Functional Redundancy of Multidrug Resistance Transporters in Yeast: Substrate Diversity and System Robustness

Yeast harbour more than ten different multiple drug resistance (MDR) genes encoding transporters that extrude xenobiotics from the cytoplasm into the environment. These transporters, belonging to the ATP-binding cassette (ABC) or major facilitator superfamily (MFS), exhibit broad and significantly overlapping substrate specificities, though the precise boundaries of their individual substrate ranges remain undefined. During evolution, genes with overlapping functions tend either to specialize or to degenerate into pseudogenes. Here, we propose several explanations for how this apparent redundancy of MDR efflux pumps benefits cells, and we discuss the potential individual roles of the full MDR efflux pump repertoire in the model organism Saccharomyces cerevisiae. We posit that individual MDR transporters may vary in stability under challenging environmental conditions, in the energetic cost of their synthesis and maintenance, and in their degree of specialization toward particular classes of xenobiotics. Furthermore, given that ABC transporters and MFS transporters exploit distinct driving forces for xenobiotic efflux, each class may have its own vulnerabilities. We argue that deciphering the distinct roles of MDR proteins will reveal critical weaknesses in the MDR system and guide the development of strategies to overcome multidrug resistance in pathogenic fungi.