1. ABC Transporters: A Smart Example of Molecular Machineries

Thorsten Jumpertz, I. Barry Holland and Lutz Schmitt

ABC transporters are fascinating molecular machines that use the energy of ATP to catalyze the transport of a tremendous variety of substrates across biological membranes in a vectorial fashion. Here, we will summarize our current knowledge of the domain organization and topology of this class of primary transporters. All ABC transporters analyzed so far are composed of two nucleotide-binding domains (NBD) and two transmembrane domains (TMD) that can be arranged in any possible combination. However, additional transmembrane segments or extended NBDs, raise the possibility that these extensions act as platforms to interact with additional proteins with functional or regulatory consequences. In the light of the recent crystal structures of isolated NBDs in different functional states and the structures of intact ABC transporters, we will also focus on the three-dimensional architecture, domain-domain interactions and putative signaling pathways in these membrane proteins that guarantee efficient substrate recognition and translocation in an ATP-dependent manner.

2. Evolution and Function of the Multidrug Resistance-linked ABC Transporters in Bacteria and Cancer Cells

Zuben E. Sauna and Suresh V. Ambudkar

The ATP-binding cassette (ABC) genes constitute one of the largest super-families in living organisms. The majority of ABC proteins encode membrane proteins that utilize the energy of ATP binding and hydrolysis to transport a diverse set of compounds. ABC systems are critical to the cellular physiology of prokaryotes, facilitating the import of nutrients, the extrusion of toxins and for DNA repair. In eukaryotes, ABC proteins function primarily as efflux pumps (or as import pumps in intracellular organelles) playing important roles in protecting organisms from xenobiotics, antigen presentation, cholesterol and lipid transport. As ABC transporters extrude an unusually large set of chemically diverse compounds, they impede the treatment of microbial infections and human cancers and are implicated in multidrug resistance (MDR). Among prokaryotes, ABC proteins segregate into 29 families belonging to three main classes approximating the functional divisions of the ABC system (importers, exporters and "other"). However, the vertebrate ABC proteins do not have the functional or organizational diversity of the prokaryotic proteins. The primary focus of this review is to discuss the evolution and structure-function relationship of microbial and mammalian ABC transporters responsible for MDR and the current status of substrate specificity and mechanistic aspects of mammalian ABC drug transporters.

3. Structure-function Relationships in ABC Multidrug Transporters

Daniel A. P. Gutmann and Hendrik W. van Veen

Investigations on ATP binding cassette (ABC) transporters have not only been an academic pursuit; these membrane transporters are central to many inherited disorders and are associated with multidrug resistance in cancer cells and pathogenic microorganisms. ABC multidrug transporters are capable of exporting a vast number of structurally unrelated compounds. Quantitative structure-activity relationship (QSAR) analyses of transported substrates, site-directed mutagenesis and use of thiol-reactive or photo-activatable drug analogues have provided useful information regarding the determinants of drug binding by ABC transporters. Recent studies on MsbA from Escherichia coli indicated that residues important for drug selectivity are located in the membrane domains of the transporter close to the leaflet-leaflet interface of the phospholipid bilayer. These and other insights in substrate binding, substrate translocation and energy-coupling by ABC extrusion systems will be reviewed.

4. Can ABC Proteins Confer Drug Resistance in Microorganisms without Being Export Pumps?

James M. Dorrian and Ian D. Kerr

Inhibition of bacterial protein translation is a key point of anti-microbial intervention therapy, with numerous antibiotics functioning as inhibitors of protein synthesis. The macrolide, lincosamide and streptogramin antibiotics (MLS antibiotics) all cause inhibition of protein synthesis by binding to the 50S ribosomal subunit. Resistance to this class of antibiotics is mediated by many different mechanisms, at least one of which involves an unusual class of ABC proteins. These antibiotic resistance element (ARE) type ABC proteins do not include membrane spanning segments within the polypeptide and are not linked in operons to membrane spanning regions. A straightforward transport-based explanation for the function of these ABC proteins in antibiotic resistance is therefore difficult to support. We will illustrate the diversity of these ABC proteins, discuss their clinical relevance, and illustrate three potential modes of action.

