Maltose Uptake Research Articles

Maltose accumulation-induced cell death in Saccharomyces cerevisiae.

Pretreatment of lignocellulose yields a complex sugar mixture that potentially can be converted into bioethanol and other chemicals by engineered yeast. One approach to overcome competition between sugars for uptake and metabolism is the use of a consortium of specialist strains capable of efficient conversion of single sugars. Here, we show that maltose inhibits cell growth of a xylose-fermenting specialist strain IMX730.1 that is unable to utilize glucose because of the deletion of all hexokinase genes. The growth inhibition cannot be attributed to a competition between maltose and xylose for uptake. The inhibition is enhanced in a strain lacking maltase enzymes (dMalX2) and completely eliminated when all maltose transporters are deleted. High-level accumulation of maltose in the dMalX2 strain is accompanied by a hypotonic-like transcriptional response, while cells are rescued from maltose-induced cell death by the inclusion of an extracellular osmolyte such as sorbitol. These data suggest that maltose-induced cell death is due to high levels of maltose uptake causing hypotonic-like stress conditions and can be prevented through engineering of the maltose transporters. Transporter engineering should be included in the development of stable microbial consortia for the efficient conversion of lignocellulosic feedstocks.

Maltose-Derivatized Fluorescence Turn-On Imaging Probe for Bacteria Detection.

We report a maltose-derivatized fluorescence turn-on imaging probe, Mal-Cz, to detect E. coli and Staphylococci. The fluorescence turn-on is achieved through an intramolecular C-H insertion reaction of the perfluoroaryl azide-functionalized carbazole to give a fluorescent product. Confocal fluorescence microscopy confirmed the successful uptake of Mal-Cz by E. coli and Staphylococci upon photoactivation. The Mal-Cz probe could selectively detect E. coli and S. epidermidis in the presence of P. aeruginosa and M. smegmatis without interference from these bacteria. Both the photoactivation and bacteria detection can be accomplished using a hand-held UV lamp at 365 nm, with the limit of detection of 103 CFU/mL by the naked eye. Mal-Cz could also be used to detect E. coli and S. epidermidis spiked in milk by the naked eye under a hand-held UV lamp. The uptake of Mal-Cz requires metabolically active bacteria: the uptake was reduced in stationary phase bacteria and was diminished in bacteria that were killed by heating or treating with antibiotics or sodium azide. The uptake decreased with increasing concentration of added free maltose, indicating that Mal-Cz hijacked the maltose uptake pathways. In E. coli, the maltose transport systems, including maltoporin LamB, maltose binding protein MBP, and the maltose ATP binding cassette (ABC) transporter MalFGK2, are all critical for the transport of Mal-Cz. The uptake was diminished in the deletion mutants ΔLamB, ΔMalE, ΔMalF, and ΔMalK.

Transport and Utilization of Glycogen Breakdown Products by Gardnerella spp. from the Human Vaginal Microbiome.

