Covalent modification of cysteine 193 impairs ATPase function of nucleotide-binding domain of a Candida drug efflux pump

Thiomers and their potential applications in drug delivery

INTRODUCTION

Thiomers are the product of the immobilization of sulfhydryl-bearing ligands onto the polymer backbone of a conventional polymer, which results in a significant improvement in mucoadhesion; in situ gelation and efflux inhibition compare with unchanged polymers. Because of thiol groups, thiomers have more reactivity and enhanced protection against oxidation. Since the late 1990s, extensive work has been conducted on these promising polymeric excipients in the pharmaceutical field. Areas covered: This review covers thiomers, their classification and their different properties. Various techniques for the synthesis, purification and characterization of thiomers are described in detail. This review also encompasses their various properties such as mucoadhesion, permeation enhancement, in situ gelation and efflux inhibition, as well as different formulations based on thiomers. In addition to the use of thiomers as multifunctional excipients, this review also encompasses their use as drugs. Expert opinion: The synthesis is realized by linkage of sulfhydryl-bearing ligands but reported methods give low yields. Higher degrees of modification are not necessary and would probably lead to extreme changes in properties. Nevertheless, an accurate characterization of the final product is important. The scale-up procedure for industrial manufacturing has been adapted to produce GMP materials; Lacrimera® eye drops have already entered the European market.

Mutation of G234 amino acid residue in candida albicans drug-resistance-related protein Rta2p is associated with fluconazole resistance and dihydrosphingosine transport

Widespread and repeated use of azoles has led to the rapid development of drug resistance in Candida albicans. Our previous study found Rta2p, a membrane protein with 7 transmembrane domains, was involved in calcineurin-mediated azole resistance and sphingoid long-chain base release in C. albicans. Conserved amino acids in the transmembrane domain of Rta2p were subjected to site-directed mutagenesis. The sensitivity of C. albicans to fluconazole in vitro was examined by minimum inhibitory concentration and killing assay, and the therapeutic efficacy of fluconazole in vivo was performed by systemic mice candidiasis model. Furthermore, dihydrosphingosine transport activity was detected by NBD labeled D-erythro-dihydrosphingosine uptake and release assay, and the sensitivity to sphingolipid biosynthesis inhibitors. We successfully constructed 14 mutant strains of Rta2p, screened them by minimum inhibitory concentration and found Ca(2+) did not completely induce fluconazole resistance with G158E and G234S mutations. Furthermore, we confirmed that G234S mutant enhanced the therapeutic efficacy of fluconazole against systemic candidiasis and significantly increased the accumulation of dihydrosphingosine by decreasing its release. However, G158E mutant didn't affect drug therapeutic efficacy in vivo and dihydrosphingosine transport in C. albicans. G234 of Rta2p in C. albicans is crucial in calcineurin-mediated fluconazole resistance and dihydrosphingosine transport.

The putative ABC transporter encoded by the orf19.4531 plays a role in the sensitivity of Candida albicans cells to azole antifungal drugs

ATP-binding cassette (ABC) transporters constitute a large superfamily of integral membrane proteins in prokaryotic and eukaryotic cells. In the human fungal pathogen Candida albicans, there are 28 genes encoding ABC transporters and many of them have not been characterized so far. The orf19.4531 (also known as IPF7530) encodes a putative ABC transporter. In this study, we have demonstrated that disruption of orf19.4531 causes C. albicans cells to become tolerant to azoles, but not to polyene antifungals and terbinafine. Therefore, the protein encoded by orf19.4531 is involved in azole sensitivity and we name it as ROA1, the regulator of azole sensitivity 1 gene. Consistently, we show that the expression of ROA1 is responsive to treatment of either fluconazole or ketoconazole inC. albicans In addition, through a GFP tagging approach, Roa1 is localized in a small punctuate compartment adjacent to the vacuolar membrane. However, ROA1 is not essential for the in vitro filamentation of C. albicans cells.