5. ABC-type Multidrug Resistance Transporters and their Role in Survival of Bacteria

Patrick J. Bakkes, H. Bart van den Berg van Saparoea and Arnold J.M. Driessen

Multidrug transporters are membrane proteins that actively catalyze the extrusion of structurally and functionally unrelated drugs from the cell. They are considered the major contributors to multidrug resistance (MDR) of bacterial cells. The majority of the identified MDR transporters were shown to rely on ion motive forces for their extrusion activity, while only few systems characterized thus far use ATP hydrolysis to drive efflux. There is accumulating evidence that MDR transporters also have natural physiological roles in addition to the extrusion of man-made synthetic drugs. MDR efflux pumps have been implicated in the protection against toxic compounds present in the natural environment such as bile acids, host-defense molecules and hormones. Here we summarize recent advances in the characterization of ATP-binding cassette ABC-type transporters involved in bacterial multidrug resistance with implications for their physiological functions. In addition, we will discuss possible pitfalls in the functional analysis of ABC-type MDR-like transporters in heterologous systems, and the need for functional characterization in the authentic host.

6. ABC Transporters in Plasmodium falciparum and their Involvement in Resistance to Antimalarial Drugs

Bruno Pradines, Véronique Parquet and Eve Orlandi-Pradines

Malaria is by far the world's most important parasitic disease. Unfortunately, the extensive use of drugs led to the emergence of drug resistance. Quinoline resistance is associated with decreased accumulation of drugs in intracellular vacuoles, due to reduced drug uptake into the cell, increased efflux from the cell or a combination of both. A number of candidate transporters in P. falciparum, including ABC transporters, such as Pgh1 (PfMDR1) and PfMRP, have been proposed to be involved in antimalarial resistance. Targeting these transport proteins with specific drugs may permit to reverse quinoline resistance. Compounds, belonging to different pharmacological classes have demonstrated in the past decade a promising capability to reverse the antimalarial resistance in vitro of parasite isolates, as well as in animal models and human malaria. Herein we will present the progress made in biochemical and genetic basis of antimalarial resistance, emphasizing the recent developments on drugs, which interfere with transmembrane proteins involved in drug efflux, or uptake.

7. Cellular Functions of ABC Proteins in Trypanosomatidae

Philippe Leprohon, Danielle Légaré and Marc Ouellette

The ATP-binding cassette (ABC) protein superfamily is a ubiquitous and functionally versatile family of proteins that is conserved from archae to man. In eukaryotes, most of these proteins are implicated in the transport of a variety of molecules across cellular membranes, whereas others are involved in biological processes unrelated to transport. ABC proteins have been reported in several parasitic protozoa, including parasites of the Trypanosomatidae family, which gathers evolutionary related protozoa of medical relevance like Leishmania spp., Trypanosoma cruzi and Trypanosoma brucei. A recent survey of the genome sequences of three trypanosomatid parasites indicated the presence of a complete set of ABC genes, with representative members of every subfamily described in eukaryotes (ABCA-ABCH). The biological functions of several members of the ABCA, ABCB, ABCC, ABCE and ABCG subfamilies have been described in protozoan parasites and include vesicular trafficking, phospholipids movement, translation and drug resistance. This chapter will review the ABC proteins that have been described in Trypanosomatidae and will present our current understanding on their functions in the biology of these medically important protozoan parasites.

