The multidrug/oligosaccharidyl-lipid/polysaccharide (MOP) exporter superfamily

The Role of Protonation in the PfMATE Transporter Protein Structural Transitions

Multi-antimicrobial extrusion (MATE) transporter membrane proteins provide drug and toxin resistivity by expelling compounds from cells. MATE proteins can be pictured as V-shaped. To regulate its functioning, the protein structure can switch between outward-facing (OF) and inward-facing (IF). Pyrococcus furiosus MATE (PfMATE) is the only member of the multidrug/oligosaccharidyl-lipid/polysaccharide (MOP) superfamily that has available both the IF and OF crystal structures. With the availability of both the IF and OF structures, we are able to perform computational investigations to determine how protonation of specific amino acids causes a cascade of changes in the protein conformation that allow PfMATE to change its state from OF to IF in order to regulate its antiporter function. Using a variety of computational and theoretical techniques, we investigated four different systems of IF and OF PfMATE along with the native archaeal lipid bilayer, without or with protonation at the experimentally determined locations within the protein. We performed molecular dynamics (MD) simulations to investigate the flexibility of the four different PfMATE structures and also performed targeted molecular dynamics (TMD) simulations, during which we observed occluded conformations. Our analysis of hydrogen bond changes, potential of mean force, dynamic network analysis, and transfer entropy analysis provides information on how protonation can induce cascading structural changes responsible for the transition between the IF and OF configurations.

Combinatorial iterative method for metabolic engineering of Yarrowia lipolytica: Application for betanin biosynthesis

Combinatorial library-based metabolic engineering approaches allow lower cost and faster strain development. We developed a genetic toolbox EXPRESSYALI for combinatorial engineering of the oleaginous yeast Yarrowia lipolytica. The toolbox enables consecutive rounds of engineering, where up to three combinatorially assembled gene expression cassettes can be integrated into each yeast clone per round. The cassettes are integrated into distinct intergenic sites or an open reading frame of a target gene if a simultaneous gene knockout is desired. We demonstrate the application of the toolbox by optimizing the Y. lipolytica to produce the red beet color betanin via six consecutive rounds of genome editing and screening. The library size varied between 24 and 360. Library screening was facilitated by automated color-based colony picking. In the first round, betanin pathway genes were integrated, resulting in betanin titer of around 20 mg/L. Through the following five consecutive rounds, additional biosynthetic genes were integrated, and the precursor supply was optimized, resulting in a titer of 70 mg/L. Three beta-glucosidases were deleted to prevent betanin deglycosylation, which led to a betanin titer of 130 mg/L in a small scale and a titer of 1.4 g/L in fed-batch bioreactors. The EXPRESSYALI toolbox can facilitate metabolic engineering efforts in Y. lipolytica (available via AddGene Cat. Nr. 212682-212704, Addgene kit ID # 1000000245).

Molecular characterization of Rft1, an ER membrane protein associated with congenital disorder of glycosylation RFT1-CDG

The oligosaccharide needed for protein N-glycosylation is assembled on a lipid carrier via a multistep pathway. Synthesis is initiated on the cytoplasmic face of the endoplasmic reticulum (ER) and completed on the luminal side after transbilayer translocation of a heptasaccharide lipid intermediate. More than 30 congenital disorders of glycosylation (CDGs) are associated with this pathway, including RFT1-CDG which results from defects in the membrane protein Rft1. Rft1 is essential for the viability of yeast and mammalian cells and was proposed as the transporter needed to flip the heptasaccharide lipid intermediate across the ER membrane. However, other studies indicated that Rft1 is not required for heptasaccharide lipid flipping in microsomes or unilamellar vesicles reconstituted with ER membrane proteins, nor is it required for the viability of at least one eukaryote. It is therefore not known what essential role Rft1 plays in N-glycosylation. Here, we present a molecular characterization of human Rft1, using yeast cells as a reporter system. We show that it is a multispanning membrane protein located in the ER, with its N and C termini facing the cytoplasm. It is not N-glycosylated. The majority of RFT1-CDG mutations map to highly conserved regions of the protein. We identify key residues that are important for Rft1's ability to support N-glycosylation and cell viability. Our results provide a necessary platform for future work on this enigmatic protein.

Molecular characterization of Rft1, an ER membrane protein associated with congenital disorder of glycosylation RFT1-CDG

The oligosaccharide needed for protein N-glycosylation is assembled on a lipid carrier via a multi-step pathway. Synthesis is initiated on the cytoplasmic face of the endoplasmic reticulum (ER) and completed on the luminal side after transbilayer translocation of a heptasaccharide lipid intermediate. More than 30 Congenital Disorders of Glycosylation (CDGs) are associated with this pathway, including RFT1-CDG which results from defects in the membrane protein Rft1. Rft1 is essential for the viability of yeast and mammalian cells and was proposed as the transporter needed to flip the heptasaccharide lipid intermediate across the ER membrane. However, other studies indicated that Rft1 is not required for heptasaccharide lipid flipping in microsomes or unilamellar vesicles reconstituted with ER membrane proteins, nor is it required for the viability of at least one eukaryote. It is therefore not known what essential role Rft1 plays in N-glycosylation. Here, we present a molecular characterization of human Rft1, using yeast cells as a reporter system. We show that it is a multi-spanning membrane protein located in the ER, with its N and C-termini facing the cytoplasm. It is not N-glycosylated. The majority of RFT1-CDG mutations map to highly conserved regions of the protein. We identify key residues that are important for Rft1's ability to support N-glycosylation and cell viability. Our results provide a necessary platform for future work on this enigmatic protein.

Antimicrobial Resistance in Livestock: A Serious Threat to Public Health

Antimicrobial resistance represents an alarming public health problem; its importance is related to the significant clinical implications (increased morbidity, mortality, disease duration, development of comorbidities, and epidemics), as well as its economic effects on the healthcare sector. In fact, therapeutic options are severely limited by the advent and spread of germs resistant to many antibiotics. The situation worldwide is worrying, especially in light of the prevalence of Gram-negative bacteria-Klebsiella pneumoniae and Acinetobacter baumannii-which are frequently isolated in hospital environments and, more specifically, in intensive care units. The problem is compounded by the ineffective treatment of infections by patients who often self-prescribe therapy. Resistant bacteria also show resistance to the latest generation antibiotics, such as carbapenems. In fact, superbacteria, grouped under the acronym extended-spectrum betalactamase (ESBL), are becoming common. Antibiotic resistance is also found in the livestock sector, with serious repercussions on animal production. In general, this phenomenon affects all members of the biosphere and can only be addressed by adopting a holistic "One Health" approach. In this literature overview, a stock is taken of what has been learned about antibiotic resistance, and suggestions are proposed to stem its advance.

Major facilitator superfamily efflux pumps in human pathogens: Role in multidrug resistance and beyond

The major facilitator superfamily (MFS) of proteins constitutes a large group of related solute transporters found across all known living taxa of organisms. The transporters of the MFS contain an extremely diverse array of substrates, including ions, molecules of intermediary metabolism, and structurally different antimicrobial agents. First discovered over 30 years ago, the MFS represents an important collection of integral membrane transporters. Bacterial microorganisms expressing multidrug efflux pumps belonging to the MFS are considered serious pathogens, accounting for alarming morbidity and mortality numbers annually. This review article considers recent advances in the structure-function relationships, the transport mechanism, and modulation of MFS multidrug efflux pumps within the context of drug resistance mechanisms of bacterial pathogens of public health concerns.

