SUMMARY Fungi cause serious infections in the immunocompromised and debilitated, and the incidence of invasive mycoses has increased significantly over the last 3 decades. Slow diagnosis and the relatively few classes of antifungal drugs result in high attributable mortality for systemic fungal infections. Azole antifungals are commonly used for fungal infections, but azole resistance can be a problem for some patient groups. High-level, clinically significant azole resistance usually involves overexpression of plasma membrane efflux pumps belonging to the ATP-binding cassette (ABC) or the major facilitator superfamily class of transporters. The heterologous expression of efflux pumps in model systems, such Saccharomyces cerevisiae, has enabled the functional analysis of efflux pumps from a variety of fungi. Phylogenetic analysis of the ABC pleiotropic drug resistance family has provided a new view of the evolution of this important class of efflux pumps. There are several ways in which the clinical significance of efflux-mediated antifungal drug resistance can be mitigated. Alternative antifungal drugs, such as the echinocandins, that are not efflux pump substrates provide one option. Potential therapeutic approaches that could overcome azole resistance include targeting efflux pump transcriptional regulators and fungal stress response pathways, blockade of energy supply, and direct inhibition of efflux pumps.

Bitopic integral membrane proteins with a single transmembrane helix play diverse roles in catalysis, cell signaling, and morphogenesis. Complete monospanning protein structures are needed to show how interaction between the transmembrane helix and catalytic domain might influence association with the membrane and function. We report crystal structures of full-length Saccharomyces cerevisiae lanosterol 14α-demethylase, a membrane monospanning cytochrome P450 of the CYP51 family that catalyzes the first postcyclization step in ergosterol biosynthesis and is inhibited by triazole drugs. The structures reveal a well-ordered N-terminal amphipathic helix preceding a putative transmembrane helix that would constrain the catalytic domain orientation to lie partly in the lipid bilayer. The structures locate the substrate lanosterol, identify putative substrate and product channels, and reveal constrained interactions with triazole antifungal drugs that are important for drug design and understanding drug resistance.M embrane proteins that span the lipid bilayer once constitute around 50% of all integral membrane proteins (1). Although monospanning membrane proteins carry out numerous key biological functions, including environmental sensing, organellespecific catalysis, and the regulation of cell morphology, only individual domains or subdomains are currently represented in the Protein Data Bank, and structural information about interactions between their transmembrane domains and extramembranous components is lacking. Cytochrome P450 proteins are prominent enzymes with orthologs found in all kingdoms of life. In eukaryotes, microsomal members of this major family of mixed-function mono-oxygenases contain a single transmembrane helix and can be grouped in two broad functional categories: biodefense, such as the first phase of xenobiotic detoxification, and core metabolism including reactions in sterol biosynthesis and fatty acid oxidation (2).The lanosterol 14α-demethylases or CYP51 enzymes, probably the most genetically ancient of the cytochrome P450 families, play a central role in cholesterol or ergosterol biosynthesis (3). CYP51s carry out three consecutive mono-oxygenase reaction cycles to remove the 14α-methyl group from lanosterol to yield 4,4-dimethyl-cholesta-8,14,24-trienol, a key precursor in cholesterol and ergosterol biosynthesis, releasing water and formic acid (3). Because of the key roles that CYP51s play in yeast, filamentous fungi, and some parasitic protozoa, these enzymes are therapeutic targets for antimicrobial agents, including fluconazole (FLC), voriconazole (VCZ), and itraconazole (ITC) (4). Fungal infections play an increasingly significant role in disease, impacting agriculture ecosystems and human health, especially in immunocompromised individuals (5-7) for whom antifungal resistance continually poses a threat (8). In humans CYP51 is being tested as a target for cholesterol-lowering drugs (9) and in antiangiogenic cancer therapies (10). A limited set of cytochrome P450 isoforms (1A2, 2C8, 2C...

The study of eukaryotic membrane proteins has been hampered by a paucity of systems that achieve consistent high-level functional protein expression. We report the use of a modified membrane protein hyperexpression system to characterize three classes of fungal membrane proteins (ABC transporters Pdr5p, CaCdr1p, CaCdr2p, CgCdr1p, CgPdh1p, CkAbc1p, and CneMdr1p, the major facilitator superfamily transporter CaMdr1p, and the cytochrome P450 enzyme CaErg11p) that contribute to the drug resistance phenotypes of five pathogenic fungi and to express human P glycoprotein (HsAbcb1p). The hyperexpression system consists of a set of plasmids that direct the stable integration of a single copy of the expression cassette at the chromosomal PDR5 locus of a modified host Saccharomyces cerevisiae strain, AD⌬. Overexpression of heterologous proteins at levels of up to 29% of plasma membrane protein was achieved. Membrane proteins were expressed with or without green fluorescent protein (GFP), monomeric red fluorescent protein, His, FLAG/His, Cys, or His/Cys tags. Most GFP-tagged proteins tested were correctly trafficked within the cell, and His-tagged proteins could be affinity purified. Kinetic analysis of ABC transporters indicated that the apparent K m value and the V max value of ATPase activities were not significantly affected by the addition of His tags. The efflux properties of seven fungal drug pumps were characterized by their substrate specificities and their unique patterns of inhibition by eight xenobiotics that chemosensitized S. cerevisiae strains overexpressing ABC drug pumps to fluconazole. The modified hyperexpression system has wide application for the study of eukaryotic membrane proteins and could also be used in the pharmaceutical industry for drug screening.The resolution and exploitation of protein structure and function are among the greatest biological challenges in the postgenomic era. These challenges, and their potential dividends, are greatest for membrane proteins, which are notoriously difficult to functionally express and purify in the quantities and forms needed for drug discovery or for high-resolution X-ray crystallography (1, 16). About a quarter of the cellular proteome consists of membrane proteins (5), which often play vital physiological roles: from environmental sensing to energy transduction, from nutrient uptake to drug efflux, and from cellular proliferation to programmed cell death. Membrane proteins are involved in many prominent diseases, including cystic fibrosis (48), type 2 diabetes (49), heart disease (52), and the drug resistance of numerous cancers (57). Hence, they are the targets for many therapies and constitute up to 70% of the drug targets used in medicine today. Membrane proteins also play key roles in drug modification, detoxification, and resistance in a wide variety of prokaryotic and eukaryotic systems (7). A fundamental understanding of cell biology, cell physiology, and cell-drug interactions therefore requires a detailed analysis of membrane protein function. Fu...

