植物PDR型ABC转运蛋白的结构及功能

ATP-结合盒(ATP-binding cassette,ABC)转运蛋白是目前已知最大、功能最广泛的蛋白质家族。多向耐药性(pleiotropic drug resistance,PDR)蛋白是该家族中仅存于植物和真菌中的一个亚族,结构域与其他亚族相反,即核苷酸结合域(nucleotide-binding domain,NBD)位于跨膜结构域(trans-membrane domain,TMD)的N端。目前已发现PDR型转运蛋白具有转运次生代谢产物和参与胁迫反应等方面的功能。植物PDR基因分为5个亚族:I族基因涉及多种生物和非生物胁迫反应,II￣V族基因功能研究甚少。植物PDR基因在器官水平、化学及环境因素影响下具有特异性较好的表达谱。本文系统阐述了植物PDR型转运蛋白基因的进化、结构及其功能,为理解植物PDR型转运蛋白在生物分子转运和复杂生理功能方面提供一个基础框架。     

Plant ABC transporters

The ATP binding cassette (ABC) superfamily is a large, ubiquitous and diverse group of proteins, most of which mediate transport across biological membranes. ABC transporters have been shown to function not only as ATP-dependent pumps, but also as ion channels and channel regulators. Whilst members of this gene family have been extensively characterised in mammalian and microbial systems, the study of plant ABC transporters is a relatively new field of investigation. Sequences of over 20 plant ABC proteins have been published and include homologues of P-glycoprotein, MRP, PDR5 and organellar transporters. At present, functions have been assigned to a small proportion of these genes and only the MRP subclass has been extensively characterised. This review aims to summarise literature relevant to the study of plant ABC transporters, to review methods of cloning, to discuss the utility of yeast and mammalian systems as models and to speculate on possible roles of uncharacterised ABC transporters in plants.     

植物PDR蛋白转运机制及功能的研究进展

植物多向耐药性（pleiotropic drug resistance,PDR）基因亚家族是ATP结合盒（ATP-binding cassette,ABC）基因家族的一员,其编码的PDR蛋白通过水解ATP释放能量、引起蛋白构象变化实现物质跨膜转运。PDR蛋白可以转运萜类物质、植物生长素和金属离子以应答外界生物和非生物胁迫。综述植物PDR蛋白结构、转运机制及其功能,为克隆植物PDR基因并深入研究其结构与功能提供基础知识。     

Plant pleiotropic drug resistance transporters:Transport mechanism,gene expression,and function

Pleiotropic drug resistance(PDR) transporters belonging to the ABCG subfamily of ATP-binding cassette(ABC)transporters are identified only in fungi and plants. Members of this family are expressed in plants in response to various biotic and abiotic stresses and transport a diverse array of molecules across membranes. Although their detailed transport mechanism is largely unknown, they play important roles in detoxification processes, preventing water loss, transport of phytohormones,and secondary metabolites. This review provides insights into transport mechanisms of plant PDR transporters, their expression profiles, and multitude functions in plants.     

Inventory and Comparative Evolution of the ABC Superfamily in the Genomes of <Emphasis Type="Italic">Phytophthora ramorum</Emphasis> and <Emphasis Type="Italic">Phytophthora sojae</Emphasis>

Automated and manual annotation of the ATP binding cassette (ABC) superfamily in the Phytophthora ramorum and P. sojae genomes has identified 135 and 136 members, respectively, indicating that this family is comparable in size to the Arabidopsis thaliana and rice genomes, and significantly larger than that of two fungal pathogens, Fusarium graminearum and Magnaporthe grisea. The high level of synteny between these oomycete genomes extends to the ABC superfamily, where 108 orthologues were identified by phylogenetic analysis. The largest subfamilies include those most often associated with multidrug resistance. The P. ramorum genome contains 22 multidrug resistance-associated protein (MRP) genes and 49 pleiotropic drug resistance (PDR) genes, while P. sojae contains 20 MRP and 49 PDR genes. Tandem duplication events in the last common ancestor appear to account for much of the expansion of these subfamilies. Recent duplication events in the PDR and ABCG families in both the P. ramorum and the P. sojae genomes indicate that selective expansion of ABC transporters may still be occurring. In other kingdoms, subfamilies define both domain arrangements and proteins having a common phylogenetic origin, but this is not the case for several subfamilies in oomycetes. At least one ABCG type transporter is derived from a PDR transporter, while transporters in the ABCB-half family cluster with transporters from bacterial, plant, and metazoan genomes. Additional examples of transporters that appear to be derived from horizontal transfer events from bacterial genomes include components of transporters associated with iron uptake and DNA repair. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

