the internalization of AMPs to the cytoplasm by the Sap transporter, followed by proteolytic digestion. In this review, we focus on AMP transport as a resistance mechanism compiling all the experimental evidence for the involvement of efflux in AMP resistance in Gram-negative bacteria and combine this information with the analysis of the structures of the efflux systems involved. Finally, we expose some open questions with the aim of arousing the interest of the scientific community towards the AMPs-efflux pumps interactions. All the collected information broadens our understanding of AMP removal by efflux pumps and gives some clues to assist the rational design of AMP-derivatives as inhibitors of the efflux pumps.

The Pseudomonas aeruginosa homeostasis enzyme AlgL clears the periplasmic space of accumulated alginate during polymer biosynthesis

Pseudomonas aeruginosa is an opportunistic human pathogen and a leading cause of chronic infection in the lungs of individuals with cystic fibrosis. After colonization, P. aeruginosa often undergoes a phenotypic conversion to mucoidy, characterized by overproduction of the alginate exopolysaccharide. This conversion is correlated with poorer patient prognoses. The majority of genes required for alginate synthesis, including the alginate lyase, algL, are located in a single operon. Previous investigations of AlgL have resulted in several divergent hypotheses regarding the protein’s role in alginate production. To address these discrepancies, we determined the structure of AlgL and, using multiple sequence alignments, identified key active site residues involved in alginate binding and catalysis. In vitro enzymatic analysis of active site mutants highlights R249 and Y256 as key residues required for alginate lyase activity. In a genetically engineered P. aeruginosa strain where alginate biosynthesis is under arabinose control, we found that AlgL is required for cell viability and maintaining membrane integrity during alginate production. We demonstrate that AlgL functions as a homeostasis enzyme to clear the periplasmic space of accumulated polymer. Constitutive expression of the AlgU/T sigma factor mitigates the effects of an algL deletion during alginate production, suggesting that an AlgU/T-regulated protein or proteins can compensate for an algL deletion. Together, our study demonstrates the role of AlgL in alginate biosynthesis, explains the discrepancies observed previously across other P. aeruginosa ΔalgL genetic backgrounds, and clarifies the existing divergent data regarding the function of AlgL as an alginate degrading enzyme.

Bacteria with a mouth: Discovery and new insights into cell surface structure and macromolecule transport

A bacterium with a "mouth"-like pit structure isolated for the first time in the history of microbiology was a Gram-negative rod, containing glycosphingolipids in the cell envelope, and named Sphingomonas sp. strain A1. The pit was dynamic, with repetitive opening and closing during growth on alginate, and directly included alginate concentrated around the pit, particularly by flagellins, an alginate-binding protein localized on the cell surface. Alginate incorporated into the periplasm was subsequently transferred to the cytoplasm by cooperative interactions of periplasmic solute-binding proteins and an ATP-binding cassette transporter in the cytoplasmic membrane. The mechanisms of assembly, functions, and interactions between the above-mentioned molecules were clarified using structural biology. The pit was transplanted into other strains of sphingomonads, and the pitted recombinant cells were effectively applied to the production of bioethanol, bioremediation for dioxin removal, and other tasks. Studies of the function of the pit shed light on the biological significance of cell surface structures and macromolecule transport in bacteria.

The role of calcium binding to the EF-hand-like motif in bacterial solute-binding protein for alginate import

Gram-negative Sphingomonas sp. A1 incorporates acidic polysaccharide alginate into the cytoplasm via a cell-surface alginate-binding protein (AlgQ2)-dependent ATP-binding cassette transporter (AlgM1M2SS). We investigated the function of calcium bound to the EF-hand-like motif in AlgQ2 by introducing mutations at the calcium-binding site. The X-ray crystallography of the AlgQ2 mutant (D179A/E180A) demonstrated the absence of calcium binding and significant disorder of the EF-hand-like motif. Distinct from the wild-type AlgQ2, the mutant was quite unstable at temperature of strain A1 growth, although unsaturated alginate oligosaccharides stabilized the mutant by formation of substrate/protein complex. In the assay of ATPase and alginate transport by AlgM1M2SS reconstructed in the liposome, the wild-type and mutant AlgQ2 induced AlgM1M2SS ATPase activity in the presence of unsaturated alginate tetrasaccharide. These results indicate that the calcium bound to EF-hand-like motif stabilizes the substrate-unbound AlgQ2 but is not required for the complexation of substrate-bound AlgQ2 and AlgM1M2SS.