Deorphanizing solute carriers in Saccharomyces cerevisiae for secondary uptake of xenobiotic compounds

The exchange of small molecules between the cell and the environment happens through transporter proteins. Besides nutrients and native metabolic products, xenobiotic molecules are also transported, however it is not well understood which transporters are involved. In this study, by combining exo-metabolome screening in yeast with transporter characterization in Xenopus oocytes, we mapped the activity of 30 yeast transporters toward six small non-toxic substrates. Firstly, using LC-MS, we determined 385 compounds from a chemical library that were imported and exported by S. cerevisiae. Of the 385 compounds transported by yeast, we selected six compounds (viz. sn-glycero-3-phosphocholine, 2,5-furandicarboxylic acid, 2-methylpyrazine, cefadroxil, acrylic acid, 2-benzoxazolol) for characterization against 30 S. cerevisiae xenobiotic transport proteins expressed in Xenopus oocytes. The compounds were selected to represent a diverse set of chemicals with a broad interest in applied microbiology. Twenty transporters showed activity toward one or more of the compounds. The tested transporter proteins were mostly promiscuous in equilibrative transport (i.e., facilitated diffusion). The compounds 2,5-furandicarboxylic acid, 2-methylpyrazine, cefadroxil, and sn-glycero-3-phosphocholine were transported equilibratively by transporters that could transport up to three of the compounds. In contrast, the compounds acrylic acid and 2-benzoxazolol, were strictly transported by dedicated transporters. The prevalence of promiscuous equilibrative transporters of non-native substrates has significant implications for strain development in biotechnology and offers an explanation as to why transporter engineering has been a challenge in metabolic engineering. The method described here can be generally applied to study the transport of other small non-toxic molecules. The yeast transporter library is available at AddGene (ID 79999).

Efflux pumps as potential targets for biofilm inhibition

Biofilms account for a great deal of infectious diseases and contribute significantly to antimicrobial resistance. Efflux pumps confer antimicrobial resistance to microorganisms and involve multiple processes of biofilm formation. Efflux pump inhibitors (EPIs) are attracting considerable attention as a biofilm inhibition strategy. The regulatory functions of efflux pumps in biofilm formation such as mediating adherence, quorum sensing (QS) systems, and the expression of biofilm-associated genes have been increasingly identified. The versatile properties confer efflux pumps both positive and negative effects on biofilm formation. Furthermore, the expression and function of efflux pumps in biofilm formation are species-specific. Therefore, this review aims to detail the double-edged sword role of efflux pumps in biofilm formation to provide potential inhibition targets and give an overview of the effects of EPIs on biofilm formation.

Dynamics of efflux pumps in antimicrobial resistance, persistence, and community living of Vibrionaceae

The marine bacteria of the Vibrionaceae family are significant from the point of view of their role in the marine geochemical cycle, as well as symbionts and opportunistic pathogens of aquatic animals and humans. The well-known pathogens of this group, Vibrio cholerae, V. parahaemolyticus, and V. vulnificus, are responsible for significant morbidity and mortality associated with a range of infections from gastroenteritis to bacteremia acquired through the consumption of raw or undercooked seafood and exposure to seawater containing these pathogens. Although generally regarded as susceptible to commonly employed antibiotics, the antimicrobial resistance of Vibrio spp. has been on the rise in the last two decades, which has raised concern about future infections by these bacteria becoming increasingly challenging to treat. Diverse mechanisms of antimicrobial resistance have been discovered in pathogenic vibrios, the most important being the membrane efflux pumps, which contribute to antimicrobial resistance and their virulence, environmental fitness, and persistence through biofilm formation and quorum sensing. In this review, we discuss the evolution of antimicrobial resistance in pathogenic vibrios and some of the well-characterized efflux pumps' contributions to the physiology of antimicrobial resistance, host and environment survival, and their pathogenicity.

An Orphan VrgG Auxiliary Module Related to the Type VI Secretion Systems from Pseudomonas ogarae F113 Mediates Bacterial Killing

The model rhizobacterium Pseudomonas ogarae F113, a relevant plant growth-promoting bacterium, encodes three different Type VI secretion systems (T6SS) in its genome. In silico analysis of its genome revealed the presence of a genetic auxiliary module containing a gene encoding an orphan VgrG protein (VgrG5a) that is not genetically linked to any T6SS structural cluster, but is associated with genes encoding putative T6SS-related proteins: a possible adaptor Tap protein, followed by a putative effector, Tfe8, and its putative cognate immunity protein, Tfi8. The bioinformatic analysis of the VgrG5a auxiliary module has revealed that this cluster is only present in several subgroups of the P. fluorescens complex of species. An analysis of the mutants affecting the vgrG5a and tfe8 genes has shown that the module is involved in bacterial killing. To test whether Tfe8/Tfi8 constitute an effector-immunity pair, the genes encoding Tfe8 and Tfi8 were cloned and expressed in E. coli, showing that the ectopic expression of tfe8 affected growth. The growth defect was suppressed by tfi8 ectopic expression. These results indicate that Tfe8 is a bacterial killing effector, while Tfi8 is its cognate immunity protein. The Tfe8 protein sequence presents homology to the proteins of the MATE family involved in drug extrusion. The Tfe8 effector is a membrane protein with 10 to 12 transmembrane domains that could destabilize the membranes of target cells by the formation of pores, revealing the importance of these effectors for bacterial interaction. Tfe8 represents a novel type of a T6SS effector present in pseudomonads.

Functional Roles of the Conserved Amino Acid Sequence Motif C, the Antiporter Motif, in Membrane Transporters of the Major Facilitator Superfamily

The biological membrane surrounding all living cells forms a hydrophobic barrier to the passage of biologically important molecules. Integral membrane proteins called transporters circumvent the cellular barrier and transport molecules across the cell membrane. These molecular transporters enable the uptake and exit of molecules for cell growth and homeostasis. One important collection of related transporters is the major facilitator superfamily (MFS). This large group of proteins harbors passive and secondary active transporters. The transporters of the MFS consist of uniporters, symporters, and antiporters, which share similarities in structures, predicted mechanism of transport, and highly conserved amino acid sequence motifs. In particular, the antiporter motif, called motif C, is found primarily in antiporters of the MFS. The antiporter motif's molecular elements mediate conformational changes and other molecular physiological roles during substrate transport across the membrane. This review article traces the history of the antiporter motif. It summarizes the physiological evidence reported that supports these biological roles.

Lipopolysaccharide as an antibiotic target

Gram-negative bacteria, including Escherichia coli, Pseudomonas aeruginosa and Acinetobacter baumannii are amongst the highest priority drug-resistant pathogens, for which new antibiotics are urgently needed. Whilst antibiotic drug development is inherently challenging, this is particularly true for Gram-negative bacteria due to the presence of the outer membrane, a highly selective permeability barrier that prevents the ingress of several classes of antibiotic. This selectivity is largely due to an outer leaflet composed of the glycolipid lipopolysaccharide (LPS), which is essential for the viability of almost all Gram-negative bacteria. This essentiality, coupled with the conservation of the synthetic pathway across species and recent breakthroughs in our understanding of transport and membrane homeostasis has made LPS an attractive target for novel antibiotic drug development. Several different targets have been explored and small molecules developed that show promising activity in vitro. However, these endeavours have met limited success in clinical testing and the polymyxins, discovered more than 70 years ago, remain the only LPS-targeting drugs to enter the clinic thus far. In this review, we will discuss efforts to develop therapeutic inhibitors of LPS synthesis and transport and the reasons for limited success, and explore new developments in understanding polymyxin mode of action and the identification of new analogues with reduced toxicity and enhanced activity.

