Sucrose transport through maltoporin mutants of Escherichia coli

The C2 entity of chitosugars is crucial in molecular selectivity of the Vibrio campbellii chitoporin

The marine bacterium Vibrio campbellii expresses a chitooligosaccharide-specific outer-membrane channel (chitoporin) for the efficient uptake of nutritional chitosugars that are externally produced through enzymic degradation of environmental host shell chitin. However, the principles behind the distinct substrate selectivity of chitoporins are unclear. Here, we employed black lipid membrane (BLM) electrophysiology, which handles the measurement of the flow of ionic current through porins in phospholipid bilayers for the assessment of porin conductivities, to investigate the pH dependency of chitosugar-chitoporin interactions for the bacterium's natural substrate chitohexaose and its deacetylated form, chitosan hexaose. We show that efficient passage of the N-acetylated chitohexaose through the chitoporin is facilitated by its strong affinity for the pore. In contrast, the deacetylated chitosan hexaose is impermeant; however, protonation of the C2 amino entities of chitosan hexaose allows it to be pulled through the channel in the presence of a transmembrane electric field. We concluded from this the crucial role of C2-substitution as the determining factor for chitoporin entry. A change from N-acetylamino- to amino-substitution effectively abolished the ability of approaching molecules to enter the chitoporin, with deacetylation leading to loss of the distinctive structural features of nanopore opening and pore access of chitosugars. These findings provide further understanding of the multistep pathway of chitin utilization by marine Vibrio bacteria and may guide the development of solid-state or genetically engineered biological nanopores for relevant technological applications.

Manipulation of charge distribution in the arginine and glutamate clusters of the OmpG pore alters sugar specificity and ion selectivity

OmpG is a general diffusion pore in the E. coli outer membrane with a molecular architecture comprising a 14-stranded β-barrel scaffold and unique structural features. In contrast to other non-specific porins, OmpG lacks a central constriction zone and has an exceptionally wide pore diameter of about 13 Å. The equatorial plane of OmpG harbors an annulus of four alternating basic and acidic patches whose function is only poorly characterized. We have investigated the role of charge distribution for ion selectivity and sugar transport with the help of OmpG variants mutated in the annulus. Substituting the glutamate residues of the annulus for histidines or alanines led to a strong reduction in cation selectivity. Replacement of the glutamates in the annulus by histidine residues also disfavored the passage of pentoses and hexoses relative to disaccharides. Our results demonstrate that despite the wide pore diameter, an annulus only consisting of two opposing basic patches confers reduced cation and monosaccharide transport compared to OmpG wild type. Furthermore, randomization of charged residues in the annulus had the potential to abolish pH-dependency of sugar transport. Our results indicate that E¹⁵, E³¹, R⁹², R¹¹¹ and R²¹¹ in the annulus form electrostatic interactions with R²²⁸, E²²⁹ and D²³² in loop L6 that influence pH-dependency of sugar transport.

Chitoporin from Vibrio harveyi, a channel with exceptional sugar specificity

Chitoporin (VhChiP) is a sugar-specific channel responsible for the transport of chitooligosaccharides through the outer membrane of the marine bacterium Vibrio harveyi. Single channel reconstitution into black lipid membrane allowed single chitosugar binding events in the channel to be resolved. VhChiP has an exceptionally high substrate affinity, with a binding constant of K = 5.0 × 10(6) M(-1) for its best substrate (chitohexaose). The on-rates of chitosugars depend on applied voltages, as well as the side of the sugar addition, clearly indicating the inherent asymmetry of the VhChiP lumen. The binding affinity of VhChiP for chitohexaose is 1-5 orders of magnitude larger than that of other known sugar-specific porins for their preferred substrates. Thus, VhChiP is the most potent sugar-specific channel reported to date, with its high efficiency presumably reflecting the need for the bacterium to take up chitin-containing nutrients promptly under turbulent aquatic conditions to exploit them efficiently as its sole source of energy.