Multiple Gardnerella species frequently cooccur in vaginal microbiomes, and several factors, including competition for nutrients such as glycogen could determine their population structure. Although Gardnerella spp. can hydrolyze glycogen to produce glucose, maltose, maltotriose, and maltotetraose, how these sugars are transported and utilized for growth is unknown. We determined the distribution of genes encoding transporter proteins associated with the uptake of glucose, maltose, and malto-oligosaccharides and maltodextrins among Gardnerella species. A total of five different ABC transporters were identified in Gardnerella spp. of which MusEFGK2I and MalXFGK were conserved across all 15 Gardnerella isolates. RafEFGK and TMSP (trehalose, maltose, sucrose, and palatinose) operons were specific to G. vaginalis while the MalEFG transporter was identified in G. leopoldii only. Although no glucose specific sugar-symporters were identified, putative "glucose/galactose porters" and components of a phosphotransferase system were identified. In laboratory experiments, all Gardnerella isolates grew more in the presence of glucose, maltose, maltotriose, and maltotetraose compared to unsupplemented media. In addition, most isolates (10/15) showed significantly more growth on maltotetraose compared to glucose (Kruskal Wallis, P < 0.05) suggesting their preference for longer chain malto-oligosaccharides. Our findings show that although putative MusEFGK2I and MalXFGK transporters are found in all Gardnerella spp., some species-specific transporters are also present. Observed distribution of genes encoding transporter systems was consistent with laboratory observations that Gardnerella spp. grow better on longer chain malto-oligosaccharides. IMPORTANCE Increased abundance of Gardnerella spp. is a diagnostic characteristic of bacterial vaginosis, an imbalance in the human vaginal microbiome associated with troubling symptoms and negative reproductive health outcomes, including increased transmission of sexually transmitted infections and preterm birth. Competition for nutrients is likely an important factor in causing dramatic shifts in the vaginal microbial community. Gardnerella produces enzymes to digest glycogen, an important nutrient source for vaginal bacteria, but little is known about the mechanisms in Gardnerella for uptake of the products of this digestion, or whether Gardnerella use some or all of the products. Our results indicate that Gardnerella may have evolved to preferentially use a subset of the glycogen breakdown products, which would help them reduce direct competition with some other bacteria in the vagina.

Bacterial chemotaxis to saccharides is governed by a trade-off between sensing and uptake

To swim up gradients of nutrients, E. coli senses nutrient concentrations within its periplasm. For small nutrient molecules, periplasmic concentrations typically match extracellular concentrations. However, this is not necessarily the case for saccharides, such as maltose, which are transported into the periplasm via a specific porin. Previous observations have shown that, under various conditions, E. coli limits maltoporin abundance so that, for extracellular micromolar concentrations of maltose, there are predicted to be only nanomolar concentrations of free maltose in the periplasm. Thus, in the micromolar regime, the total uptake of maltose from the external environment into the cytoplasm is limited not by the abundance of cytoplasmic transport proteins but by the abundance of maltoporins. Here, we present results from experiments and modeling suggesting that this porin-limited transport enables E. coli to sense micromolar gradients of maltose despite having a high-affinity ABC transport system that is saturated at these micromolar levels. We used microfluidic assays to study chemotaxis of E. coli in various gradients of maltose and methyl-aspartate and leveraged our experimental observations to develop a mechanistic transport-and-sensing chemotaxis model. Incorporating this model into agent-based simulations, we discover a trade-off between uptake and sensing: although high-affinity transport enables higher uptake rates at low nutrient concentrations, it severely limits the range of dynamic sensing. We thus propose that E. coli may limit periplasmic uptake to increase its chemotactic sensitivity, enabling it to use maltose as an environmental cue.

The mRNA derived MalH sRNA contributes to alternative carbon source utilization by tuning maltoporin expression in E. coli

ABSTRACT Previous high-throughput studies in Gram-negative bacteria identified a large number of 3ʹUTR fragments that potentially function as sRNAs. Here we extensively characterize the MalH sRNA. We show that MalH is a stable degradation intermediate derived from the 3ʹ end of malG, which is part of the maltose uptake operon transcript malEFG. Unlike the majority of bacterial sRNAs, MalH is transiently expressed during the transition from the exponential to the stationary growth phase, suggesting that it contributes to adaptation to changes in nutrient availability. Over-expression of MalH reduces expression of general outer membrane porins and MicA, a repressor of the high-affinity maltose/maltodextrin transporter LamB. Disrupting MalH production and function significantly reduces lamB accumulation when maltose is the only available carbon source, presumably due to the accumulation of the MicA repressor. We propose that MalH is part of a regulatory network that, during the transition phase, directly or indirectly promotes accumulation of high-affinity maltose transporters in the outer membrane by dampening competing pathways.

Extracellular maltotriose hydrolysis by Saccharomyces cerevisiae cells lacking the AGT1 permease.

