A 60-year journey with a fungal transporter: from classical genetics to functional, structural, and evolutionary...

SUMMARYTransporters are transmembrane proteins that mediate the selective translocation of metabolites, ions, and drugs across biological membranes. Their activity is essential for cellular communication, nutrition, detoxification, homeostasis, and responses to stress. Despite their fundamental biological and biomedical importance, particularly their involvement in genetic diseases and multidrug resistance in microbes and cancer cells, transporters remained relatively understudied until recently. This was largely due to the technical challenges of isolating and functionally characterizing these dynamic proteins, whose structure, function, and cellular expression continuously depends on specific interactions with membrane lipids. Nevertheless, since the mid-1960s, several transporters have been identified and extensively characterized at the genetic and physiological levels in model microorganisms, including fungi. In this review, we trace the 60-year research journey of UapA, a uric acid-xanthine transporter from the filamentous fungus Aspergillus nidulans (Ascomycota). The UapA story spans from early classical genetic analyses to the recent determination of its high-resolution structure. We describe the development of genetic, molecular, and cellular tools and how these enabled the functional dissection of UapA and were subsequently used for other A. nidulans transporters, including studies on cellular expression and turnover regulation. We additionally highlight up-to-date structural approaches that refined our knowledge on the transport mechanism, how substrate specificity is determined, and on membrane trafficking pathways underlying biogenesis and endocytosis of UapA and other transporters. The concepts developed through 60 years of persistent work on UapA have established important paradigms relevant to fungal physiology that have proven to be broadly applicable to understanding transporter-linked processes in other eukaryotic organisms.