Abnormal glycosylation in glioma: related changes in biology, biomarkers and targeted therapy

Glioma is a rapidly growing and aggressive primary malignant tumor of the central nervous system that can diffusely invade the brain tissue around, and the prognosis of patients is not significantly improved by traditional treatments. One of the most general posttranslational modifications of proteins is glycosylation, and the abnormal distribution of this modification in gliomas may shed light on how it affects biological behaviors of glioma cells, including proliferation, migration, and invasion, which may be produced by regulating protein function, cell-matrix and cell‒cell interactions, and affecting receptor downstream pathways. In this paper, from the perspective of regulating protein glycosylation changes and abnormal expression of glycosylation-related proteins (such as glycosyltransferases in gliomas), we summarize how glycosylation may play a crucial role in the discovery of novel biomarkers and new targeted treatment options for gliomas. Overall, the mechanistic basis of abnormal glycosylation affecting glioma progression remains to be more widely and deeply explored, which not only helps to inspire researchers to further explore related diagnostic and prognostic markers but also provides ideas for discovering effective treatment strategies and improving glioma patient survival and prognosis.

Inhibition of Multidrug Efflux Pumps Belonging to the Major Facilitator Superfamily in Bacterial Pathogens

Bacterial pathogens resistant to multiple structurally distinct antimicrobial agents are causative agents of infectious disease, and they thus constitute a serious concern for public health. Of the various bacterial mechanisms for antimicrobial resistance, active efflux is a well-known system that extrudes clinically relevant antimicrobial agents, rendering specific pathogens recalcitrant to the growth-inhibitory effects of multiple drugs. In particular, multidrug efflux pump members of the major facilitator superfamily constitute central resistance systems in bacterial pathogens. This review article addresses the recent efforts to modulate these antimicrobial efflux transporters from a molecular perspective. Such investigations can potentially restore the clinical efficacy of infectious disease chemotherapy.

Role of bacterial efflux pumps in antibiotic resistance, virulence, and strategies to discover novel efflux pump inhibitors

The problem of antibiotic resistance among pathogenic bacteria has reached a crisis level. The treatment options against infections caused by multiple drug-resistant bacteria are shrinking gradually. The current pace of the discovery of new antibacterial entities is lagging behind the rate of development of new resistance. Efflux pumps play a central role in making a bacterium resistant to multiple antibiotics due to their ability to expel a wide range of structurally diverse compounds. Besides providing an escape from antibacterial compounds, efflux pumps are also involved in bacterial stress response, virulence, biofilm formation, and altering host physiology. Efflux pumps are unique yet challenging targets for the discovery of novel efflux pump inhibitors (EPIs). EPIs could help rejuvenate our currently dried pipeline of antibacterial drug discovery. The current article highlights the recent developments in the field of efflux pumps, challenges faced during the development of EPIs and potential approaches for their development. Additionally, this review highlights the utility of resources such as natural products and machine learning to expand our EPIs arsenal using these latest technologies.

Genome wide identification and characterization of MATE family genes in mangrove plants

Multidrug and Toxic Compound Extrusion (MATE) proteins are essential transporters that extrude metabolites and participate in plant development and cellular detoxification. MATE transporters, which play crucial roles in the survival of mangrove plants under highly challenged environments, by specialized salt extrusion mechanisms, are mined from their genomes and reported here for the first time. Through homology search and domain prediction in the genome assemblies of Avicennia marina, Bruguiera sexangula, Ceriops zippeliana, Kandelia obovata, Rhizophora apiculata and Ceriops tagal, 74, 68, 66, 66, 63 and 64 MATE proteins, respectively were identified. The phylogenetic analysis divided the identified proteins into five major clusters and following the clustering pattern of the functionally characterized proteins, functions of the transporters in each cluster were predicted. Amino acid sequences, exon-intron structure, motif details and subcellular localization pattern for all the 401 proteins are described. The custom designed repeat masking libraries generated for each of these genomes, which will be of extensive use for the researchers worldwide, are also provided in this paper. This is the first study on the MATE genes in mangroves and the results provide comprehensive information on the molecular mechanisms enabling the survival of mangroves under hostile conditions.

Characterization and expression analysis of MATEs in Cannabis sativa L. reveals genes involving in cannabinoid synthesis

The medicinal plant Cannabis sativa L. (C. sativa) accumulates plant cytotoxic but medicinally important cannabinoids in glandular trichomes and flowers of female plants. Although the major biosynthetic pathway of cannabinoids has been revealed, their transportation mechanism is still unknown. Multidrug and toxic compound extrusion proteins (MATEs) can transport plant metabolites, ions and phytohormones intra and inter-cellularly. MATEs could have the potential to translocate cannabinoids or their synthetic intermediates to cellular compartment, thus protecting them from unwanted modifications and cytotoxicity. In this study, we performed a genome-wide identification and expression analysis of Cannabis sativa MATEs (CsMATEs) and revealed 42 CsMATEs that were classified phylogenetically into four conserved subfamilies. Forty-two CsMATEs were unevenly distributed on 10 chromosomes, with 50% CsMATEs were physically adjacent to at least one another CsMATEs and 83% CsMATEs localized on plasma membrane. Tandem duplication is the major evolutionary driving force for CsMATEs expansion. Real-time quantitative PCR revealed CsMATE23, CsMATE28 and CsMATE34 mainly expressed in flower, whereas CsMATE17 and CsMATE27 showed strong transcription in root. Light responsive cis-acting element was most abundant in promoters of CsMATE23, CsMATE28 and CsMATE34. Finally, the contents of cannabinoids and corresponding biosynthetic intermediates as well as expressions of CsMATE28 and CsMATE34 were determined under UV-B treatment, among which strong correlation was found. Our results indicates that CsMATEs might involve in biosynthesis of cannabinoids and has the potential to be used in heterologous production of cannabinoids.

Genome-Wide Analysis of Multidrug and Toxic Compound Extruction Transporters in Grape

Grape (Vitis vinifera L.) is an important fruit crop in the world. It is used as a table grape and is also used for raisin and wine production. Grape berries accumulate secondary metabolites, such as anthocyanins, tannins, and resveratrol, which are known as functional compounds for human health. Multidrug and toxic compound extrusion transporter (MATEs) transport secondary metabolites. MATEs also transport other solutes, including organic acids, and toxic xenobiotics, depending on cation gradient and play various roles in plants. MATE comprises 300-500 amino acid residues and possesses a MATE domain and 8-12 transmembrane domains. In the present study, 59 MATE genes were identified in the grape genome, and phylogenetic analysis revealed the presence of four groups of grape MATEs (Group 1-4). Their information, such as gene structures, protein motifs, predicted subcellular localizations, and gene IDs of four genome annotations, that is, CRIBI v1, CRIBI v2, Genoscope, and Vcost v3, were annotated. The transport substrates and physiological functions of grape MATEs were estimated based on their homology with the analyzed MATEs in other plant species. Group 1 may transport toxic compounds and alkaloids, Group 2 may transport polyphenolic compounds, Group 3 may transport organic acids, and Group 4 may transport plant hormones related to signal transduction. In addition to the known anthocyanin transporters, VvMATE37 and VvMATE39, a novel anthocyanin transporter, VvMATE38 in Group 2, was suggested as a key transporter for anthocyanin accumulation in grape berry skin. VvMATE46, VvMATE47, and VvMATE49 in Group 3 may contribute to Al³⁺ detoxification and Fe²⁺/Fe³⁺ translocation via organic acid transport. This study provides helpful and fundamental information for grape MATE studies and resolves the confusion of gene IDs in different genome annotations.