Mutational Analysis of Intracellular Loops Identify Cross Talk with Nucleotide Binding Domains of Yeast ABC Transporter Cdr1p

The ABC transporter Cdr1 protein (Cdr1p) of Candida albicans, which plays a major role in antifungal resistance, has two transmembrane domains (TMDs) and two nucleotide binding domains (NBDs) that are interconnected by extracellular (ECLs) and intracellular (ICLs) loops. To examine the communication interface between the NBDs and ICLs of Cdr1p, we subjected all four ICLs to alanine scanning mutagenesis, replacing each of the 85 residues with an alanine. The resulting ICL mutant library was analyzed by biochemical and phenotypic mapping. Only 18% of the mutants from this library displayed enhanced drug susceptibility. Most of the drug-susceptible mutants displayed uncoupling between ATP hydrolysis and drug transport. The two drug-susceptible ICL1 mutants (I574A and S593A) that lay within or close to the predicted coupling helix yielded two chromosomal suppressor mutations that fall near the Q-loop of NBD2 (R935) and in the Walker A motif (G190) of NBD1. Based on a 3D homology model and kinetic analysis of drug transport, our data suggest that large distances between ICL residues and their respective chromosomal suppressor mutations rule out a direct interaction between them. However, they impact the transport cycle by restoring the coupling interface via indirect downstream signaling.

Preactivated thiomers: their role in drug delivery

INTRODUCTION

Since thiolated polymers - known as thiomers - have entered the pharmaceutical arena in the late 1990 s, more and more academic and industrial research groups have started to work with these promising polymeric excipients. Meanwhile, various thiomers are the subject of clinical trials and the first product based on thiolated chitosan will reach the market in 2015. Due to the formation of disulfide bonds with mercaptopyridine substructures, thiol groups of thiomers are on the one hand more reactive and on the other hand are protected toward oxidation. These so-called preactivated thiomers representing the second generation of thiomers are subject of this review.

AREAS COVERED

Within this review, preactivated thiomers are classified and their mode of action is described. Furthermore, different synthetic pathways, purification and chemical characterization methods of preactivated thiomers are explained. Their properties including mucoadhesive, permeation-enhancing, efflux pump inhibitory and in situ gelling properties are described. In addition, various formulations based on preactivated thiomers are introduced.

EXPERT OPINION

The first-generation thiomers have already shown great potential resulting in various product developments. Preactivated thiomers - representing the second generation of thiomers - offer the additional advantage of even comparatively more reactive sulfhydryl ligands and of stability toward oxidation. According to this, they are promising novel polymeric excipients for various applications.

Thiomers--from bench to market

Thiolated polymers or designated thiomers are obtained by immobilization of sulhydryl bearing ligands on the polymeric backbone of well-established polymers such as poly(acrylates) or chitosans. This functionalization leads to significantly improved mucoadhesive properties compared to the corresponding unmodified polymers, as disulfide bonds between thiol groups of thiomers and cysteine-rich glycoproteins of the mucus gel layer are formed. Furthermore, enzyme- and efflux-pump inhibiting as well as improved permeation-enhancing properties are advantages of thiolization. By the covalent attachment of mercaptonicotinamide substructures via disulfide bonds to thiolated polymers these properties are even substantially further improved and stability towards oxidation even in aqueous media can be provided. Meanwhile, more than 50 research groups worldwide are working on thiolated polymers. For certain thiomers the scale up process for industrial production has already been done and GMP material is available. Furthermore, safety of thiolated poly(acrylic acid), thiolated chitosan and thiolated hyaluronic acid could be demonstrated via orientating studies in human volunteers and via various clinical trials. The first product (Lacrimera® eye drops, Croma-Pharma) containing a chitosan-N-acetylcysteine conjugate for treatment of dry eye syndrome will enter the European market this year. It is the only product providing a sustained protective effect on the ocular surface due to its comparatively much more prolonged residence time worldwide. Various further products utilizing, for instance, thiolated hyaluronic acid in ocular surgery are in the pipeline.

Thiomers: promising platform for macromolecular drug delivery

The application of macromolecules as therapeutic agents holds great promise for several major disorders such as cancer and cardiovascular disease. However, their use is limited by the lack of efficient, safe and specific delivery strategies. A promising strategy to overcome these challenges might be the use of thiolated polymers or designated thiomers. Thiomers are synthesized by immobilization of sulfhydryl bearing ligands on a polymeric backbone of well-established polymers. These multifunctional polymeric excipients show advantages in mucoadhesion, enzyme and efflux pump inhibition in comparison to unmodified polymers. One obstacle in the use of thiomers is that they are prone to oxidation at lower pH but this could be solved by introducing a completely new generation of thiomers, namely, the preactivated thiomer generation. Preactivated thiomers are mixed disulfides, which exhibit oxidation resistance and, beyond that, improved thiomer features. This review summarizes recent findings of polymeric excipients for macromolecular drug delivery as well as their synthesis and distinctive features.