Maltotriose is the second most abundant sugar present in brewing. However, many yeast strains have difficulties to consume maltotriose, mainly because of its low uptake rate by the yeast cells when compared to glucose and maltose uptake. The AGT1 permease is required for efficient maltotriose fermentation, but some strains deleted in this gene are still able to grow on maltotriose after an extensive lag phase. This manuscript shows that such delayed growth on maltotriose is a consequence of extracellular hydrolysis of the sugar. Our results also indicate that the IMA5-encoded α-glucosidase is likely the enzyme responsible for this phenotype.

Protoplast-Esculin Assay as a New Method to Assay Plant Sucrose Transporters: Characterization of AtSUC6 and AtSUC7 Sucrose Uptake Activity in Arabidopsis Col-0 Ecotype.

The best characterized function of sucrose transporters of the SUC family in plants is the uptake of sucrose into the phloem for long-distance transport of photoassimilates. This important step is usually performed by one specific SUC in every species. However, plants possess small families of several different SUCs which are less well understood. Here, we report on the characterization of AtSUC6 and AtSUC7, two members of the SUC family in Arabidopsis thaliana. Heterologous expression in yeast (Saccharomyces cerevisiae) revealed that AtSUC6Col-0 is a high-affinity H+-symporter that mediates the uptake of sucrose and maltose across the plasma membrane at exceptionally low pH values. Reporter gene analyses revealed a strong expression of AtSUC6Col-0 in reproductive tissues, where the protein product might contribute to sugar uptake into pollen tubes and synergid cells. A knockout of AtSUC6 did not interfere with vegetative development or reproduction, which points toward physiological redundancy of AtSUC6Col-0 with other sugar transporters. Reporter gene analyses showed that AtSUC7Col-0 is expressed in roots and pollen tubes and that this sink specific expression of AtSUC7Col-0 is regulated by intragenic regions. Transport activity of AtSUC7Col-0 could not be analyzed in baker’s yeast or Xenopus oocytes because the protein was not correctly targeted to the plasma membrane in both heterologous expression systems. Therefore, a novel approach to analyze sucrose transporters in planta was developed. Plasma membrane localized SUCs including AtSUC6Col-0 and also sucrose specific SWEETs were able to mediate transport of the fluorescent sucrose analog esculin in transformed mesophyll protoplasts. In contrast, AtSUC7Col-0 is not able to mediate esculin transport across the plasma membrane which implicates that AtSUC7Col-0 might be a non-functional pseudogene. The novel protoplast assay provides a useful tool for the quick and quantitative analysis of sucrose transporters in an in planta expression system.

Proton-solute coupling mechanism of the maltose transporter from Saccharomyces cerevisiae

Mal11 catalyzes proton-coupled maltose transport across the plasma membrane of Saccharomyces cerevisiae. We used structure-based design of mutants and a kinetic analysis of maltose transport to determine the energy coupling mechanism of transport. We find that wildtype Mal11 is extremely well coupled and allows yeast to rapidly accumulate maltose to dangerous levels, resulting under some conditions in self-lysis. Three protonatable residues lining the central membrane-embedded cavity of Mal11 were identified as having potential roles in proton translocation. We probed the mechanistic basis for proton coupling with uphill and downhill transport assays and found that single mutants can still accumulate maltose but with a lower coupling efficiency than the wildtype. Next, we combined the individual mutations and created double and triple mutants. We found some redundancy in the functions of the acidic residues in proton coupling and that no single residue is most critical for proton coupling to maltose uptake, unlike what is usually observed in related transporters. Importantly, the triple mutants were completely uncoupled but still fully active in downhill efflux and equilibrium exchange. Together, these results depict a concerted mechanism of proton transport in Mal11 involving multiple charged residues.