Crystal structure of the lipid flippase MurJ in a "squeezed" form distinct from its inward- and outward-facing forms

The bacterial peptidoglycan enclosing the cytoplasmic membrane is a fundamental cellular architecture. The integral membrane protein MurJ plays an essential role in flipping the cell wall building block Lipid II across the cytoplasmic membrane for peptidoglycan biosynthesis. Previously reported crystal structures of MurJ have elucidated its V-shaped inward- or outward-facing forms with an internal cavity for substrate binding. MurJ transports Lipid II using its cavity through conformational transitions between these two forms. Here, we report two crystal structures of inward-facing forms from Arsenophonus endosymbiont MurJ and an unprecedented crystal structure of Escherichia coli MurJ in a "squeezed" form, which lacks a cavity to accommodate the substrate, mainly because of the increased proximity of transmembrane helices 2 and 8. Subsequent molecular dynamics simulations supported the hypothesis that the squeezed form is an intermediate conformation. This study fills a gap in our understanding of the Lipid II flipping mechanism.

A Systematic Phylogenomic Classification of the Multidrug and Toxic Compound Extrusion Transporter Gene Family in Plants

Multidrug and toxic compound extrusion (MATE) transporters comprise a multigene family that mediates multiple functions in plants through the efflux of diverse substrates including organic molecules, specialized metabolites, hormones, and xenobiotics. MATE classification based on genome-wide studies remains ambiguous, likely due to a lack of large-scale phylogenomic studies and/or reference sequence datasets. To resolve this, we established a phylogeny of the plant MATE gene family using a comprehensive kingdom-wide phylogenomic analysis of 74 diverse plant species. We identified more than 4,000 MATEs, which were classified into 14 subgroups based on a systematic bioinformatics pipeline using USEARCH, blast+ and synteny network tools. Our classification was performed using a four-step process, whereby MATEs sharing ≥ 60% protein sequence identity with a ≤ 1E-05 threshold at different sequence lengths (either full-length, ≥ 60% length, or ≥ 150 amino acids) or retaining in the similar synteny blocks were assigned to the same subgroup. In this way, we assigned subgroups to 95.8% of the identified MATEs, which we substantiated using synteny network clustering analysis. The subgroups were clustered under four major phylogenetic groups and named according to their clockwise appearance within each group. We then generated a reference sequence dataset, the usefulness of which was demonstrated in the classification of MATEs in additional species not included in the original analysis. Approximately 74% of the plant MATEs exhibited synteny relationships with angiosperm-wide or lineage-, order/family-, and species-specific conservation. Most subgroups evolved independently, and their distinct evolutionary trends were likely associated with the development of functional novelties or the maintenance of conserved functions. Together with the systematic classification and synteny network profiling analyses, we identified all the major evolutionary events experienced by the MATE gene family in plants. We believe that our findings and the reference dataset provide a valuable resource to guide future functional studies aiming to explore the key roles of MATEs in different aspects of plant physiology. Our classification framework can also be readily extendable to other (super) families.

The Bacterial Cell Wall: From Lipid II Flipping to Polymerization

The peptidoglycan (PG) cell wall is an extra-cytoplasmic glycopeptide polymeric structure that protects bacteria from osmotic lysis and determines cellular shape. Since the cell wall surrounds the cytoplasmic membrane, bacteria must add new material to the PG matrix during cell elongation and division. The lipid-linked precursor for PG biogenesis, Lipid II, is synthesized in the inner leaflet of the cytoplasmic membrane and is subsequently translocated across the bilayer so that the PG building block can be polymerized and cross-linked by complex multiprotein machines. This review focuses on major discoveries that have significantly changed our understanding of PG biogenesis in the past decade. In particular, we highlight progress made toward understanding the translocation of Lipid II across the cytoplasmic membrane by the MurJ flippase, as well as the recent discovery of a novel class of PG polymerases, the SEDS (shape, elongation, division, and sporulation) glycosyltransferases RodA and FtsW. Since PG biogenesis is an effective target of antibiotics, these recent developments may lead to the discovery of much-needed new classes of antibiotics to fight bacterial resistance.

Structure and Mechanism of the Lipid Flippase MurJ

Biosynthesis of many important polysaccharides (including peptidoglycan, lipopolysaccharide, and N-linked glycans) necessitates the transport of lipid-linked oligosaccharides (LLO) across membranes from their cytosolic site of synthesis to their sites of utilization. Much of our current understanding of LLO transport comes from genetic, biochemical, and structural studies of the multidrug/oligosaccharidyl-lipid/polysaccharide (MOP) superfamily protein MurJ, which flips the peptidoglycan precursor lipid II. MurJ plays a pivotal role in bacterial cell wall synthesis and is an emerging antibiotic target. Here, we review the mechanism of LLO flipping by MurJ, including the structural basis for lipid II flipping and ion coupling. We then discuss inhibition of MurJ by antibacterials, including humimycins and the phage M lysis protein, as well as how studies on MurJ could provide insight into other flippases, both within and beyond the MOP superfamily. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Mining for novel cyclomaltodextrin glucanotransferases unravels the carbohydrate metabolism pathway via cyclodextrins in Thermoanaerobacterales

Carbohydrate metabolism via cyclodextrins (CM-CD) is an uncommon starch-converting pathway that thoroughly depends on extracellular cyclomaltodextrin glucanotransferases (CGTases) to transform the surrounding starch substrate to α-(1,4)-linked oligosaccharides and cyclodextrins (CDs). The CM-CD pathway has emerged as a convenient microbial adaptation to thrive under extreme temperatures, as CDs are functional amphipathic toroids with higher heat-resistant values than linear dextrins. Nevertheless, although the CM-CD pathway has been described in a few mesophilic bacteria and archaea, it remains obscure in extremely thermophilic prokaryotes (Topt ≥ 70 °C). Here, a new monophyletic group of CGTases with an exceptional three-domain ABC architecture was detected by (meta)genome mining of extremely thermophilic Thermoanaerobacterales living in a wide variety of hot starch-poor environments on Earth. Functional studies of a representative member, CldA, showed a maximum activity in a thermoacidophilic range (pH 4.0 and 80 °C) with remarkable product diversification that yielded a mixture of α:β:γ-CDs (34:62:4) from soluble starch, as well as G3-G7 linear dextrins and fermentable sugars as the primary products. Together, comparative genomics and predictive functional analysis, combined with data of the functionally characterized key proteins of the gene clusters encoding CGTases, revealed the CM-CD pathway in Thermoanaerobacterales and showed that it is involved in the synthesis, transportation, degradation, and metabolic assimilation of CDs.

Cellular export of sugars and amino acids: role in feeding other cells and organisms

Sucrose, hexoses, and raffinose play key roles in the plant metabolism. Sucrose and raffinose, produced by photosynthesis, are translocated from leaves to flowers, developing seeds and roots. Translocation occurs in the sieve elements or sieve tubes of angiosperms. But how is sucrose loaded into and unloaded from the sieve elements? There seem to be two principal routes: one through plasmodesmata and one via the apoplasm. The best-studied transporters are the H+/SUCROSE TRANSPORTERs (SUTs) in the sieve element-companion cell complex. Sucrose is delivered to SUTs by SWEET sugar uniporters that release these key metabolites into the apoplasmic space. The H+/amino acid permeases and the UmamiT amino acid transporters are hypothesized to play analogous roles as the SUT-SWEET pair to transport amino acids. SWEETs and UmamiTs also act in many other important processes-for example, seed filling, nectar secretion, and pollen nutrition. We present information on cell type-specific enrichment of SWEET and UmamiT family members and propose several members to play redundant roles in the efflux of sucrose and amino acids across different cell types in the leaf. Pathogens hijack SWEETs and thus represent a major susceptibility of the plant. Here, we provide an update on the status of research on intercellular and long-distance translocation of key metabolites such as sucrose and amino acids, communication of the plants with the root microbiota via root exudates, discuss the existence of transporters for other important metabolites and provide potential perspectives that may direct future research activities.