S-protected thiolated chitosan for oral delivery of hydrophilic macromolecules: evaluation of permeation enhancing and efflux pump inhibitory properties

The objective of this study was the investigation of permeation enhancing and P-glycoprotein (P-gp) inhibition effects of a novel thiolated chitosan, the so-named S-protected thiolated chitosan. Mediated by a carbodiimide, increasing amounts of thioglycolic acid (TGA) were covalently bound to chitosan (CS) in the first step of modification. In the second step, these thiol groups of thiolated chitosan were protected by disulfide bond formation with the thiolated aromatic residue 6-mercaptonicotinamide (6-MNA). Mucoadhesive properties of all conjugates were evaluated in vitro on porcine intestinal mucosa based on tensile strength investigations. Permeation enhancing effects were evaluated ex vivo using rat intestinal mucosa and in vitro via Caco-2 cells using the hydrophilic macromolecule FD(4) as the model drug. Caco-2 cells were further used to show P-gp inhibition effects by using Rho-123 as P-gp substrate. Apparent permeability coefficients (P(app)) were calculated and compared to values obtained from each buffer control. Three different thiolated chitosans were generated in the first step of modification, which displayed increasing amounts of covalently attached free thiol groups on the polymer backbone. In the second modification step, more than 50% of these free thiol groups were covalently linked with 6-MNA. Within 3 h of permeation studies on excised rat intestine, P(app) values of all S-protected chitosans were at least 1.3-fold higher compared to those of corresponding thiomers and more than twice as high as that of unmodified chitosan. Additional permeation studies on Caco-2 cells confirmed these results. Because of the chemical modification and higher amount of reactive thiol groups, all S-protected thiolated chitosans exhibit at least 1.4-fold pronounced P-gp inhibition effects in contrast to their corresponding thiomers. These features approve S-protected thiolated chitosan as a promising excipient for various drug delivery systems providing improved permeation enhancing and efflux inhibition effects.

Fungal PDR transporters: Phylogeny, topology, motifs and function

The overexpression of pleiotropic drug resistance (PDR) efflux pumps of the ATP-binding cassette (ABC) transporter superfamily frequently correlates with multidrug resistance. Phylogenetic analysis of 349 full-size ( approximately 160kDa) PDR proteins (Pdrps) from 55 fungal species, including major fungal pathogens, identified nine separate protein clusters (A-G, H1a/H1b and H2). Fungal, plant and human ABCG-family Pdrps possess a nucleotide-binding domain [NBD] and a transmembrane domain [TMD] in a family-defining 'reverse' ABC transporter topology [NBD-TMD] that is duplicated [NBD-TMD](2) in full-size fungal and plant Pdrps. Although full-size Pdrps have similar halves indicating early gene duplication/fusion, they show asymmetry of their NBDs and extracellular loops (ELs). Members of cluster F are most symmetric and may be closely related to the evolutionary ancestor of Pdrps. Unique structural elements are predicted, new PDR-specific motifs identified, and the significance of these and other structural features discussed.

Pyranonaphthoquinone lactones: a new class of AKT selective kinase inhibitors alkylate a regulatory loop cysteine

The naturally occurring pyranonaphthoquinone (PNQ) antibiotic lactoquinomycin and related aglycones were found to be selective inhibitors of the serine-threonine kinase AKT. A set of synthetic PNQs were prepared and a minimum active feature set and preliminary SAR were determined. PNQ lactones inhibit the proliferation of human tumor cell lines containing constitutively activated AKT and show expected effects on cellular biomarkers. Biochemical data are presented supporting a proposed bioreductive alkylation mechanism of action.