8. ATP-binding Cassette (ABC) Transporters in Yeasts, their Role in Multidrug Resistance and Survival

Hina Sanwal, Sneh Lata Panwar and Rajendra Prasad

Complete genome sequencing of the model yeast Saccharomyces cerevisiae and many other pathogenic fungi such as Candida albicans, has led to the identification and classification of proteins belonging to the ATP-binding cassette (ABC) superfamily. Usually these ABC proteins encode transporters. These transporters provide an advantage to yeasts living in competitive environments by actively secreting toxic compounds, thereby serving as a first-line of defense. Amongst the different subfamilies of the ABC transporters, proteins of the multi drug resistance (MDR) and the pleiotropic drug resistance (PDR) subfamilies are well characterized. Most members of the PDR subfamily are drug transporters involved in the development of antifungal drug resistance caused by the over expression of genes encoding these drug efflux pump proteins. As compared to any other yeast, the role of efflux pumps in drug resistance has been extensively studied in Saccharomyces cerevisiae. However, several studies in the past decade have given momentum to the research pertaining to ABC drug transporters in pathogenic yeasts as well. The presence of large number of ABC members in yeast genomes suggests indispensability of these proteins and suggests that they are physiologically relevant.

9. ABC Transporter Blockers and Reversal of Drug Resistance in Microorganisms

Alicia Ponte-Sucre, Maritza Padrón-Nieves and Emilia Díaz

One of the major causes of drug resistance and chemotherapeutic failure in both cancer and anti-infective therapies is the decrease of effective drug concentrations at their target place, due to the increased action of ATP-binding cassette (ABC) transporters. ABC transporters encompass membrane proteins that couple the energy derived from ATP hydrolysis to the translocation of solutes across biological membranes. Their functions include ancient and conserved mechanisms related to physiological mechanisms in prokaryotes and eukaryotes, emphasizing the biological relevance of ABC transporters. Since ABC transporter blockers can be used in combination with current drugs to increase their effective intracellular concentration, the possible impact of ABC transporter inhibitors is of great clinical interest. Special concern exists since scarce ABC transporter blockers that may be useful to increase the efficacy of current drugs have entered clinical trials and are available to be used in therapeutic regimes. Herein we review the progress made in recent years in the identification, design, availability and applicability of compounds that may work as ABC transporter blockers, as well as the roles of these compounds not only by directly blocking ABC transporters but also in the pharmacokinetics and pharmacodynamics of therapeutic drugs used against infectious diseases. These data may be helpful in the design of strategies to circumvent drug resistance in microorganisms including parasites in clinical circumstances.

10. ABC Transporters as Target for RNA Interference-mediated Reversal of Multidrug Resistance: Implications in Microorganisms

Hermann Lage

Drug resistance is a common clinical problem that occurs in patients suffering from infectious diseases, and in patients suffering from cancer. Prokaryotic and eukaryotic microorganisms as well as neoplastic cells are often found to be refractory to multiple structurally unrelated compounds used during chemotherapy. This phenomenon has been termed multidrug resistance (MDR). In both, prokaryotic and eukaryotic organisms, MDR is frequently associated with overexpression of trans-membrane xenobiotic transport molecules belonging to the superfamily of ATP-binding cassette (ABC)-transporters. Inhibition of ABC-transporters by low-molecular weight compounds has been extensively investigated in cancer patients; however, the clinical results have been disappointing. Consequently, innovative experimental therapeutic strategies for overcoming MDR are urgently needed and are under investigation. These strategies include the application of the novel RNA interference (RNAi) technology. Various RNAi strategies have been applied to reverse MDR in different tumor models in vitro and in vivo indicating that this technology is tremendously effective in reversing ABC-transporter-mediated MDR in cancer and is therefore a promising strategy for overcoming MDR by gene therapeutic applications. The RNAi pathway is restricted to eukaryotic cells. This observation suggests that the RNAi technology appears to be only useful for reversal of ABC-transporter-mediated MDR in eukaryotic infection disease-causing microorganisms, such as parasites. However, besides the data that has been obtained using this technology in pathogenic parasites, efforts have been made to trigger gene silencing by RNAi in prokaryotic cells. Should this approach be successful in the future, RNAi technology could also be considered for overcoming MDR in infection diseases mediated by prokaryotic pathogens.