Candida albicans is a commensal yeast normally present in small numbers in the oral flora of a large proportion of humans. Colonization of the oral cavity by C. albicans involves the acquisition and maintenance of a stable yeast population. Micro-organisms are continually being removed from the oral cavity by host clearance mechanisms, and so, in order to survive and inhabit this eco-system, C. albicans cells have to adhere and replicate. The oral cavity presents many niches for C. albicans colonization, and the yeast is able to adhere to a plethora of ligands. These include epithelial and bacterial cell-surface molecules, extracellular matrix proteins, and dental acrylic. In addition, saliva molecules, including basic proline-rich proteins, adsorbed to many oral surfaces promote C. albicans adherence. Several adhesins present in the C. albicans cell wall have now been partially characterized. Adherence involves lectin, protein-protein, and hydrophobic interactions. As C. albicans cells evade host defenses and colonize new environments by penetrating tissues, they are exposed to new adherence receptors and respond by expressing alternative adhesins. The relatively small number of commensal Candida cells in the oral flora raises the possibility that strategies can be devised to prevent oral colonization and infection. However, the variety of oral niches and the complex adherence mechanisms of the yeast mean that such a goal will remain elusive until more is known about the contribution of each mechanism to colonization.

Analysis of the transport functions of individual Candida albicans plasma membrane drug efflux pumps is hampered by the multitude of endogenous transporters. We have stably expressed C. albicans Cdr1p, the major pump implicated in multiple-drug-resistance phenotypes, from the genomic PDR5 locus in a Saccharomyces cerevisiae mutant (AD1-8u ؊ ) from which seven major transporters of the ATP-binding cassette (ABC) family have been deleted. High-level expression of Cdr1p, under the control of the S. cerevisiae PDR5 promoter and driven by S. cerevisiae Pdr1p transcriptional regulator mutation pdr1-3, was demonstrated by increased levels of mRNA transcription, increased levels of nucleoside triphosphatase activity, and immunodetection in plasma membrane fractions. S. cerevisiae AD1-8u؊ was hypersensitive to azole antifungals (the MICs at which 80% of cells were inhibited [MIC 80 s] were 0.625 g/ml for fluconazole, <0.016 g/ml for ketoconazole, and <0.016 g/ml for itraconazole), whereas the strain (AD1002) that overexpressed C. albicans Cdr1p was resistant to azoles (MIC 80 s of fluconazole, ketoconazole, and itraconazole, 30, 0.5, and 4 g/ml, respectively). Drug resistance correlated with energy-dependent drug efflux. AD1002 demonstrated resistance to a variety of structurally unrelated chemicals which are potential drug pump substrates. The controlled overexpression of C. albicans Cdr1p in an S. cerevisiae background deficient in other pumps allows the functional analysis of pumping specificity and mechanisms of a major ABC transporter involved in drug efflux from an important human pathogen.Candida albicans is an asexual diploid fungus that causes opportunistic infections commonly seen in immunocompromised and debilitated patients (9, 30). An estimated 33 to 55% of patients with human immunodeficiency virus infection and AIDS contract oropharyngeal candidosis (34), and the synthetic triazole fluconazole has been the mainstay of their treatment. The widespread use of prolonged fluconazole therapy has increased the incidence of treatment failure due to fluconazole-resistant C. albicans (3,14,21,34,42). A number of studies have identified the major azole resistance mechanisms (1,20,38,41,42,(44)(45)(46). These include overexpression of, or mutations in, the drug target, 14␣-sterol demethylase; mutations in other parts of the sterol biosynthesis pathway; and, most commonly, overexpression of drug efflux proteins.C. albicans possesses transporters such as Cdr1p and Cdr2p with homology to proteins of the ATP-binding cassette (ABC) family (10,16,18,19,31), as well as Ben r p, which has homology to the major facilitator superfamily (MFS) class of drugproton antiport efflux pumps (1,5,36,46). The BEN r gene encodes a transporter associated with resistance to benomyl and methotrexate when it is expressed in Saccharomyces cerevisiae. The C. albicans CDR1 gene is a homologue of S. cerevisiae PDR5, which encodes a multidrug efflux pump, and CDR1 is the gene most often associated with energy-dependent drug efflux in fluconazole-r...

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 (~160 kDa) PDR proteins (Pdrps) from 55 fungal species, including major fungal pathogens, identified 9 separate protein clusters (A, B, C, D, E, F, G, H1a/H1b and H2).

Hyperexpression of the

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