ABC A-subfamily transporters: structure, function and disease

ABC transporters constitute a family of evolutionarily highly conserved multispan proteins that mediate the translocation of defined substrates across membrane barriers. Evidence has accumulated during the past years to suggest that a subgroup of 12 structurally related "full-size" transporters, referred to as ABC A-subfamily transporters, mediates the transport of a variety of physiologic lipid compounds. The emerging importance of ABC A-transporters in human disease is reflected by the fact that as yet four members of this protein family (ABCA1, ABCA3, ABCR/ABCA4, ABCA12) have been causatively linked to completely unrelated groups of monogenetic disorders including familial high-density lipoprotein (HDL) deficiency, neonatal surfactant deficiency, degenerative retinopathies and congenital keratinization disorders. Although the biological function of the remaining 8 ABC A-transporters currently awaits clarification, they represent promising candidate genes for a presumably equally heterogenous group of Mendelian diseases associated with perturbed cellular lipid transport. This review summarizes our current knowledge on the role of ABC A-subfamily transporters in physiology and disease and explores clinical entities which may be potentially associated with dysfunctional members of this gene subfamily.     

The plant PDR family of ABC transporters

The plant pleiotropic drug resistance (PDR) family of ATP-binding cassette (ABC) transporters has been implicated in the transport of antifungal agents. In this paper, we provide an analysis of the entire family of PDR genes present in the Arabidopsis thaliana (L.) Heynh. genome. This analysis both resolves discrepancies in published inventories of plant ABC proteins and provides an expression analysis of all the annotated Arabidopsis PDR genes. The results indicate that the Arabidopsis genome contains 15 genes encoding PDR proteins and that these genes show a spectrum of specific expression patterns, both at the organ level and in response to various hormonal, environmental and chemical factors. These data provide a scaffold for the future molecular genetic analysis of this important family of ABC transporters. In addition, we demonstrate the usefulness of such data by using them to identify an Arabidopsis PDR protein that may play a role in the extrusion of the antifungal diterpene sclareol. Electronic Supplementary Material is available if you access this article at http://dx.doi.org/10.1007/s00425-002-0889-z. On that page (frame on the left side), a link takes you directly to the supplementary material.     

ABC转运蛋白在植物花器官生长发育中的研究进展

ABC转运蛋白(ATP binding cassette transporter) 是目前发现的最大的蛋白家族之一,其广泛存在于真核生物与原核生物之中,近年来在植物研究领域正受到越来越多的关注。ABC转运蛋白的转运底物种类较为多样,该家族成员几乎作用于植物生长发育的各个阶段,并对植物花器官产生较大的影响。该文对ABC转运蛋白的基本特征及亚家族分类情况进行了总结,重点对近年来国内外有关ABC转运蛋白家族在植物花药和花序轴等花器官生长发育,以及花瓣形态、花色、花香等观赏性状方面的调控功能等方面的研究进展进行了综述,并对ABC转运蛋白在改良植物花色、花香等观赏性状方面的应用潜力进行展望,以期为植物观赏性状的改良提供一定的参考。     

Plant pleiotropic drug resistance transporters： Transport mechanism,gene expression,and function

Pleiotropic drug resistance （PDR） transporters belonging to the ABCG subfamily of ATP-binding cassette （ABC） transporters are identified only in fungi and plants. Members of this family are expressed in plants in response to various biotic and abiotic stresses and transport a diverse array of moleculesacross membranes, Although their detailed transport mechanism is largely unknown, they play important roles in detoxification processes, preventing water loss, transport of phytohormones, and secondary metabolites. This review provides insights into transport mechanisms of plant PDR transporters, their expression profiles, and multitude functions in plants.     

A new member of the NY-ESO-1 gene family is ubiquitously expressed in somatic tissues and evolutionarily conserved

Three single copy ATP-binding cassette (ABC) transporter encoding genes, designated MgAtr3, MgAtr4, and MgAtr5, were cloned and sequenced from the plant pathogenic fungus Mycosphaerella graminicola. The encoded ABC proteins all exhibit the [NBD-TMS(6)](2) configuration and can be classified as novel members of the pleiotropic drug resistance (PDR) class of ABC transporters. The three proteins are highly homologous to other fungal and yeast, ABC proteins involved in multidrug resistance or plant pathogenesis. MgAtr4 and MgAtr5 possess a conserved ABC motif at both the N- and C-terminal domain of the protein. In contrast, the Walker A motif in the N-terminal and the ABC signature in the C-terminal domain of MgAtr3, deviate significantly from the consensus sequence found in other members of the PDR class of ABC transporters. Expression of MgAtr3 could not be detected under any of the conditions tested. However, MgAtr4 and MgAtr5 displayed distinct expression profiles when treated with a range of compounds known to be either substrates or inducers of ABC transporters. These included synthetic fungitoxic compounds, such as imazalil and cyproconazole, natural toxic compounds, such as the plant defence compounds eugenol and psoralen, and the antibiotics cycloheximide and neomycin. The expression pattern of the genes was also dependent on the morphological state of the fungus. The findings suggest a role for MgAtr4 and MgAtr5 during plant pathogenesis and in protection against toxic compounds.     

