In the light of multidrug resistance (MDR) among pathogenic microbes and cancer cells, membrane transporters have gained profound clinical significance. Chemotherapeutic failure, by far, has been attributed mainly to the robust and diverse array of these proteins, which are omnipresent in every stratum of the living world. Candida albicans , one of the major fungal pathogens affecting immunocompromised patients, also develops MDR during the course of chemotherapy. The pivotal membrane transporters that C. albicans has exploited as one of the strategies to develop MDR belongs to either the ATP binding cassette (ABC) or the major facilitator superfamily (MFS) class of proteins. The ABC transporter Candida drug resistance 1 protein (Cdr1p) is a major player among these transporters that enables the pathogen to outplay the battery of antifungals encountered by it. The promiscuous Cdr1 protein fulfills the quintessential need of a model to study molecular mechanisms of multidrug transporter regulation and structure-function analyses of asymmetric ABC transporters. In this review, we cover the highlights of two decades of research on Cdr1p that has provided a platform to study its structure-function relationships and regulatory circuitry for a better understanding of MDR not only in yeast but also in other organisms.

ATP-binding cassette (ABC) superfamily members have a key role as nutrient importers and exporters in bacteria. However, their role as drug exporters in eukaryotes brought this superfamily member to even greater prominence. The capacity of ABC transporters to efflux a broad spectrum of xenobiotics represents one of the major mechanisms of clinical multidrug resistance in pathogenic fungi including Candida species. Candida auris, a newly emerged multidrug-resistant fungal pathogen of humans, has been responsible for multiple outbreaks of drug-resistant infections in hospitals around the globe. Our study has analyzed the entire complement of ABC superfamily transporters to assess whether these play a major role in drug resistance mechanisms of C. auris. Our bioinformatics analyses identified 28 putative ABC proteins encoded in the genome of the C. auris type-strain CBS 10913T; 20 of which contain transmembrane domains (TMDs). Quantitative real-time PCR confirmed the expression of all 20 TMD transporters, underlining their potential in contributing to the C. auris drug-resistant phenotype. Changes in transcript levels after short-term exposure of drugs and in drug-resistant C. auris isolates suggested their importance in the drug resistance phenotype of this pathogen. CAUR_02725 orthologous to CDR1, a major multidrug exporter in other yeasts, showed consistently higher expression in multidrug-resistant strains of C. auris. Homologs of other ABC transporter genes, such as CDR4, CDR6, and SNQ2, also displayed raised expression in a sub-set of clinical isolates. Together, our analysis supports the involvement of these transporters in multidrug resistance in C. auris.

ATP-binding cassette (ABC) transporters help export various substrates across the cell membrane and significantly contribute to drug resistance. However, a recent study reported an unusual case in which the loss of an ABC transporter in Candida albicans, orf19.4531 (previously named ROA1), increases resistance against antifungal azoles, which was attributed to an altered membrane potential in the mutant strain. To obtain further mechanistic insights into this phenomenon, here we confirmed that the plasma membrane–localized transporter (renamed CDR6/ROA1 for consistency with C. albicans nomenclature) could efflux xenobiotics such as berberine, rhodamine 123, and paraquat. Moreover, a CDR6/ROA1 null mutant, NKKY101, displayed increased susceptibility to these xenobiotics. Interestingly, fluorescence recovery after photobleaching (FRAP) results indicated that NKKY101 mutant cells exhibited increased plasma membrane rigidity, resulting in reduced azole accumulation and contributing to azole resistance. Transcriptional profiling revealed that ribosome biogenesis genes were significantly up-regulated in the NKKY101 mutant. As ribosome biogenesis is a well-known downstream phenomenon of target of rapamycin (TOR1) signaling, we suspected a link between ribosome biogenesis and TOR1 signaling in NKKY101. Therefore, we grew NKKY101 cells on rapamycin and observed TOR1 hyperactivation, which leads to Hsp90-dependent calcineurin stabilization and thereby increased azole resistance. This in vitro finding was supported by in vivo data from a mouse model of systemic infection in which NKKY101 cells led to higher fungal load after fluconazole challenge than wild-type cells. Taken together, our study uncovers a mechanism of azole resistance in C. albicans, involving increased membrane rigidity and TOR signaling.