Inhibition of Saccharomyces cerevisiae growth by simultaneous uptake of glucose and maltose

Saccharomyces cerevisiae expresses α-glucoside transporters, such as MalX1p (X=1(Agt1p), 2, 3, 4, and 6), which are proton symporters. These transporters are regulated at transcriptional and posttranslational levels in the presence of glucose. Malt wort contains glucose, maltose, and maltotriose, and the assimilation of maltose is delayed as a function of glucose concentration. With the objective of increasing beer fermentation rates, we characterized α-glucoside transporters and bred laboratory yeasts that expressed various α-glucoside transporters for the simultaneous uptake of different sugars. Mal21p was found to be the most resistant transporter to glucose-induced degradation, and strain (HD17) expressing MAL21 grew on a medium containing glucose or maltose, but not on a medium containing both sugars (YPDM). This unexpected growth defect was observed on a medium containing glucose and >0.1% maltose but was not exhibited by a strain that constitutively expressed maltase. The defect depended on intracellular maltose concentration. Although maltose accumulation caused a surge in turgor pressure, addition of sorbitol to YPDM did not increase growth. When strain HD17 was cultivated in a medium containing only maltose, protein synthesis was inhibited at early times but subsequently resumed with reduction in accumulated maltose, but not if the medium was exchanged for YPDM. We conclude that protein synthesis was terminated under the accumulation of maltose, regardless of extracellular osmolarity, and HD17 could not resume growth, because the intracellular concentration of maltose did not decrease due to insufficient synthesis of maltase. Yeast should incorporate maltose after expressing adequate maltase in beer brewing.

Transcription of malP is subject to phosphotransferase system-dependent regulation in Corynebacterium glutamicum.

The Gram-positive Corynebacterium glutamicum co-metabolizes most carbon sources such as the phosphotransferase system (PTS) sugar glucose and the non-PTS sugar maltose. Maltose is taken up via the ABC-transporter MusEFGK2I, and is further metabolized to glucose phosphate by amylomaltase MalQ, maltodextrin phosphorylase MalP, glucokinase Glk and phosophoglucomutase Pgm. Surprisingly, growth of C. glutamicum strains lacking the general PTS components EI or HPr was strongly impaired on the non-PTS sugar maltose. Complementation experiments showed that a functional PTS phosphorelay is required for optimal growth of C. glutamicum on maltose, implying its involvement in the control of maltose metabolism and/or uptake. To identify the target of this PTS-dependent control, transport measurements with 14C-labelled maltose, Northern blot analyses and enzyme assays were performed. The activities of the maltose transporter and enzymes MalQ, Pgm and GlK were not decreased in PTS-deficient C. glutamicum strains, which was corroborated by comparable transcript amounts of musE, musK and musG, as well as of malQ, in C. glutamicum ΔptsH and WT. By contrast, MalP activity was significantly reduced and only residual amounts of malP transcripts were detected in C. glutamicum ΔptsH when compared to WT. Promoter activity assays with the malP promoter in C. glutamicum ΔptsH and WT confirmed that malP transcription is reduced in the PTS-deficient strain. Taken together, we show here for what is to the best of our knowledge the first time a regulatory function of the PTS in C. glutamicum and identify malP transcription as its target.

Promotion of maltose fermentation at extremely low temperatures using a cryotolerant Saccharomyces cerevisiae strain immobilized on porous cellulosic material

Advantages in maltose fermentation at extremely low temperatures (5–10°C) using an alcohol resistant and cryotolerant yeast strain (Saccharomyces cerevisiae AXAZ-1) immobilized on porous cellulosic material (or tubular cellulose, TC), produced by delignification of wood sawdust, are reported. Pure maltose and glucose media (80, 100 and 140g/l) were examined as model substrates to evaluate the potential effect of TC on the rate of fermentation of maltose containing substrates. The use of TC sharply accelerated the rate of maltose fermentation compared to free cells (FC) in suspension. Fermentation at 5°C by immobilized cells was complete, while FC were unable to ferment maltose at this temperature, in contrast to glucose, which was completely fermented. From the results of maltose and glucose fermentations at 5 and 10°C, it was concluded that the effect of TC was higher at lower fermentation temperature and that its promotional effect on fermentation rate had to be at the step of maltose uptake. Specifically, it is suggested that the presence of TC increased maltose uptake rate by the immobilized cells due to attraction by hydrogen bonding on the TC surface and continuous pumping of maltose towards the cells. Calculation of the activation energy of maltose fermentations at 5, 10 and 15°C showed that it was reduced by an average 42% when cells immobilized on TC were used.