The structure of the Aquifex aeolicus MATE family multidrug resistance transporter and sequence comparisons suggest the existence of a new subfamily

Multidrug and toxic compound extrusion (MATE) transporters are widespread in all domains of life. Bacterial MATE transporters confer multidrug resistance by utilizing an electrochemical gradient of H⁺ or Na⁺ to export xenobiotics across the membrane. Despite the availability of X-ray structures of several MATE transporters, a detailed understanding of the transport mechanism has remained elusive. Here we report the crystal structure of a MATE transporter from Aquifex aeolicus at 2.0-Å resolution. In light of its phylogenetic placement outside of the diversity of hitherto-described MATE transporters and the lack of conserved acidic residues, this protein may represent a subfamily of prokaryotic MATE transporters, which was proven by phylogenetic analysis. Furthermore, the crystal structure and substrate docking results indicate that the substrate binding site is located in the N bundle. The importance of residues surrounding this binding site was demonstrated by structure-based site-directed mutagenesis. We suggest that Aq_128 is functionally similar but structurally diverse from DinF subfamily transporters. Our results provide structural insights into the MATE transporter, which further advances our global understanding of this important transporter family.

Crystal structures of a nicotine MATE transporter provide insight into its mechanism of substrate transport

A transporter of the multidrug and toxic compound extrusion (MATE) family, Nicotiana tabacum MATE2 (NtMATE2), is located in the vacuole membrane of the tobacco plant root and is involved in the transportation of nicotine, a secondary or specialized metabolic compound in Solanaceae. Here, we report the crystal structures of NtMATE2 in its outward-facing forms. The overall structure has a bilobate V-shape with pseudo-symmetrical assembly of the N- and C-lobes. In one crystal structure, the C-lobe cavity of NtMATE2 interacts with an unidentified molecule that may partially mimic a substrate. In addition, NtMATE2-specific conformational transitions imply that an unprecedented movement of the transmembrane α-helix 7 is related to the release of the substrate into the vacuolar lumen.

Bacterial Resistance to Antimicrobial Agents

Bacterial pathogens as causative agents of infection constitute an alarming concern in the public health sector. In particular, bacteria with resistance to multiple antimicrobial agents can confound chemotherapeutic efficacy towards infectious diseases. Multidrug-resistant bacteria harbor various molecular and cellular mechanisms for antimicrobial resistance. These antimicrobial resistance mechanisms include active antimicrobial efflux, reduced drug entry into cells of pathogens, enzymatic metabolism of antimicrobial agents to inactive products, biofilm formation, altered drug targets, and protection of antimicrobial targets. These microbial systems represent suitable focuses for investigation to establish the means for their circumvention and to reestablish therapeutic effectiveness. This review briefly summarizes the various antimicrobial resistance mechanisms that are harbored within infectious bacteria.

Transporters Involved in the Biogenesis and Functionalization of the Mycobacterial Cell Envelope

The biology of mycobacteria is dominated by a complex cell envelope of unique composition and structure and of exceptionally low permeability. This cell envelope is the basis of many of the pathogenic features of mycobacteria and the site of susceptibility and resistance to many antibiotics and host defense mechanisms. This review is focused on the transporters that assemble and functionalize this complex structure. It highlights both the progress and the limits of our understanding of how (lipo)polysaccharides, (glyco)lipids, and other bacterial secretion products are translocated across the different layers of the cell envelope to their final extra-cytoplasmic location. It further describes some of the unique strategies evolved by mycobacteria to import nutrients and other products through this highly impermeable barrier.

Genome-wide identification and expression analysis of detoxification efflux carriers (DTX) genes family under abiotic stresses in flax

The detoxification efflux carriers (DTX)/multidrug and toxic compound extrusion (MATE) transporters encompass an ancient gene family of secondary transporters involved in the process of plant detoxification. A genome-wide analysis of these transporters was carried out in order to better understand the transport of secondary metabolites in flaxseed genome (Linum usitassimum). A total of 73 genes coding for DTX/MATE transporters were identified. Gene structure, protein domain and motif organization were found to be notably conserved over the distinct phylogenetic groups, showing the evolutionary significant role of each class. Gene ontology (GO) annotation revealed a link to transporter activities, response to stimulus and localizations. The presence of various hormone and stress-responsive cis-regulatory elements in promoter regions could be directly correlated with the alteration of their transcripts. Tertiary structure showed conservation for pore size and constrains in the pore, which indicate their involvement in the exclusion of toxic substances from the cell. MicroRNA target analysis revealed that LuDTXs genes were targeted by different classes of miRNA families. Twelve LuDTX genes were chosen for further quantitative real-time polymerase chain reaction analysis in response to cold, salinity and cadmium stress at 0, 6, 12 and 24 hours after treatment. Altogether, the identified members of the DTX gene family, their expression profile, phylogenetic and miRNAs analysis might provide opportunities for future functional validation of this important gene family in flax.

Spotlight on the overlapping routes and partners for anthocyanin transport in plants

Secondary metabolites are produced by plants and are classified based on their chemical structure or the biosynthetic routes through which they are synthesized. Among them, flavonoids, including anthocyanins and pro-anthocyanidins (PAs), are abundant in leaves, flowers, fruits, and seed coats in plants. The anthocyanin biosynthetic pathway has been intensively studied, but the molecular mechanism of anthocyanin transport from the synthesis site to the storage site needs attention. Although the major transporters are well defined yet, the redundancy of these transporters for structurally similar or dis-similar anthocyanins motivates additional research. Herein, we reviewed the role of membrane transporters involved in anthocyanin transport, including ATP-binding cassette, multidrug and toxic compound extrusion (MATE), Bilitranslocase-homolog (BTL), and vesicle-mediated transport. We also highlight the ability of transporters to cater distinct anthocyanins or their chemically-modified forms with overlapping transport mechanisms and sequestration into the vacuoles. Our understanding of the anthocyanin transporters could provide anthocyanin-rich crops and fruits with a benefit on human health at a large scale.

Involvement of Human Multidrug and Toxic Compound Extrusion (MATE) Transporters in Testosterone Transport

Multidrug and toxic compound extrusion (MATE) transporters are primarily expressed in the kidneys and liver, where they contribute to the excretion of organic cations. Our previous study suggested that pig MATE2 (class III) participates in testosterone secretion from Leydig cells. In humans, it is unclear which MATE class is involved in testosterone transport. In this study, we aimed to clarify whether human MATE1 (hMATE1) or human MATE2K (hMATE2K) mediates testosterone transport. To confirm that testosterone inhibits transporter-mediated tetraethylammonium (TEA) uptake, a cis-inhibition assay was performed using cells that stably expressed hMATE1 or hMATE2K. Docking simulations were performed to characterize differences in the binding of hMATE1 and hMATE2K to testosterone. Transport experiments in LLC-PK1 cells that stably expressed hMATE1 were used to test whether hMATE1 mediates testosterone transport. We detected differences between the amino acid sequences of the substrate-binding sites of hMATE1 and hMATE2K that could potentially be involved in testosterone binding. Testosterone and estradiol inhibited TEA uptake mediated by hMATE1 but not that mediated by hMATE2K. Transport experiments in LLC-PK1 cells indicated that testosterone might be transported via hMATE1. This study suggested that hMATE1, but not hMATE2K, is involved in human testosterone transport.