A structural model for (GlcNAc)2 translocation via a periplasmic chitooligosaccharide-binding protein from marine Vibrio bacteria

VhCBP is a periplasmic chitooligosaccharide-binding protein mainly responsible for translocation of the chitooligosaccharide (GlcNAc)₂ across the double membranes of marine bacteria. However, structural and thermodynamic understanding of the sugar-binding/-release processes of VhCBP is relatively less. VhCBP displayed the greatest affinity toward (GlcNAc)₂, with lower affinity for longer-chain chitooligosaccharides [(GlcNAc)_3–4]. (GlcNAc)₄ partially occupied the closed sugar-binding groove, with two reducing-end GlcNAc units extending beyond the sugar-binding groove and barely characterized by weak electron density. Mutation of three conserved residues (Trp³⁶³, Asp³⁶⁵, and Trp⁵¹³) to Ala resulted in drastic decreases in the binding affinity toward the preferred substrate (GlcNAc)₂, indicating their significant contributions to sugar binding. The structure of the W513A–(GlcNAc)₂ complex in a ‘half-open’ conformation unveiled the intermediary step of the (GlcNAc)₂ translocation from the soluble CBP in the periplasm to the inner membrane–transporting components. Isothermal calorimetry data suggested that VhCBP adopts the high-affinity conformation to bind (GlcNAc)₂, while its low-affinity conformation facilitated sugar release. Thus, chitooligosaccharide translocation, conferred by periplasmic VhCBP, is a crucial step in the chitin catabolic pathway, allowing Vibrio bacteria to thrive in oceans where chitin is their major source of nutrients.

Synbiotic Matchmaking in Lactobacillus plantarum: Substrate Screening and Gene-Trait Matching To Characterize Strain-Specific Carbohydrate Utilization

Synbiotics are food supplements that combine probiotics and prebiotics to synergistically elicit a health effect in humans. Lactobacillus plantarum exhibits remarkable genetic and phenotypic diversity, in particular in strain-specific carbohydrate utilization capacities, and several strains are marketed as probiotics. We have screened 77 L. plantarum strains for their abilities to utilize specific prebiotic fibers, revealing variable and strain-specific growth efficiencies on isomalto- and galactooligosaccharides. We identified a single strain within the screening panel that was able to effectively utilize inulin and fructooligosaccharides (FOS), which did not support efficient growth of the rest of the strains. In the panel we tested, we did not find strains that could utilize arabinoxylooligosaccharides or sulfated fucoidan. The strain-specific growth phenotype on isomaltooligosaccharides was further analyzed using high-performance anion-exchange chromatography, which revealed distinct substrate utilization phenotypes within the strain panel. The strain-specific phenotypes could be linked to the strains' genotypes by identifying gene clusters coding for carbohydrate membrane transport systems that are predicted to be involved in the utilization of isomaltose and other (unidentified) oligosaccharides in the isomaltooligosaccharide substrate.IMPORTANCE Synbiotics combine prebiotics and probiotics to synergistically enhance the health benefits associated with these ingredients. Lactobacillus plantarum is encountered as a natural inhabitant of the gastrointestinal tract, and specific strains are marketed as probiotics based on their strain-specific health-promoting activities. Strain-specific stimulation of growth through prebiotic substrates could enhance the persistence and/or activity of L. plantarum in situ Our study establishes a high-throughput screening model for prebiotic substrate utilization by individual strains of bacteria, which can be readily employed for synbiotic matchmaking approaches that aim to enhance the intestinal delivery of probiotics through strain-specific, selective growth stimulation.