Identification of a two-component signal transduction system that regulates maltose genes in Clostridium perfringens

Clostridium perfringens is a Gram-positive rod that is widely distributed in nature and is the etiological agent of several human and animal diseases. The complete genome sequence of C. perfringens strain 13 has been determined and multiple two-component signal transduction systems identified. One of these systems, designated here as the MalNO system, was analyzed in this study. Microarray analysis was used to carry out functional analysis of a malO mutant. The results, which were confirmed by quantitative reverse-transcriptase PCR, indicated that genes putatively involved in the uptake and metabolism of maltose were up-regulated in the malO mutant. These effects were reversed by complementation with the wild-type malO gene. Growth of these isogenic strains in medium with and without maltose showed that the malO mutant recovered more quickly from maltose deprivation when compared to the wild-type and complemented strains, leading to the conclusion that the MalNO system regulates maltose utilization in C. perfringens. It is postulated that this regulatory network may allow this soil bacterium and opportunistic pathogen to respond to environmental conditions where there are higher concentrations of maltose or maltodextrins, such as in the presence of decaying plant material in rich soil.

Structure determination of a sugar-binding protein from the phytopathogenic bacterium Xanthomonas citri.

The uptake of maltose and related sugars in Gram-negative bacteria is mediated by an ABC transporter encompassing a periplasmic component (the maltose-binding protein or MalE), a pore-forming membrane protein (MalF and MalG) and a membrane-associated ATPase (MalK). In the present study, the structure determination of the apo form of the putative maltose/trehalose-binding protein (Xac-MalE) from the citrus pathogen Xanthomonas citri in space group P6522 is described. The crystals contained two protein molecules in the asymmetric unit and diffracted to 2.8 Å resolution. Xac-MalE conserves the structural and functional features of sugar-binding proteins and a ligand-binding pocket with similar characteristics to eight different orthologues, including the residues for maltose and trehalose interaction. This is the first structure of a sugar-binding protein from a phytopathogenic bacterium, which is highly conserved in all species from the Xanthomonas genus.

Improvement of production of lipopeptide antibiotic iturin A using fish protein

To enhance the production of lipopeptide antibiotic iturin A, nutrient contents of the culture mediums were investigated in both submerged and biofilm fermentations. As a carbon source maltose and as nitrogen source, fish protein was used. In submerged fermentation maltose uptake was found lower (12%) compared to biofilm fermentation (15%) that was associated with higher cellular growth in biofilm. However, requirement of nitrogen (fish protein) concentration was found similar in both submerged and biofilm fermentations. Production of iturin A in submerged fermentation with 12% maltose and 5% fish protein was 4450 mg/L, and in biofilm fermentation it was 5050 mg/L when 15% maltose and 5% fish protein was used.

Carbon catabolite repression of the maltose transporter revealed by X-ray crystallography

SummaryEfficient carbon utilization is critical to the survival of microorganisms in competitive environments. To optimize energy usage, bacteria have developed an integrated control system to preferentially uptake carbohydrates that support rapid growth. The availability of a preferred carbon source such as glucose represses the synthesis and activities of proteins necessary for the transport and metabolism of secondary carbon sources. This regulatory phenomenon is defined as carbon catabolite repression (CCR)1. In enteric bacteria, the key player of CCR is a component of the glucose-specific phosphotransferase system, enzyme IIA (EIIAGlc)1,2. It is known that unphosphorylated EIIAGlc binds and inhibits a variety of transporters when glucose is available1,2. However, understanding the underlying molecular mechanism has been hindered by the complete absence of structures for any EIIAGlc-transporter complexes. Here, we present the 3.9 Å crystal structure of EIIAGlc in complex with the maltose transporter, an ATP-binding cassette (ABC) transporter. The structure shows that two EIIAGlc molecules bind to the cytoplasmic ATPase subunits, stabilizing the transporter in an inward-facing conformation and preventing the structural rearrangements necessary for ATP hydrolysis. We also show that the half maximal inhibitory concentrations of the full-length EIIAGlc and an N-terminal truncation mutant differ by 60 fold, consistent with the hypothesis that the N-terminal region, disordered in the crystal structure, functions as a membrane-anchor to increase the effective EIIAGlc concentration at the membrane3,4. Together these data suggest a model of how the central regulatory protein EIIAGlc allosterically inhibits maltose uptake in E. coli.