A novel catalytic mechanism for ATP hydrolysis employed by the N-terminal nucleotide-binding domain of Cdr1p, a multidrug ABC transporter of Candida albicans

Although essentially conserved, the N-terminal nucleotide-binding domain (NBD) of Cdr1p and other fungal transporters has some unique substitutions of amino acids which appear to have functional significance for the drug transporters. We have previously shown that the typical Cys193 in Walker A as well as Trp326 and Asp327 in the Walker B of N-terminal NBD (NBD-512) of Cdr1p has acquired unique roles in ATP binding and hydrolysis. In the present study, we show that due to spatial proximity, fluorescence resonance energy transfer (FRET) takes place between Trp326 of Walker B and MIANS [2-(4-maleimidoanilino) naphthalene-6-sulfonic acid] on Cys193 of Walker A motif. By exploiting FRET, we demonstrate how these critical amino acids are positioned within the nucleotide-binding pocket of NBD-512 to bind and hydrolyze ATP. Our results show that both Mg2+ coordination and nucleotide binding contribute to the formation of the active site. The entry of Mg2+ into the active site causes the first large conformational change that brings Trp326 and Cys193 in close proximity to each other. We also show that besides Trp326, typical Glu238 in the Q-loop also participates in coordination of Mg2+ by NBD-512. A second conformational change is induced when ATP, but not ADP, docks into the pocket. Asn328 does sensing of the gamma-phosphate of the substrate in the extended Walker B motif, which is essential for the second conformational change that must necessarily precede ATP hydrolysis. Taken together our results imply that the uniquely placed residues in NBD-512 have acquired critical roles in ATP catalysis, which drives drug extrusion.

Candida drug resistance protein 1, a major multidrug ATP binding cassette transporter of Candida albicans, translocates fluorescent phospholipids in a reconstituted system

Candida albicans drug resistance protein 1 (Cdr1p), an ATP-dependent drug efflux pump, contributes to multidrug resistance in Candida-infected immunocompromised patients. Previous cell-based assays suggested that Cdr1p also acts as a phospholipid translocator. To investigate this, we reconstituted purified Cdr1p into sealed membrane vesicles. Comparison of the ATPase activities of sealed and permeabilized proteoliposomes indicated that Cdr1p was asymmetrically reconstituted such that approximately 70% of the molecules had their ATP binding sites accessible to the extravesicular space. Fluorescent glycerophospholipids were incorporated into the outer leaflet of the proteoliposomes, and their transport into the inner leaflet was tracked with a quenching assay using membrane-impermeant dithionite. We observed ATP-dependent transport of the fluorescent lipids into the inner leaflet of the vesicles. With approximately 6 molecules of Cdr1p per vesicle on average, the half-time to reach the maximal extent of transport was approximately 15 min. Transport was reduced in vesicles reconstituted with Cdr1p variants with impaired ATPase activity and could be competed out to different levels by a molar excess of drugs such as fluconazole and miconazole that are known to be effluxed by Cdr1p. Transport was not affected by ampicillin, a compound that is not effluxed by Cdr1p. Our results suggest a direct link between the ability of Cdr1p to translocate fluorescent phospholipids and efflux drugs. We note that only a few members of the ABC superfamily of Candida have a well-defined role as drug exporters; thus, lipid translocation mediated by Cdr1p could reflect its cellular function.

Amino acid residues affecting drug pump function in Candida albicans--C. albicans drug pump function

Membrane-located drug transporters are important components in the multidrug resistance of microbial cells and human tissues. In fungi, clinically important resistance to antifungal drugs most often results from the over-expression of efflux pump proteins in the plasma membrane of the resistant cell. This review describes studies of the ATP binding cassette (ABC) family of membrane efflux pumps in the opportunistic human pathogen Candida albicans and, in particular, examines how changes in the polypeptide sequence can affect pump function. The identification of amino acid residues affecting pump function can provide new insights into efflux pump mechanisms and the relationship between structure and function. Such information will be important for the design of pump inhibitors which could supplement existing antifungal drugs.