Kinetic Modelling of Transport Inhibition by Substrates in ABC Importers

Prokaryotic ATP-binding cassette (ABC) importers require a substrate-binding protein (SBP) for the capture and delivery of the cognate substrate to the transmembrane domain (TMD) of the transporter. Various biochemical compounds have been identified that bind to the SBP but are not transported. The mechanistic basis for the "non-cognate" substrates not being transported differs. Some non-cognate substrates fail to trigger the appropriate conformational change in the SBP, resulting in loss of affinity for the TMD or the inability to allosterically activate transport. In another mechanism, the SBP cannot release the bound non-cognate substrate. Here, we used rate equations to derive the steady-state transport rate of cognate substrates of an ABC importer and investigated how non-cognate substrates influence this rate. We found that under limiting non-cognate substrate concentrations, the transport rate remains unaltered for each of the mechanisms. In contrast, at saturating substrate and SBP concentrations, the effect of the non-cognate substrate depends heavily on the respective mechanism. For instance, the transport rate becomes zero when the non-cognate substrate cannot be released by the SBP. Yet it remains unaffected when substrate release is possible but the SBP cannot dock onto the TMDs. Our work shows how the different mechanisms of substrate inhibition impact the transport kinetics, which is relevant for understanding and manipulating solute fluxes and hence the propagation of cells in nutritionally complex milieus.

Structural and biochemical characterisation of a novel alginate lyase from Paenibacillus sp. str. FPU-7

A novel alginate lyase, PsAly, with a molecular mass of 33 kDa and whose amino acid sequence shares no significant similarity to other known proteins, was biochemically and structurally characterised from Paenibacillus sp. str. FPU-7. The maximum PsAly activity was obtained at 65 °C, with an optimum pH of pH 7-7.5. The activity was enhanced by divalent cations, such as Mg²⁺, Mn²⁺, or Co²⁺, and inhibited by a metal chelator, ethylenediaminetetraacetic acid. The reaction products indicated that PsAly is an endolytic enzyme with a preference for polymannuronate. Herein, we report a detailed crystal structure of PsAly at a resolution of 0.89 Å, which possesses a β-helix fold that creates a long cleft. The catalytic site was different from that of other polysaccharide lyases. Site-directed mutational analysis of conserved residues predicted Tyr184 and Lys221 as catalytic residues, abstracting from the C5 proton and providing a proton to the glycoside bond, respectively. One cation was found to bind to the bottom of the cleft and neutralise the carboxy group of the substrate, decreasing the pK_a of the C5 proton to promote catalysis. Our study provides an insight into the structural basis for the catalysis of alginate lyases and β-helix polysaccharide lyases.

A temperature-controlled cold-gas humidifier and its application to protein crystals with the humid-air and glue-coating method

The room-temperature experiment has been revisited for macromolecular crystallography. Despite being limited by radiation damage, such experiments reveal structural differences depending on temperature, and it is expected that they will be able to probe structures that are physiologically alive. For such experiments, the humid-air and glue-coating (HAG) method for humidity-controlled experiments is proposed. The HAG method improves the stability of most crystals in capillary-free experiments and is applicable at both cryogenic and ambient temperatures. To expand the thermal versatility of the HAG method, a new humidifier and a protein-crystal-handling workbench have been developed. The devices provide temperatures down to 4°C and successfully maintain growth at that temperature of bovine cytochrome c oxidase crystals, which are highly sensitive to temperature variation. Hence, the humidifier and protein-crystal-handling workbench have proved useful for temperature-sensitive samples and will help reveal temperature-dependent variations in protein structures.