Principles of Alternating Access in Multidrug and Toxin Extrusion (MATE) Transporters

The multidrug and toxin extrusion (MATE) transporters catalyze active efflux of a broad range of chemically- and structurally-diverse compounds including antimicrobials and chemotherapeutics, thus contributing to multidrug resistance in pathogenic bacteria and cancers. Multiple methodological approaches have been taken to investigate the structural basis of energy transduction and substrate translocation in MATE transporters. Crystal structures representing members from all three MATE subfamilies have been interpreted within the context of an alternating access mechanism that postulates occupation of distinct structural intermediates in a conformational cycle powered by electrochemical ion gradients. Here we review the structural biology of MATE transporters, integrating the crystallographic models with biophysical and computational studies to define the molecular determinants that shape the transport energy landscape. This holistic analysis highlights both shared and disparate structural and functional features within the MATE family, which underpin an emerging theme of mechanistic diversity within the framework of a conserved structural scaffold.

Physiological Functions of Bacterial "Multidrug" Efflux Pumps

Bacterial multidrug efflux pumps have come to prominence in human and veterinary pathogenesis because they help bacteria protect themselves against the antimicrobials used to overcome their infections. However, it is increasingly realized that many, probably most, such pumps have physiological roles that are distinct from protection of bacteria against antimicrobials administered by humans. Here we undertake a broad survey of the proteins involved, allied to detailed examples of their evolution, energetics, structures, chemical recognition, and molecular mechanisms, together with the experimental strategies that enable rapid and economical progress in understanding their true physiological roles. Once these roles are established, the knowledge can be harnessed to design more effective drugs, improve existing microbial production of drugs for clinical practice and of feedstocks for commercial exploitation, and even develop more sustainable biological processes that avoid, for example, utilization of petroleum.

Racing to build a wall: glycoconjugate assembly in Gram-positive and Gram-negative bacteria

The last two years have seen major advances in understanding the structural basis of bacterial cell envelope glycoconjugate biosynthesis, including capsules, lipopolysaccharide, teichoic acid, cellulose, and peptidoglycan. The recent crystal and cryo-electron microscopy structures of proteins involved in the initial glycosyltransferase steps in the cytoplasm, the transport of large and small lipid-linked glycoconjugates across the inner membrane, the polymerization of glycans in the periplasm, and the export of molecules from the cell have shed light on the mechanisms by which cell envelope glycoconjugates are made. We discuss these recent advances and highlight remaining unanswered questions.

Peptidoglycan: Structure, Synthesis, and Regulation

Peptidoglycan is a defining feature of the bacterial cell wall. Initially identified as a target of the revolutionary beta-lactam antibiotics, peptidoglycan has become a subject of much interest for its biology, its potential for the discovery of novel antibiotic targets, and its role in infection. Peptidoglycan is a large polymer that forms a mesh-like scaffold around the bacterial cytoplasmic membrane. Peptidoglycan synthesis is vital at several stages of the bacterial cell cycle: for expansion of the scaffold during cell elongation and for formation of a septum during cell division. It is a complex multifactorial process that includes formation of monomeric precursors in the cytoplasm, their transport to the periplasm, and polymerization to form a functional peptidoglycan sacculus. These processes require spatio-temporal regulation for successful assembly of a robust sacculus to protect the cell from turgor and determine cell shape. A century of research has uncovered the fundamentals of peptidoglycan biology, and recent studies employing advanced technologies have shed new light on the molecular interactions that govern peptidoglycan synthesis. Here, we describe the peptidoglycan structure, synthesis, and regulation in rod-shaped bacteria, particularly Escherichia coli, with a few examples from Salmonella and other diverse organisms. We focus on the pathway of peptidoglycan sacculus elongation, with special emphasis on discoveries of the past decade that have shaped our understanding of peptidoglycan biology.

Membrane transporters: the key drivers of transport of secondary metabolites in plants

This review summarizes the recent updates in the area of transporters of plant secondary metabolites, including their applied aspects in metabolic engineering of economically important secondary metabolites. Plants have evolved biosynthetic pathways to produce structurally diverse secondary metabolites, which serve distinct functions, including defense against pathogens and herbivory, thereby playing a pivotal role in plant ecological interactions. These compounds often display interesting bioactivities and, therefore, have been used as repositories of natural drugs and phytoceuticals for humans. At an elevated level, plant secondary metabolites could be cytotoxic to the plant cell itself; therefore, plants have developed sophisticated mechanisms to sequester these compounds to prevent cytotoxicity. Many of these valuable natural compounds and their precursors are biosynthesized and accumulated at diverse subcellular locations, and few are even transported to sink organs via long-distance transport, implying the involvement of compartmentalization via intra- and intercellular transport mechanisms. The transporter proteins belonging to different families of transporters, especially ATP binding cassette (ABC) and multidrug and toxic compound extrusion (MATE) have been implicated in membrane-mediated transport of certain plant secondary metabolites. Despite increasing reports on the characterization of transporter proteins and their genes, our knowledge about the transporters of several medicinally and economically important plant secondary metabolites is still enigmatic. A comprehensive understanding of the molecular mechanisms underlying the whole route of secondary metabolite transportome, in addition to the biosynthetic pathways, will aid in systematic and targeted metabolic engineering of high-value secondary metabolites. The present review embodies a comprehensive update on the progress made in the elucidation of transporters of secondary metabolites in view of basic and applied aspects of their transport mechanism.

Decoding capsule synthesis in Streptococcus pneumoniae

Streptococcus pneumoniae synthesizes more than one hundred types of capsular polysaccharides (CPS). While the diversity of the enzymes and transporters involved is enormous, it is not limitless. In this review, we summarized the recent progress on elucidating the structure-function relationships of CPS, the mechanisms by which they are synthesized, how their synthesis is regulated, the host immune response against them, and the development of novel pneumococcal vaccines. Based on the genetic and structural information available, we generated provisional models of the CPS repeating units that remain unsolved. In addition, to facilitate cross-species comparisons and assignment of glycosyltransferases, we illustrated the biosynthetic pathways of the known CPS in a standardized format. Studying the intricate steps of pneumococcal CPS assembly promises to provide novel insights for drug and vaccine development as well as improve our understanding of related pathways in other species.

Identification and Expression of the Multidrug and Toxic Compound Extrusion (MATE) Gene Family in Capsicum annuum and Solanum tuberosum

Multidrug and Toxic Compound Extrusion (MATE) proteins are essential transporters that extrude metabolites and participate in plant development and the detoxification of toxins. Little is known about the MATE gene family in the Solanaceae, which includes species that produce a broad range of specialized metabolites. Here, we identified and analyzed the complement of MATE genes in pepper (Capsicum annuum) and potato (Solanum tuberosum). We classified all MATE genes into five groups based on their phylogenetic relationships and their gene and protein structures. Moreover, we discovered that tandem duplication contributed significantly to the expansion of the pepper MATE family, while both tandem and segmental duplications contributed to the expansion of the potato MATE family, indicating that MATEs took distinct evolutionary paths in these two Solanaceous species. Analysis of ω values showed that all potato and pepper MATE genes experienced purifying selection during evolution. In addition, collinearity analysis showed that MATE genes were highly conserved between pepper and potato. Analysis of cis-elements in MATE promoters and MATE expression patterns revealed that MATE proteins likely function in many stages of plant development, especially during fruit ripening, and when exposed to multiple stresses, consistent with the existence of functional differentiation between duplicated MATE genes. Together, our results lay the foundation for further characterization of pepper and potato MATE gene family members.