Conserved Asp327 of walker B motif in the N-terminal nucleotide binding domain (NBD-1) of Cdr1p of Candida albicans has acquired a new role in ATP hydrolysis

The Walker A and B motifs of nucleotide binding domains (NBDs) of Cdr1p though almost identical to all ABC transporters, has unique substitutions. We have shown in the past that Trp326 of Walker B and Cys193 of Walker A motifs of N-terminal NBD of Cdr1p have distinct roles in ATP binding and hydrolysis, respectively. In the present study, we have examined the role of a well conserved Asp327 in the Walker B motif of the N-terminal NBD, which is preceded (Trp326) and followed (Asn328) by atypical amino acid substitutions and compared it with its equivalent well conserved Asp1026 of the C-terminal NBD of Cdr1p. We observed that the removal of the negative charge by D327N, D327A, D1026N, D1026A, and D327N/D1026N substitutions, resulted in Cdr1p mutant variants that were severely impaired in ATPase activity and drug efflux. Importantly, all of the mutant variants showed characteristics similar to those of the wild type with respect to cell surface expression and photoaffinity drug analogue [125I] IAAP and [3H] azidopine labeling. Although the Cdr1p D327N mutant variant showed comparable binding with [alpha-32P] 8-azido ATP, Cdr1p D1026N and Cdr1p D327N/D1026N mutant variants were crippled in nucleotide binding. That the two conserved carboxylate residues Asp327 and Asp1026 are functionally different was further evident from the pH profile of ATPase activity. The Cdr1p D327N mutant variant showed approximately 40% enhancement of its residual ATPase activity at acidic pH, whereas no such pH effect was seen with the Cdr1p D1026N mutant variant. Our experimental data suggest that Asp327 of N-terminal NBD has acquired a new role to act as a catalytic base in ATP hydrolysis, a role normally conserved for Glu present adjacent to the conserved Asp in the Walker B motif of all the non-fungal transporters.

Structure and function analysis of CaMdr1p, a major facilitator superfamily antifungal efflux transporter protein of Candida albicans: identification of amino acid residues critical for drug/H+ transport

We have cloned and overexpressed multidrug transporter CaMdr1p as a green fluorescent protein-tagged protein to show its capability to extrude drug substrates. The drug extrusion was sensitive to pH and energy inhibitors and displayed selective substrate specificity. CaMdr1p has a unique and conserved antiporter motif, also called motif C [G(X6)G(X3)GP(X2)GP(X2)G], in its transmembrane segment 5 (TMS 5). Alanine scanning of all the amino acids of the TMS 5 by site-directed mutagenesis highlighted the importance of the motif, as well as that of other residues of TMS 5, in drug transport. The mutant variants of TMS 5 were placed in four different categories. The first category had four residues, G244, G251, G255, and G259, which are part of the conserved motif C, and their substitution with alanine resulted in increased sensitivity to drugs and displayed impaired efflux of drugs. Interestingly, first category mutants, when replaced with leucine, resulted in more dramatic loss of drug resistance and efflux. Notwithstanding the location in the core motif, the second category included residues which are part of the motif, such as P260, and those which were not part of the motif, such as L245, W248, P256, and F262, whose substitution with alanine resulted in a severe loss of drug resistance and efflux. The third category included G263, which is a part of motif C, but unlike other conserved glycines, its replacement with alanine or leucine showed no change in the phenotype. The replacement of the remaining 11 residues of the fourth category did not result in any change. The putative helical wheel projection showed clustering of functionally critical residues to one side and thus suggests an asymmetric nature of TMS 5.

Heterozygosity and functional allelic variation in the Candida albicans efflux pump genes CDR1 and CDR2

Elevated expression of the plasma membrane drug efflux pump proteins Cdr1p and Cdr2p was shown to accompany decreased azole susceptibility in Candida albicans clinical isolates. DNA sequence analysis revealed extensive allelic heterozygosity, particularly of CDR2. Cdr2p alleles showed different abilities to transport azoles when individually expressed in Saccharomyces cerevisiae. Loss of heterozygosity, however, did not accompany decreased azole sensitivity in isogenic clinical isolates. Two adjacent non-synonymous single nucleotide polymorphisms (NS-SNPs), G1473A and I1474V in the putative transmembrane (TM) helix 12 of CDR2, were found to be present in six strains including two isogenic pairs. Site-directed mutagenesis showed that the TM-12 NS-SNPs, and principally the G1473A NS-SNP, contributed to functional differences between the proteins encoded by the two Cdr2p alleles in a single strain. Allele-specific PCR revealed that both alleles were equally frequent among 69 clinical isolates and that the majority of isolates (81%) were heterozygous at the G1473A/I1474V locus, a significant (P < 0.001) deviation from the Hardy-Weinberg equilibrium. Phylogenetic analysis by maximum likelihood (Paml) identified 33 codons in CDR2 in which amino acid allelic changes showed a high probability of being selectively advantageous. In contrast, all codons in CDR1 were under purifying selection. Collectively, these results indicate that possession of two functionally different CDR2 alleles in individual strains may confer a selective advantage, but that this is not necessarily due to azole resistance.