The ABCA subclass of mammalian transporters

We describe here a subclass of mammalian ABC transporters, the ABCA subfamily. This is a unique group that, in contrast to any other human ABC transporters, lacks a structural counterpart in yeast. The structural hallmark of the ABCA subfamily is the presence of a stretch of hydrophobic amino acids thought to span the membrane within the putative regulatory (R) domain. As for today, four ABCA transporters have been fully characterised but 11 ABCA-encoding genes have been identified. ABCA-specific motifs in the nucleotide binding folds can be detected when analysing the conserved sequences among the different members. These motifs may reveal functional constraints exclusive to this group of ABC transporters.     

The Arabidopsis thaliana ABC protein superfamily, a complete inventory

We describe the first complete inventory of ATP-binding cassette (ABC) proteins from a multicellular organism, the model plant Arabidopsis thaliana. By the application of several search criteria, Arabidopsis was found to contain a total of 129 open reading frames (ORFs) capable of encoding ABC proteins, of which 103 possessed contiguous transmembrane spans and were identified as putative intrinsic membrane proteins. Fifty-two of the putative intrinsic membrane proteins contained at least two transmembrane domains (TMDs) and two nucleotide-binding folds (NBFs) and could be classified as belonging to one of five subfamilies of full-molecule transporters. The other 51 putative membrane proteins, all of which were half-molecule transporters, fell into five subfamilies. Of the remaining ORFs identified, all of which encoded proteins lacking TMDs, 11 could be classified into three subfamilies. There were no obvious homologs in other organisms for 15 of the ORFs which encoded a heterogeneous group of non-intrinsic ABC proteins (NAPs). Unrooted phylogenetic analyses substantiated the subfamily designations. Notable features of the Arabidopsis ABC superfamily was the presence of a large yeast-like PDR subfamily, and the absence of genes encoding bona fide cystic fibrosis transmembrane conductance regulator (CFTR), sulfonylurea receptor (SUR), and heavy metal tolerance factor 1 (HMT1) homologs. Arabidopsis was unusual in its large allocation of ORFs (a minimum of 0.5%) to members of the ABC protein superfamily.     

ABCG transporters and disease

ATP-binding cassette (ABC) transporters form a large family of transmembrane proteins that facilitate the transport of specific substrates across membranes in an ATP-dependent manner. Transported substrates include lipids, lipopolysaccharides, amino acids, peptides, proteins, inorganic ions, sugars and xenobiotics. Despite this broad array of substrates, the physiological substrate of many ABC transporters has remained elusive. ABC transporters are divided into seven subfamilies, A-G, based on sequence similarity and domain organization. Here we review the role of members of the ABCG subfamily in human disease and how the identification of disease genes helped to determine physiological substrates for specific ABC transporters. We focus on the recent discovery of mutations in ABCG2 causing hyperuricemia and gout, which has led to the identification of urate as a physiological substrate for ABCG2.     

A gain-of-function mutation in the Arabidopsis pleiotropic drug resistance transporter PDR9 confers resistance to auxinic herbicides

Arabidopsis (Arabidopsis thaliana) contains 15 genes encoding members of the pleiotropic drug resistance (PDR) family of ATP-binding cassette transporters. These proteins have been speculated to be involved in the detoxification of xenobiotics, however, little experimental support of this hypothesis has been obtained to date. Here we report our characterization of the Arabidopsis PDR9 gene. We isolated a semidominant, gain-of-function mutant, designated pdr9-1, that exhibits increased tolerance to the auxinic herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). Reciprocally, loss-of-function mutations in PDR9 confer 2,4-D hypersensitivity. This altered auxin sensitivity defect of pdr9 mutants is specific for 2,4-D and closely related compounds as these mutants respond normally to the endogenous auxins indole-3-acetic acid and indole-butyric acid. We demonstrate that 2,4-D, but not indole-3-acetic acid transport is affected by mutations in pdr9, suggesting that the PDR9 transporter specifically effluxes 2,4-D out of plant cells without affecting endogenous auxin transport. The semidominant pdr9-1 mutation affects an extremely highly conserved domain present in all known plant PDR transporters. The single amino acid change results in increased PDR9 abundance and provides a novel approach for elucidating the function of plant PDR proteins.     