Periplasmic solute-binding proteins: Structure classification and chitooligosaccharide recognition

Periplasmic solute-binding proteins (SBPs) serve as molecular shuttles that assist the transport of small solutes from the outer membrane to the inner membrane of all Gram-negative bacteria. Based on the available crystal structures, SBPs are classified into seven clusters, A-G, and are further divided into subclusters, IV. This minireview is focused on the classification, structure and substrate specificity of a distinct class of SBPs specific for chitooligosaccharides (CBPs). To date, only two structures of CBP homologues, VhCBP and VcCBP, have been reported in the marine Vibrio species, with exposition of their limited function. The Vibrio CBPs are structurally classified as cluster C/subcluster IV SBPs that exclusively recognize β-1,4- or β-1,3-linked linear oligosaccharides. The overall structural feature of the Vibrios CBPs is most similar to the cellobiose-binding orthologue from the hyperthermophilic bacterium Thermotoga maritima. This similarity provides an opportunity to engineer the substrate specificity of the proteins and to control the uptake of chitinous and cellulosic nutrients in marine bacteria.

Structural studies on bacterial system used in the recognition and uptake of the macromolecule alginate

Alginate is an acidic heteropolysaccharide produced by brown seaweed and certain kinds of bacteria. The cells of Sphingomonas sp. strain A1, a gram-negative bacterium, have several alginate-degrading enzymes in their cytoplasm and efficiently utilize this polymer for their growth. Sphingomonas sp. strain A1 cells can directly incorporate alginate into their cytoplasm through a transport system consisting of a "pit" on their cell surface, substrate-binding proteins in their periplasm, and an ATP-binding cassette transporter in their inner membrane. This review deals with the structural and functional aspects of bacterial systems necessary for the recognition and uptake of alginate.

Evidence from Mutational Analysis for a Single Transmembrane Substrate Binding Site in the Histidine ATP-Binding Cassette Transporter of Salmonella enterica Serovar Typhimurium

The histidine ATP-binding cassette (ABC) transporter of Salmonella enterica serovar Typhimurium is among the best-studied type I ABC import systems. The transporter consists of two transmembrane subunits, HisQ and HisM, and a homodimer of the nucleotide-binding subunit, HisP. Substrates are delivered by two periplasmic solute binding proteins, HisJ and LAO, with preferences for histidine and for lysine, arginine, and ornithine, respectively. A homology model was built by using the arginine-bound crystal structure of the closely related Art(QN)₂ transporter of Thermoanaerobacter tengcongensis as the template. In the homodimeric Art(QN)₂, one substrate molecule is bound to each of the ArtQ subunits, whereas the structural model and sequence alignments predict only one substrate molecule in contact with HisM. To address the question whether one or two binding sites exist in heterodimeric HisQM, we have studied the functional consequences of mutations by monitoring (i) the complementation of growth on d-histidine of auxotrophic tester strains, (ii) the growth of tester strains on arginine as a nitrogen source, and (iii) ATPase activity of purified variants in a lipid environment. Our results demonstrate that two negatively charged residues, namely, HisM-E166 and HisQ-D61, are indispensable for function. Furthermore, the complete reconstruction of an ArtQ-like binding site in HisQ resulted in an inactive transporter. Likewise, switching the positions of both negatively charged residues between HisQ and HisM caused transport-deficient phenotypes. Thus, we propose that one substrate molecule is primarily liganded by residues of HisM while HisQ-D61 forms a crucial salt bridge with the α-amino group of the substrate.IMPORTANCE Canonical ATP-binding cassette (ABC) importers are major players in the translocation of numerous nutrients, vitamins, and growth factors to the cytoplasm of prokaryotes. Moreover, some ABC importers have been identified as virulence factors in bacterial pathogenesis. Thus, a full understanding of their mode of action is considered a prerequisite, among others, for the development of novel antibacterial drugs. However, mainly owing to the lack of structural information, the knowledge of the chemical nature and number of substrate binding sites formed by the transmembrane subunits of ABC importers is scarce. Here, we provide evidence from mutational analyses that, in contrast to homologous homodimeric systems, the heterodimeric histidine transporter of Salmonella enterica serovar Typhimurium is liganding only one substrate molecule between its transmembrane subunits, HisM and HisQ.

