ABSTRACT
CONTENTS 18.1 Aspergillus nidulans: A Champion of Transporters ............................................................... 301 18.2 Methodological Approaches to Study Aspergillus nidulans Transporters ............................. 303
18.2.1 Growth Tests ............................................................................................................. 303 18.2.2 Uptake Measurements ............................................................................................... 303 18.2.3 Fluorescent Microscopic Imaging ............................................................................ 304 18.2.4 Epitope Tagging and Immunodetection .................................................................... 304 18.2.5 Mutations: Random, Directed, Cys-Scanning .......................................................... 305 18.2.6 Chimeras ................................................................................................................... 306
18.3 Integration of Multiple Physiological Signals at the Level of Transcription of Transporters ....................................................................................................................... 307
18.4 Sensing the Growth Milieu ..................................................................................................... 308 18.5 Transporters on the “Air”: Not Just Food Suppliers? .............................................................. 309 18.6 Transport in Sex ...................................................................................................................... 310 18.7 An Emerging Role of Regulated Traffi cking and Endocytosis of Transporters ..................... 310 18.8 Paradigms of Transporter Structure-Function Analysis in Aspergillus ................................. 311 18.9 Aspergillus as a Novel System for Studying Transporters from Complex Organisms ............ 313 18.10 Epilogue ................................................................................................................................... 314 Acknowledgments .............................................................................................................................. 315 References .......................................................................................................................................... 315
18.1 Aspergillus nidulans: A Champion of Transporters The plasma membrane of all cells and internal membranes of eukaryotes contain a wide variety of proteins that ensure transmembrane solute transport. Mechanistically, they can be classifi ed as transporters (or permeases) and channels (https://www.tcdb.org/). Transporters are classifi ed as primary active transporters, secondary active transporters, and facilitators, depending on their energetic requirements. Active transporters catalyse the transport of metabolites up an electrochemical gradient using either ATP hydrolysis (primary transport) or the movement of another species, most commonly an ion (H+, Na+, K+), down an electrochemical gradient (secondary transport). Secondary active transporters can be symporters (transport of a substrate and ions in the same direction) or antiporters (transport of a substrate and ions in the opposite direction). Facilitators (uniporters) are energy-independent or passive, transporters mediating the movement of a solute across the plasma membrane along its concentration gradient. Transporters catalyse the uptake or effl ux of most metabolites (amino acids, nucleobases, nucleosides, sugars, nitrogenous solutes, vitamins, etc.). In contrast, channels mediate passive transport of ions by forming an aqueous diffusion pore. Despite their structural similarity, consisting of a highly modular structure with, usually, 10-14 repeated (polytopic) hydrophobic or amphipathic α-helices, two properties distinguish channels from transporters: (1) ion fl ow is extremely fast, and (2) ion channels are gated, their opening frequency being regulated by changes in membrane potential, by binding of a specifi c ligand, or by mechanical constraints such as membrane stretching. In contrast, transporters undergo
reversible conformational changes that expose their solute-binding site alternately on each side of the membrane. However, transporter-like channels or channel-like transporters have been described (Wadiche and Kavanaugh, 1998; Boyd et al., 2003; and references therein).