Virulence Genetics of an Erwinia amylovora Putative Polysaccharide Transporter Family Member

The Gram-negative enterobacterium Erwinia amylovora causes fire blight disease in apple and pear trees. Lipopolysaccharides and the exopolysaccharide amylovoran are essential E. amylovora virulence factors. We found that mutations in rfbX disrupted amylovoran production and virulence in apple fruits and tree shoots and that the deletion of yibD suppressed the rfbX mutant phenotype. The level of expression of yibD was about 10-fold higher in the ΔrfbX mutant than the wild type. A forward genetic suppressor screen in the ΔrfbX mutant uncovered multiple mutations in yibD and supported the conclusion that the virulence defect of rfbX mutants is due to reduced amylovoran production. The yibD and rfbX genes are expressed as a two-gene operon, yibD rfbX The rfbX gene encodes a previously uncharacterized putative polysaccharide subunit transporter, while yibD encodes a predicted glycosyltransferase. Mutation of rfbX did not have a detectable effect on lipopolysaccharide patterns; however, the overexpression of yibD in both the wild-type and ΔyibD ΔrfbX genetic backgrounds disrupted both amylovoran and lipopolysaccharide production. Additionally, the overexpression of yibD in the ΔyibD ΔrfbX mutant inhibited bacterial growth in amylovoran-inducing medium. This growth inhibition phenotype was used in a forward genetic suppressor screen and reverse-genetics tests to identify several genes involved in lipopolysaccharide production, which, when mutated, restored the ability of the ΔyibD ΔrfbX mutant overexpressing yibD to grow in amylovoran-inducing medium. Remarkably, all the lipopolysaccharide gene mutants tested were defective in lipopolysaccharide and amylovoran production. These results reveal a genetic connection between amylovoran and lipopolysaccharide production in E. amylovora IMPORTANCE This study discovered previously unknown genetic connections between exopolysaccharide and lipopolysaccharide production in the fire blight pathogen Erwinia amylovora This represents a step forward in our understanding of the biology underlying the production of these two macromolecules. Fire blight is an economically important disease that impacts the production of apples and pears worldwide. Few fire blight control measures are available, and growers rely heavily on antibiotic applications at bloom time. Both exopolysaccharide and lipopolysaccharide are E. amylovora virulence factors. Our results indicate that the overexpression of the yibD gene in E. amylovora disrupts both lipopolysaccharide production and exopolysaccharide production. This effect could potentially be used as the basis for the development of an antivirulence treatment for the prevention of fire blight disease.

Comparative Genomics of the Transport Proteins of Ten Lactobacillus Strains

The genus Lactobacillus includes species that may inhabit different anatomical locations in the human body, but the greatest percentage of its species are inhabitants of the gut. Lactobacilli are well known for their probiotic characteristics, although some species may become pathogenic and exert negative effects on human health. The transportome of an organism consists of the sum of the transport proteins encoded within its genome, and studies on the transportome help in the understanding of the various physiological processes taking place in the cell. In this communication we analyze the transport proteins and predict probable substrate specificities of ten Lactobacillus strains. Six of these strains (L. brevis, L. bulgaricus, L. crispatus, L. gasseri, L. reuteri, and L. ruminis) are currently believed to be only probiotic (OP). The remaining four strains (L. acidophilus, L. paracasei, L. planatarum, and L. rhamnosus) can play dual roles, being both probiotic and pathogenic (PAP). The characteristics of the transport systems found in these bacteria were compared with strains (E. coli, Salmonella, and Bacteroides) from our previous studies. Overall, the ten lactobacilli contain high numbers of amino acid transporters, but the PAP strains contain higher number of sugar, amino acid and peptide transporters as well as drug exporters than their OP counterparts. Moreover, some of the OP strains contain pore-forming toxins and drug exporters similar to those of the PAP strains, thus indicative of yet unrecognized pathogenic potential. The transportomes of the lactobacilli seem to be finely tuned according to the extracellular and probiotic lifestyles of these organisms. Taken together, the results of this study help to reveal the physiological and pathogenic potential of common prokaryotic residents in the human body.

A Genome-Wide Survey of MATE Transporters in Brassicaceae and Unveiling Their Expression Profiles under Abiotic Stress in Rapeseed

The multidrug and toxic compound extrusion (MATE) protein family is important in the export of toxins and other substrates, but detailed information on this family in the Brassicaceae has not yet been reported compared to Arabidopsis thaliana. In this study, we identified 57, 124, 81, 85, 130, and 79 MATE genes in A. thaliana, Brassica napus, Brassica oleracea, Brassica rapa, Brassica juncea, and Brassica nigra, respectively, which were unevenly distributed on chromosomes owing to both tandem and segmental duplication events. Phylogenetic analysis showed that these genes could be classified into four subgroups, shared high similarity and conservation within each group, and have evolved mainly through purifying selection. Furthermore, numerous B. napusMATE genes showed differential expression between tissues and developmental stages and between plants treated with heavy metals or hormones and untreated control plants. This differential expression was especially pronounced for the Group 2 and 3 BnaMATE genes, indicating that they may play important roles in stress tolerance and hormone induction. Our results provide a valuable foundation for the functional dissection of the different BnaMATE homologs in B. napus and its parental lines, as well as for the breeding of more stress-tolerant B. napus genotypes.

Changes in the gene expression profile of Arabidopsis thaliana under chromium stress

Based on previous studies and preliminary test results, 200 μM was used as the test concentration of chromium (Cr), and changes in the gene expression profile of Arabidopsis thaliana in response to 24-h treatments of Cr(III) and Cr(VI) were analyzed using the Arabidopsis ATH1 Genome Array. The results were as follows. There were 238 upregulated genes and 858 downregulated genes in response to treatments with Cr(III) and Cr(VI). For Cr(III) and Cr(VI) treatments, there were 185 and 587 specifically upregulated genes as well as 220 and 956 specifically downregulated genes, respectively. Among the common differentially expressed genes (DEGs), the expression levels of genes involved in redox, secondary metabolism, and energy metabolism processes were significantly downregulated, while those of genes related to the stress response, photosynthesis, and sulfur metabolism were significantly upregulated. These findings indicated that Cr seriously affected the normal activities of A. thaliana cells. Some genes associated with stress and regulation were upregulated to adapt to the stress caused by Cr. Among the unique DEGs, the expression levels of genes involved in indole-3-acetic acid (IAA) regulatory pathway were significantly increased in response to Cr(III) treatment; the expression levels of genes involved in the abscisic acid (ABA) regulation pathway and carotenoid synthesis were significantly increased following Cr(VI) treatment. These results revealed some differences in response to Cr(III) and Cr(VI) in A. thaliana.

Detection of Transport Intermediates in the Peptidoglycan Flippase MurJ Identifies Residues Essential for Conformational Cycling

Bacterial cell wall synthesis is an essential process in bacteria and one of the best targets for antibiotics. A critical step on this pathway is the export of the lipid-linked cell wall monomer, Lipid II, by its transporter MurJ. The mechanism by which MurJ mediates the transbilayer movement of Lipid II is not understood because intermediate states of this process have not been observed. Here we demonstrate a method to capture and detect interactions between MurJ and its substrate Lipid II by photo-cross-linking and subsequent biotin-tagging. We show that this method can be used to covalently capture intermediate transport states of Lipid II on MurJ in living cells. Using this strategy we probed several lethal arginine mutants and found that they retain appreciable substrate-binding ability despite being defective in Lipid II transport. We propose that Lipid II binding to these residues during transport induces a conformational change in MurJ required to proceed through the Lipid II transport cycle. The methods described to detect intermediate transport states of MurJ will be useful for characterizing mechanisms of inhibitors.