The diversity of ABC transporter genes across the Phylum Nematoda

It is estimated that one billion people globally are infected by parasitic nematodes, with children, pregnant women, and the elderly particularly susceptible to morbidity from infection. Control methods are limited to de-worming, which is hampered by rapid re-infection and the inevitable development of anthelmintic resistance. One family of proteins that has been implicated in nematode anthelmintic resistance are the ATP binding cassette (ABC) transporters. ABC transporters are characterized by a highly conserved ATP-binding domain and variable transmembrane regions. A growing number of studies have associated ABC transporters in anthelmintic resistance through a protective mechanism of drug efflux. Genetic deletion of P glycoprotein type ABC transporters in Caenorhabditis elegans demonstrated increased sensitivity to anthelmintics, while in the livestock parasite, Haemonchus contortus, anthelmintic use has been shown to increase the expression of ATP transporter genes. These studies as well as others, provide evidence for a potential role of ABC transporters in drug resistance in nematodes. In order to understand more about the family of ABC transporters, we used hidden Markov models to predict ABC transporter proteins from 108 species across the phylum Nematoda and use these data to analyze patterns of diversification and loss in diverse nematode species. We also examined temporal patterns of expression for the ABC transporter family within the filarial nematode Brugia malayi and identify cases of differential expression across diverse life-cycle stages. Taken together, our data provide a comprehensive overview of ABC transporters in diverse nematode species and identify examples of gene loss and diversification in nematodes based on lifestyle and taxonomy.     

Transport proteins of the ABC systems superfamily and their role in drug action and resistance in nematodes

The completion of a number of nematode genomes has provided significant information on ABC systems in these organisms. Nematodes have more ABC systems genes and greater diversity than do mammalian species. Class 1 and class 2 ABC systems, more commonly known as ABC transporters, are present. As in other organisms, nematode ABC systems are characterized by a highly conserved ATP-binding domain (ABC_2) and a less conserved transmembrane domain (ABC_TM1/TM1F). Studies of drug resistance in nematodes have suggested that ABC transporters are part of the resistance mechanism. Evidence in support of this has been obtained from genetic studies where an association between anthelmintic selection and ABC transporters was shown by comparisons between unselected and drug selected, or resistant, populations of parasitic nematodes. In drug resistant populations, genetic polymorphism and diversity, genotype patterns, and linkage disequilibrium were disrupted. Multidrug resistance (MDR) reversing agents that inhibit ABC function improve efficacy in sensitive nematode populations and restore sensitivity in resistant populations. Similar to the situation in clinical oncology, overexpression of ABC systems occurs in drug resistant and sensitive populations following drug exposure, particularly those in the P-glycoprotein (PGP) subfamily. Deletion or disruption of ABC genes, particularly PGP and the multidrug resistance associated protein (MRP), increases sensitivity to some drugs, particularly ivermectin. These studies provide evidence that ABC transporters play a role in drug action and resistance in nematodes.     

The ABC transporter SpTUR2 confers resistance to the antifungal diterpene sclareol

PDR5-like proteins represent one group of the ABC superfamily of transporters. Members of this group are present in plants and, due to the function of PDR5-related proteins in fungi in the excretion of xenobiotics (including antifungal agents), it has been proposed that they might play a similar role in plants in the response to and detoxification of herbicides and fungicides. However, until now no functional data has been presented showing an altered plant response to any herbicide or fungicide as a result of manipulating the expression of a PDR5-like gene in plants. In this paper, we show that the plant SpTUR2 PDR5-like ABC transporter is localised to the plasma membrane and that expression of this protein in Arabidopsis leads to the acquisition of resistance to the diterpenoid antifungal agent sclareol. These data both define a possible endogenous substrate for this transporter and highlight the potential of manipulating plant chemical resistance via modulating the expression of specific PDR5-like transporters.     

Multifunctionality of plant ABC transporters – more than just detoxifiers

The ABC-transporter superfamily is one of the largest protein families, and members can be found in bacteria, fungi, plants and animals. The first reports on plant ABC transporters showed that they are implicated in detoxification processes. The recent completion of the genomic sequencing of Arabidopsis thaliana (L.) Heynh. [Arabidopsis Genome Initiative (2000) Nature 408:796-815] showed that Arabidopsis contains more than 100 ABC-type proteins; 53 genes code for so-called full-size transporters, which are large proteins of about 150 kDa consisting of two hydrophobic and two hydrophilic domains. The large number of genes in the MDR/MRP and PDR5-like sub-clusters and the strong sequence homology found in many cases suggest functional redundancy. One reason for the high number of genes can be attributed to the duplication of large segments of Arabidopsis chromosomes. Recent results indicate that the function of this protein family is not restricted to detoxification processes. Plant ABC transporters have been demonstrated to participate in chlorophyll biosynthesis, formation of Fe/S clusters, stomatal movement, and probably ion fluxes; hence they may play a central role in plant growth and developmental processes.