Chimeras of the ABC drug transporter Cdr1p reveal functional indispensability of transmembrane domains and nucleotide-binding domains, but transmembrane segment 12 is replaceable with the corresponding homologous region of the non-drug transporter Cdr3p

The molecular basis of the broad substrate recognition and the transport of substrates by Cdr1p, a major drug efflux protein of Candida albicans, is not well understood. To investigate the role of transmembrane domains and nucleotide-binding domains (NBDs) of Cdr1p in drug transport, two sets of protein chimeras were constructed: one set between homologous regions of Cdr1p and the non-drug transporter Cdr3p, and another set consisting of Cdr1p variants comprising either two N- or two C-terminal NBDs of Cdr1p. The replacement of either the N- or the C-terminal half of Cdr1p by the homologous segments of Cdr3p resulted in non-functional recombinant strains expressing chimeric proteins. The results suggest that the chimeric protein could not reach the plasma membrane, probably because of misfolding and subsequent cellular trafficking problems, or the rapid degradation of the chimeras. As an exception, the replacement of transmembrane segment 12 (TMS12) of Cdr1p by the corresponding region of Cdr3p resulted in a functional chimera which displayed unaltered affinity for all the tested substrates. The variant protein comprising either two N-terminal or two C-terminal NBDs of Cdr1p also resulted in non-functional recombinant strains. However, the N-terminal NBD variant, which also showed poor cell surface localization, could be rescued to cell surface, if cells were grown in the presence of drug substrates. The rescued chimera remained non-functional, as was evident from impaired ATPase and efflux activities. Taken together, the results suggest that the two NBDs of Cdr1p are asymmetric and non-exchangeable and that the drug efflux by Cdr1p involves complex interactions between the two halves of the protein.

ABC multidrug transporter Cdr1p of Candida albicans has divergent nucleotide-binding domains which display functional asymmetry

In order to ascertain the molecular basis of ATP-mediated drug extrusion by Cdr1p, a multidrug transporter of Candida albicans, we recently have reported that the Walker A motif of the N-terminal nucleotide biding domain (NBD) of this protein contains an uncommon cysteine residue (C193; GXXGXGCS/T) which is indispensable for ATP hydrolysis. This residue is exceptionally conserved in N-terminal NBDs of fungal ABC transporters and hence makes these transporters an evolutionarily divergent group. However, the presence of a conventional lysine residue at a similar position in the Walker A motif of the C-terminal NBD warrants the individual contribution of both the NBDs in the ATP-driven efflux function of such transporters. In this study we have investigated the contribution of this divergent Walker A motif in the context of the full Cdr1p protein under in vivo conditions by swapping these two crucial amino acids (C193K in Walker A motif of N-terminal NBD and K901C in Walker A motif of C-terminal NBD) between the two NBDs. Both the native and the mutant variants of Cdr1p were integrated at the PDR5 locus as GFP-tagged fusion proteins and were hyper-expressed. Our study shows that both C193K- and K901C-expressing cells elicit a severe impairment of Cdr1p's ATPase function. However, both these mutations have distinct phenotypes with respect to other functional parameters such as substrate efflux and drug resistance profiles. In contrast to C193K, K901C mutant cells were substantially hypersensitive to the tested drugs (fluconazole, ansiomycin, miconazole and cycloheximide) and were unable to expel rhodamine 6G. Our results for the first time show that both NBDs influence the Cdr1p function asymmetrically, and that the positioning of the cysteine and lysine residues within the respective Walker A motifs is functionally not interchangeable.

Alanine scanning of transmembrane helix 11 of Cdr1p ABC antifungal efflux pump of Candida albicans: identification of amino acid residues critical for drug efflux

OBJECTIVES

To investigate the role of transmembrane segment 11 (TMS11) of Candida albicans drug resistance protein (Cdr1p) in drug extrusion.