GtcA is required for LTA glycosylation in Listeria monocytogenes serovar 1/2a and Bacillus subtilis

The cell wall polymers wall teichoic acid (WTA) and lipoteichoic acid (LTA) are often modified with glycosyl and D-alanine residues. Recent studies have shown that a three-component glycosylation system is used for the modification of LTA in several Gram-positive bacteria including Bacillus subtilis and Listeria monocytogenes. In the L. monocytogenes 1/2a strain 10403S, the cytoplasmic glycosyltransferase GtlA is thought to use UDP-galactose to produce the C₅₅-P-galactose lipid intermediate, which is transported across the membrane by an unknown flippase. Next, the galactose residue is transferred onto the LTA backbone on the outside of the cell by the glycosyltransferase GtlB. Here we show that GtcA is necessary for the glycosylation of LTA in L. monocytogenes 10403S and B. subtilis 168 and we hypothesize that these proteins act as C₅₅-P-sugar flippases. With this we revealed that GtcA is involved in the glycosylation of both teichoic acid polymers in L. monocytogenes 10403S, namely WTA with N-acetylglucosamine and LTA with galactose residues. These findings indicate that the L. monocytogenes GtcA protein can act on different C₅₅-P-sugar intermediates. Further characterization of GtcA in L. monocytogenes led to the identification of residues essential for its overall function as well as residues, which predominately impact WTA or LTA glycosylation.

Sequence and structural determinants of ligand-dependent alternating access of a MATE transporter

Multidrug and toxic compound extrusion (MATE) transporters are ubiquitous ion-coupled antiporters that extrude structurally and chemically dissimilar cytotoxic compounds and have been implicated in conferring multidrug resistance. Here, we integrate double electron-electron resonance (DEER) with functional assays and site-directed mutagenesis of conserved residues to illuminate principles of ligand-dependent alternating access of PfMATE, a proton-coupled MATE from the hyperthermophilic archaeon Pyrococcus furiosus Pairs of spin labels monitoring the two sides of the transporter reconstituted into nanodiscs reveal large-amplitude movement of helices that alter the orientation of a putative substrate binding cavity. We found that acidic pH favors formation of an inward-facing (IF) conformation, whereas elevated pH (>7) and the substrate rhodamine 6G stabilizes an outward-facing (OF) conformation. The lipid-dependent PfMATE isomerization between OF and IF conformation is driven by protonation of a previously unidentified intracellular glutamate residue that is critical for drug resistance. Our results can be framed in a mechanistic model of transport that addresses central aspects of ligand coupling and alternating access.

Functional and Structural Roles of the Major Facilitator Superfamily Bacterial Multidrug Efflux Pumps

Pathogenic microorganisms that are multidrug-resistant can pose severe clinical and public health concerns. In particular, bacterial multidrug efflux transporters of the major facilitator superfamily constitute a notable group of drug resistance mechanisms primarily because multidrug-resistant pathogens can become refractory to antimicrobial agents, thus resulting in potentially untreatable bacterial infections. The major facilitator superfamily is composed of thousands of solute transporters that are related in terms of their phylogenetic relationships, primary amino acid sequences, two- and three-dimensional structures, modes of energization (passive and secondary active), and in their mechanisms of solute and ion translocation across the membrane. The major facilitator superfamily is also composed of numerous families and sub-families of homologous transporters that are conserved across all living taxa, from bacteria to humans. Members of this superfamily share several classes of highly conserved amino acid sequence motifs that play essential mechanistic roles during transport. The structural and functional importance of multidrug efflux pumps that belong to the major facilitator family and that are harbored by Gram-negative and -positive bacterial pathogens are considered here.

The multitasking abilities of MATE transporters in plants

As sessile organisms, plants constantly monitor environmental cues and respond appropriately to modulate their growth and development. Membrane transporters act as gatekeepers of the cell regulating both the inflow of useful materials as well as exudation of harmful substances. Members of the multidrug and toxic compound extrusion (MATE) family of transporters are ubiquitously present in almost all forms of life including prokaryotes and eukaryotes. In bacteria, MATE proteins were originally characterized as efflux transporters conferring drug resistance. There are 58 MATE transporters in Arabidopsis thaliana, which are also known as DETOXIFICATION (DTX) proteins. In plants, these integral membrane proteins are involved in a diverse array of functions, encompassing secondary metabolite transport, xenobiotic detoxification, aluminium tolerance, and disease resistance. MATE proteins also regulate overall plant development by controlling phytohormone transport, tip growth processes, and senescence. While most of the functional characterizations of MATE proteins have been reported in Arabidopsis, recent reports suggest that their diverse roles extend to numerous other plant species. The wide array of functions exhibited by MATE proteins highlight their multitasking ability. In this review, we integrate information related to structure and functions of MATE transporters in plants. Since these transporters are central to mechanisms that allow plants to adapt to abiotic and biotic stresses, their study can potentially contribute to improving stress tolerance under changing climatic conditions.

Identification of genomic loci associated with genotypic and phenotypic variation among Pseudomonas aeruginosa clinical isolates from pneumonia

In this work, a genotype-phenotype survey of a highly diversified Pseudomonas aeruginosa collection was conducted, aiming to detail pathogen-associated scenarios that clinicians face nowadays. Genetic relation based on RAPD-PCR of 705 isolates, retrieved from 424 patients and several clinical contexts, reported an almost isolate-specific molecular-pattern. Pneumonia-associated isolates HB13 and HB15, clustered in the same RAPD-PCR group, were selected to evaluate the genomic background underlying their contrasting antibiotic resistance and virulence. The HB13 genome harbors antibiotic-inactivating enzymes-coding genes (e.g. aac(3)-Ia, arr, bla_VIM-2) and single-nucleotide variations (SNVs) in antibiotic targets, likely accounting for its pan-resistance, whereas HB15 susceptibility correlated to predicted dysfunctional alleles. Isolate HB13 showed the unprecedented rhl-cluster absence and variations in other pathogen competitiveness contributors. Conversely, HB15 genome comprises exoenzyme-coding genes and SNVs linked to increased virulence. Secretome analysis identified signatures features with unknown function as potential novel pathogenic (e.g. a MATE-protein in HB13, a protease in HB15) and antibiotic resistance (a HlyD-like secretion protein in HB13) determinants. Detection of active prophages, proteases (including protease IV and alkaline metalloproteinase), a porin and a peptidase in HB15 highlights the secreted arsenal likely essential for its virulent behavior. The presented phenotype-genome association will contribute to the current knowledge on Pseudomonas aeruginosa pathogenomics.

ECOD: identification of distant homology among multidomain and transmembrane domain proteins

The manual classification of protein domains is approaching its 20th anniversary. ECOD is our mixed manual-automatic domain classification. Over time, the types of proteins which require manual curation has changed. Depositions with complex multidomain and multichain arrangements are commonplace. Transmembrane domains are regularly classified. Repeatedly, domains which are initially believed to be novel are found to have homologous links to existing classified domains. Here we present a brief summary of recent manual curation efforts in ECOD generally combined with specific case studies of transmembrane and multidomain proteins wherein manual curation was useful for discovering new homologous relationships. We present a new taxonomy for the classification of ABC transporter transmembrane domains. We examine alternate topologies of the leucine-specific (LS) domain of Leucine tRNA-synthetase. Finally, we elaborate on a distant homologous links between two helical dimerization domains.