Growth independent rhamnolipid production from glucose using the non-pathogenic Pseudomonas putida KT2440

Background

Rhamnolipids are potent biosurfactants with high potential for industrial applications. However, rhamnolipids are currently produced with the opportunistic pathogen Pseudomonas aeruginosa during growth on hydrophobic substrates such as plant oils. The heterologous production of rhamnolipids entails two essential advantages: Disconnecting the rhamnolipid biosynthesis from the complex quorum sensing regulation and the opportunity of avoiding pathogenic production strains, in particular P. aeruginosa. In addition, separation of rhamnolipids from fatty acids is difficult and hence costly.

Results

Here, the metabolic engineering of a rhamnolipid producing Pseudomonas putida KT2440, a strain certified as safety strain using glucose as carbon source to avoid cumbersome product purification, is reported. Notably, P. putida KT2440 features almost no changes in growth rate and lag-phase in the presence of high concentrations of rhamnolipids (> 90 g/L) in contrast to the industrially important bacteria Bacillus subtilis, Corynebacterium glutamicum, and Escherichia coli. P. putida KT2440 expressing the rhlAB-genes from P. aeruginosa PAO1 produces mono-rhamnolipids of P. aeruginosa PAO1 type (mainly C₁₀:C₁₀). The metabolic network was optimized in silico for rhamnolipid synthesis from glucose. In addition, a first genetic optimization, the removal of polyhydroxyalkanoate formation as competing pathway, was implemented. The final strain had production rates in the range of P. aeruginosa PAO1 at yields of about 0.15 g/g_glucose corresponding to 32% of the theoretical optimum. What's more, rhamnolipid production was independent from biomass formation, a trait that can be exploited for high rhamnolipid production without high biomass formation.

Conclusions

A functional alternative to the pathogenic rhamnolipid producer P. aeruginosa was constructed and characterized. P. putida KT24C1 pVLT31_rhlAB featured the highest yield and titer reported from heterologous rhamnolipid producers with glucose as carbon source. Notably, rhamnolipid production was uncoupled from biomass formation, which allows optimal distribution of resources towards rhamnolipid synthesis. The results are discussed in the context of rational strain engineering by using the concepts of synthetic biology like chassis cells and orthogonality, thereby avoiding the complex regulatory programs of rhamnolipid production existing in the natural producer P. aeruginosa.

Plant carbohydrate scavenging through tonB-dependent receptors: a feature shared by phytopathogenic and aquatic bacteria

TonB-dependent receptors (TBDRs) are outer membrane proteins mainly known for the active transport of iron siderophore complexes in Gram-negative bacteria. Analysis of the genome of the phytopathogenic bacterium Xanthomonas campestris pv. campestris (Xcc), predicts 72 TBDRs. Such an overrepresentation is common in Xanthomonas species but is limited to only a small number of bacteria. Here, we show that one Xcc TBDR transports sucrose with a very high affinity, suggesting that it might be a sucrose scavenger. This TBDR acts with an inner membrane transporter, an amylosucrase and a regulator to utilize sucrose, thus defining a new type of carbohydrate utilization locus, named CUT locus, involving a TBDR for the transport of substrate across the outer membrane. This sucrose CUT locus is required for full pathogenicity on Arabidopsis, showing its importance for the adaptation to host plants. A systematic analysis of Xcc TBDR genes and a genome context survey suggested that several Xcc TBDRs belong to other CUT loci involved in the utilization of various plant carbohydrates. Interestingly, several Xcc TBDRs and CUT loci are conserved in aquatic bacteria such as Caulobacter crescentus, Colwellia psychrerythraea, Saccharophagus degradans, Shewanella spp., Sphingomonas spp. or Pseudoalteromonas spp., which share the ability to degrade a wide variety of complex carbohydrates and display TBDR overrepresentation. We therefore propose that TBDR overrepresentation and the presence of CUT loci designate the ability to scavenge carbohydrates. Thus CUT loci, which seem to participate to the adaptation of phytopathogenic bacteria to their host plants, might also play a very important role in the biogeochemical cycling of plant-derived nutrients in marine environments. Moreover, the TBDRs and CUT loci identified in this study are clearly different from those characterized in the human gut symbiont Bacteroides thetaiotaomicron, which allow glycan foraging, suggesting a convergent evolution of TBDRs in Proteobacteria and Bacteroidetes.