METHODS

We replaced each of the 21 putative residues of TMS11 with alanine by site-directed mutagenesis. The Saccharomyces cerevisiae AD1-8u(-) strain was used to overexpress the green fluorescent protein tagged wild-type and mutant variants of TMS11 of Cdr1p. The cells expressing mutant variants were functionally characterized.

RESULTS

Out of 21 residues of TMS11, substitution of seven residues, i.e. A1346G, A1347G, T1351A, T1355A, L1358A, F1360A and G1362A, affected differentially the substrate specificity of Cdr1p, while 14 mutants had no significant effect on Cdr1p function. TMS11 projection in an alpha-helical configuration revealed with few exceptions (A1346 and F1360), a distinct segregation of mutation-sensitive residues (A1347, T1351, T1355, L1358 and G1362) towards the more hydrophilic face. Interestingly, mutation-insensitive residues seem to cluster towards the hydrophobic side of the helix. Competition of rhodamine 6G efflux, in the presence of excess of various substrates in the cells expressing native Cdr1p, revealed for the first time the overlapping binding site between azoles (such as ketoconazole, miconazole and itraconazole) and rhodamine 6G. The ability of these azoles to compete with rhodamine 6G was completely lost in mutants F1360A and G1362A, while it was selectively lost in other variants of Cdr1p. We further confirmed that fungicidal synergism of calcineurin inhibitor FK520 with azoles is mediated by Cdr1p; wherein in addition to conserved T1351, substitution of T1355, L1358 and G1362 of TMS11 also resulted in abrogation of synergism.

CONCLUSIONS

Our study for the first time provides an insight into the possible role of TMS11 of Cdr1p in drug efflux.

Phosphorylation of Candida glabrata ATP-binding Cassette Transporter Cdr1p Regulates Drug Efflux Activity and ATPase Stability

Fungal ATP-binding cassette transporter regulation was investigated using Candida glabrata Cdr1p and Pdh1p expressed in Saccharomyces cerevisiae. Rephosphorylation of Pdh1p and Cdr1p was protein kinase A inhibitor-sensitive but responded differentially to Tpk isoforms, stressors, and glucose concentration. Cdr1p Ser(307), which borders the nucleotide binding domain 1 ABC signature motif, and Ser(484), near the membrane, were dephosphorylated on glucose depletion and independently rephosphorylated during glucose exposure or under stress. The S484A enzyme retained half the wild type ATPase activity without affecting azole resistance, but the S307A enzyme was unstable to plasma membrane isolation. Studies of pump function suggested conformational interaction between Ser(484) and Ser(307). An S307A/S484A double mutant, which failed to efflux the Cdr1p substrate rhodamine 6G, had a fluconazole susceptibility 4-fold greater than the Cdr1p expressing strain, twice that of the S307A mutant, but 64-fold less than the control null strain. Stable intragenic suppressors indicative of homodimer nucleotide binding domain 1-nucleotide binding domain 1 interactions partially restored rhodamine 6G pumping and increased fluconazole and rhodamine 6G resistance in the S307A/S484A mutant. Nucleotide binding domain 1 of Cdr1p is a sensor of important physiological stimuli.

Multidrug Resistance in Yeast Candida

The opportunistic human pathogens Candida albicans and other non-albicans species have acquired considerable significance in the recent past due to the enhanced susceptibility of immunocompromised patients. These pathogenic species of Candida derive their importance not only from the severity of their infections but also from their ability to develop resistance against antifungals. Widespread and prolonged use of azoles has led to the rapid development of the phenomenon of multidrug resistance (MDR), which poses a major hurdle in antifungal therapy. Various mechanisms that contribute to the development of MDR have been implicated in Candida as well as in other human fungal pathogens, and some of these include overexpression of or mutations in the target enzyme of azoles, lanosterol 14 alpha-demethylase, and transcriptional activation of genes encoding drug efflux pump proteins belonging to ATP-binding cassette (ABC) as well as to major facilitator superfamilies (MFS) of transporters. The ABC transporters, CDR1, CDR2, and an MFS pump CaMDR1, play a key role in azole resistance as deduced from their high level of expression found in several azole-resistant clinical isolates.