Transcription ofα-glucoside transport and metabolism genes in the hybrid brewing yeastSaccharomyces pastorianuswith respect to gene provenance and fermentation temperature

The hybrid lager yeast Saccharomyces pastorianus (S. cerevisiae × S. eubayanus) contains several genes encoding proteins responsible for the uptake and metabolism of maltose and maltotriose. In many cases the genes occur as orthologues, that is, the S. cerevisiae gene exists along with the S. eubayanus gene. Prior to formation of the hybrid, these genes existed in organisms, which had been separated for tens of millions of years and were expected to show some level of genetic and functional differentiation. In this study, oligonucleotide probes were designed for TRAC analysis of transcription of the S. cerevisiae and S. eubayanus orthologues of AGT1, MALx1, MALx2 and MALx3 as well as the S. cerevisiae-derived MPH2/3 genes within the S. pastorianus genome. Specificity of probes was validated using mRNA from S. cerevisiae and from S. eubayanus. Probes were used to analyse gene expression during 15°P wort fermentations conducted at different temperatures (10–20°C). As well as differential expression of different genes, differential expression of orthologues was also observed during fermentation. The differences suggest that, where two forms of the gene exist, either one will dominate (as with AGT1) or expression will be staggered (MALx2), possibly to maximize transport and for efficient degradation of sugars. Copyright © 2013 The Institute of Brewing & Distilling.

Maltose Uptake by the Novel ABC Transport System MusEFGK 2 I Causes Increased Expression of ptsG in Corynebacterium glutamicum

The Gram-positive Corynebacterium glutamicum efficiently metabolizes maltose by a pathway involving maltodextrin and glucose formation by 4-α-glucanotransferase, glucose phosphorylation by glucose kinases, and maltodextrin degradation via maltodextrin phosphorylase and α-phosphoglucomutase. However, maltose uptake in C. glutamicum has not been investigated. Interestingly, the presence of maltose in the medium causes increased expression of ptsG in C. glutamicum by an unknown mechanism, although the ptsG-encoded glucose-specific EII permease of the phosphotransferase system itself is not required for maltose utilization. We identified the maltose uptake system as an ABC transporter encoded by musK (cg2708; ATPase subunit), musE (cg2705; substrate binding protein), musF (cg2704; permease), and musG (cg2703; permease) by combination of data obtained from characterization of maltose uptake and reanalyses of transcriptome data. Deletion of the mus gene cluster in C. glutamicum Δmus abolished maltose uptake and utilization. Northern blotting and reverse transcription-PCR experiments revealed that musK and musE are transcribed monocistronically, whereas musF and musG are part of an operon together with cg2701 (musI), which encodes a membrane protein of unknown function with no homologies to characterized proteins. Characterization of growth and [(14)C]maltose uptake in the musI insertion strain C. glutamicum IMcg2701 showed that musI encodes a novel essential component of the maltose ABC transporter of C. glutamicum. Finally, ptsG expression during cultivation on different carbon sources was analyzed in the maltose uptake-deficient strain C. glutamicum Δmus. Indeed, maltose uptake by the novel ABC transport system MusEFGK2I is required for the positive effect of maltose on ptsG expression in C. glutamicum.