One Intact Transmembrane Substrate Binding Site Is Sufficient for the Function of the Homodimeric Type I ATP-Binding Cassette Importer for Positively Charged Amino Acids Art(MP)2 of Geobacillus stearothermophilus

ATP-binding cassette (ABC) transport systems comprise two transmembrane domains/subunits that form a translocation path and two nucleotide-binding domains/subunits that bind and hydrolyze ATP. Prokaryotic canonical ABC import systems require an extracellular substrate-binding protein for function. Knowledge of substrate-binding sites within the transmembrane subunits is scarce. Recent crystal structures of the ABC importer Art(QN)₂ for positively charged amino acids of Thermoanerobacter tengcongensis revealed the presence of one substrate molecule in a defined binding pocket in each of the transmembrane subunits, ArtQ (J. Yu, J. Ge, J. Heuveling, E. Schneider, and M. Yang, Proc Natl Acad Sci U S A 112:5243-5248, 2015, https://doi.org/10.1073/pnas.1415037112). This finding raised the question of whether both sites must be loaded with substrate prior to initiation of the transport cycle. To address this matter, we first explored the role of key residues that form the binding pocket in the closely related Art(MP)₂ transporter of Geobacillus stearothermophilus, by monitoring consequences of mutations in ArtM on ATPase and transport activity at the level of purified proteins embedded in liposomes. Our results emphasize that two negatively charged residues (E153 and D160) are crucial for wild-type function. Furthermore, the variant Art[M(L67D)P]₂ exhibited strongly impaired activities, which is why it was considered for construction of a hybrid complex containing one intact and one impaired substrate-binding site. Activity assays clearly revealed that one intact binding site was sufficient for function. To our knowledge, our study provides the first biochemical evidence on transmembrane substrate-binding sites of an ABC importer.IMPORTANCE Canonical prokaryotic ATP-binding cassette importers mediate the uptake of a large variety of chemicals, including nutrients, osmoprotectants, growth factors, and trace elements. Some also play a role in bacterial pathogenesis, which is why full understanding of their mode of action is of the utmost importance. One of the unsolved problems refers to the chemical nature and number of substrate binding sites formed by the transmembrane subunits. Here, we report that a hybrid amino acid transporter of G. stearothermophilus, encompassing one intact and one impaired transmembrane binding site, is fully competent in transport, suggesting that the binding of one substrate molecule is sufficient to trigger the translocation process.

Alternative substrate-bound conformation of bacterial solute-binding protein involved in the import of mammalian host glycosaminoglycans

Glycosaminoglycans (GAGs), constituted by repeating uronate and amino sugar units, are major components of mammalian extracellular matrices. Some indigenous and pathogenic bacteria target GAGs for colonization to and/or infection of host mammalian cells. In Gram-negative pathogenic Streptobacillus moniliformis, the solute-binding protein (Smon0123)-dependent ATP-binding cassette (ABC) transporter incorporates unsaturated GAG disaccharides into the cytoplasm after depolymerization by polysaccharide lyase. Smon0123, composed of N and C domains, adopts either a substrate-free open or a substrate-bound closed form by approaching two domains at 47° in comparison with the open form. Here we show an alternative 39°-closed conformation of Smon0123 bound to unsaturated chondroitin disaccharide sulfated at the C-4 and C-6 positions of N-acetyl-d-galactosamine residue (CΔ4S6S). In CΔ4S6S-bound Smon0123, Arg204 and Lys210 around the two sulfate groups were located at different positions from those at other substrate-bound 47°-closed conformations. Therefore, the two sulfate groups in CΔ4S6S shifted substrate-binding residue arrangements, causing dynamic conformational change. Smon0123 showed less affinity with CΔ4S6S than with non-sulfated and monosulfated substrates. ATPase activity of the Smon0123-dependent ABC transporter in the presence of CΔ4S6S was lower than that in the presence of other unsaturated chondroitin disaccharides, suggesting that CΔ4S6S-bound Smon0123 was unpreferable for docking with the ABC transporter.