The crystal structure of (S)-3-O-geranylgeranylglyceryl phosphate synthase reveals an ancient fold for an ancient enzyme

We report crystal structures of the citrate and sn-glycerol-1-phosphate (G1P) complexes of (S)-3-O-geranylgeranylglyceryl phosphate synthase from Archaeoglobus fulgidus (AfGGGPS) at 1.55 and 2.0 A resolution, respectively. AfGGGPS is an enzyme that performs the committed step in archaeal lipid biosynthesis, and it presents the first triose phosphate isomerase (TIM)-barrel structure with a prenyltransferase function. Our studies provide insight into the catalytic mechanism of AfGGGPS and demonstrate how it selects for the sn-G1P isomer. The replacement of "Helix 3" by a "strand" in AfGGGPS, a novel modification to the canonical TIM-barrel fold, suggests a model of enzyme adaptation that involves a "greasy slide" and a "swinging door." We propose functions for the homologous PcrB proteins, which are conserved in a subset of pathogenic bacteria, as either prenyltransferases or being involved in lipoteichoic acid biosynthesis. Sequence and structural comparisons lead us to postulate an early evolutionary history for AfGGGPS, which may highlight its role in the emergence of Archaea.

Maltoporin: sugar for physics and biology

Maltoporin has been studied for over 50 years. This trimeric bacterial outer membrane channel allows permeation of sugars such as maltodextrins. Its structure is described and functional studies resulting in a mechanistic transport model are critically discussed.

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Mechanism of maltodextrin transport through LamB

The Gram-negative bacterial outer membrane contains several independent, biochemically distinct transport systems for the acquisition of solutes from the environment. Three or more different classes of membrane proteins exist within the porin superfamily, that facilitate the uptake of sugars, amino acids, nucleotides, vitamins and metals. In spite of crystallographic descriptions of these protein transporters over the past decade, the mechanisms by which porins catalyze solute internalization are controversial, and in some cases still obscure. For many years the research of Maurice Hofnung endeavored to explain the transport of maltose and maltodextrins by LamB, also known as maltoporin. In the shadow of recent crystal structures, his work helped outline a different picture of outer membrane transport physiology, that is a tribute to the powerful genetic approaches Maurice pioneered. These data suggest that the principal determinant of maltodextrin recognition by maltoporin derives from the configuration of aromatic amino acids in its surface loops.

Sugar transport through maltoporin of Escherichia coli: role of the greasy slide

The lining of the maltodextrin-specific maltoporin (LamB) channel exhibits a string of aromatic residues, the greasy slide, part of which has been shown previously by crystallography to be involved in substrate binding. To probe the functional role of the greasy slide, alanine scanning mutagenesis has been performed on the six greasy slide residues and Y118 at the channel constriction. The mutants were characterized by an in vivo uptake assay and sugar-induced-current-noise analysis. Crystallographic analysis of the W74A mutant showed no perturbation of the structure. All mutants showed considerably decreased maltose uptake rates in vivo (<10% of the wild-type value), indicating the functional importance of the investigated residues. Substitutions at the channel center revealed appreciably increased (up to 100-fold) in vitro half-saturation concentrations for maltotriose and maltohexaose binding to the channel. Sugar association rates, however, were significantly affected also by the mutations at either end of the slide (W74A, W358A, and F227A), an effect which became most apparent upon nonsymmetrical sugar addition. The kinetic data are discussed on the basis of an asymmetric one-site two-barrier model, which suggests that, at low substrate concentrations, as are found under physiological conditions, only the heights of the extracellular and periplasmic barriers, which are reduced by the presence of the greasy slide, determine the efficiency of this facilitated diffusion channel.