Enterococcus faecalis utilizes maltose by connecting two incompatible metabolic routes via a novel maltose 6'-phosphate phosphatase (MapP).

Similar to Bacillus subtilis, Enterococcus faecalis transports and phosphorylates maltose via a phosphoenolpyruvate (PEP):maltose phosphotransferase system (PTS). The maltose-specific PTS permease is encoded by the malT gene. However, E. faecalis lacks a malA gene encoding a 6-phospho-α-glucosidase, which in B. subtilis hydrolyses maltose 6'-P into glucose and glucose 6-P. Instead, an operon encoding a maltose phosphorylase (MalP), a phosphoglucomutase and a mutarotase starts upstream from malT. MalP was suggested to split maltose 6-P into glucose 1-P and glucose 6-P. However, purified MalP phosphorolyses maltose but not maltose 6'-P. We discovered that the gene downstream from malT encodes a novel enzyme (MapP) that dephosphorylates maltose 6'-P formed by the PTS. The resulting intracellular maltose is cleaved by MalP into glucose and glucose 1-P. Slow uptake of maltose probably via a maltodextrin ABC transporter allows poor growth for the mapP but not the malP mutant. Synthesis of MapP in a B. subtilis mutant accumulating maltose 6'-P restored growth on maltose. MapP catalyses the dephosphorylation of intracellular maltose 6'-P, and the resulting maltose is converted by the B. subtilis maltose phosphorylase into glucose and glucose 1-P. MapP therefore connects PTS-mediated maltose uptake to maltose phosphorylase-catalysed metabolism. Dephosphorylation assays with a wide variety of phospho-substrates revealed that MapP preferably dephosphorylates disaccharides containing an O-α-glycosyl linkage.

Killing two birds with one stone: an ABC transporter regulates gene expression through sequestration of a transcriptional regulator at the membrane

Transcriptional regulators are controlled through various, mostly well-understood, principles. In the study of Richet et al., published in this issue of Molecular Microbiology, fluorescence microscopy was used to uncover an unorthodox mechanism that relies on the dynamic shuttling of a gene regulator between the membrane and the chromosome. When not occupied with transport, the maltose-specific ABC transporter sequesters and thereby inactivates its cognate transcriptional regulator MalT. Upon maltose transport, MalT is released from the membrane and activates the maltose utilization and transport genes. This mechanism prevents induction of MalT by endogenously produced maltotriose, which is the inducer. Thus, the maltose uptake system is a trigger transporter with a bi-functional role in transport and regulation.

Reaction chemistry ABC-style

For a time, membrane proteins proved intractable to X-ray crystallography. There were difficulties of overexpression and purification, multiple choices of solubilizing detergent that could only be decided by trial and error, unwillingness to crystallize, and often, poor diffraction from resultant crystals. However, the tide now seems to be turning, and in PNAS, Oldham and Chen (1) present an X-ray crystallography study of the entire WT maltose uptake protein from Escherichia coli, in which four different structures are described at commendable 2.2- to 2.4-A resolution. Maltose uptake protein is a well-characterized model (2) for the large ABC transporter family, members of which occur in all living organisms. ABC transporters, whether exporters or importers, display a common architecture consisting of two transmembrane domains (TMDs) involved directly in transport and two nucleotide binding domains (NBDs) that cooperatively hydrolyze ATP (actually MgATP) to power transport (3). Internationally, numerous researchers study ABC transporters because of their wide involvement in human and plant disease and consequent importance in medicine and agriculture. Maltose uptake protein has subunit structure MalGFK2, with GF forming the TMDs and K2 forming the head to tail NBD dimer, with its two ATP binding sites. Bacterial ABC importers use a binding protein that captures the requisite transport substrate from the periplasm and delivers it to the transporter. In the work by Oldham and Chen (1), maltose binding protein is bound to the MalGFK2 complex in its open state, and the just-delivered maltose molecule is clearly seen in the transport pathway at midmembrane.