title: The yeast ammonium transport protein Mep2 and its positive regulator, the Npr1 kinase, play an important role in normal and pseudohyphal growth on various nitrogen media through retrieval of excreted ammonium.
abstract: Three ammonium transport systems of the Mep/Amt/Rh superfamily contribute to ammonium uptake for use as a nitrogen source in Saccharomyces cerevisiae. A specific sensor role has further been proposed for Mep2 in the stimulation of pseudohyphal development during ammonium limitation. Optimal ammonium transport by the Mep proteins requires the Npr1 kinase, a potential target of the target-of-rapamycin signalling pathway. We show here that the growth impairment of cells lacking Npr1 on many nitrogen sources is shared by cells deprived of the three Mep proteins and is a consequence of deficient ammonium retrieval. Expression of a newly isolated Npr1-independent and hyperactive Mep2 in cells lacking Npr1 and/or the Mep proteins restores growth on low ammonium but also on other nitrogen sources. This hyperactive Mep2 variant efficiently counteracts ammonium excretion. Hence, ammonium uptake activity plays an important role in compensating for leakage of catabolic ammonium. Our data also reveal that the requirement of Npr1 for ammonium-induced pseudohyphal growth is an indirect consequence of its necessity for Mep2-mediated ammonium transport. Finally, we show that Mep2 participates, through ammonium leakage compensation, in pseudohyphal growth induced by amino acid starvation. This argues further in favour of tight coupling of Mep2 transport and sensor functions.
authors.affiliation: Laboratoire de Physiologie Moléculaire de la Cellule, Institut de Biologie et de Médecine Moléculaires, Université Libre de Bruxelles CP300, Rue des Professeurs Jeener et Brachet 12, 6041 Gosselies, Belgium.
authors.forename: Mélanie
authors.initials: M
authors.lastname: Boeckstaens
authors.forename: Bruno
authors.initials: B
authors.lastname: André
authors.forename: Anna Maria
authors.initials: AM
authors.lastname: Marini
mesh_terms: D000596:Amino Acids / Q000494:pharmacology; D027682:Cation Transport Proteins / Q000378:metabolism*; D025301:Hyphae / Q000166:cytology / Q000187:drug effects / Q000254:growth & development*; D007700:Kinetics; D050505:Mutant Proteins / Q000378:metabolism; D009584:Nitrogen / Q000494:pharmacology*; D011494:Protein Kinases / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*; D013997:Time Factors
chemical_list: D000596:Amino Acids; D027682:Cation Transport Proteins; C088951:MEP1 protein, S cerevisiae; C107270:MEP2 protein, S cerevisiae; C107271:MEP3 protein, S cerevisiae; D050505:Mutant Proteins; D000644:Quaternary Ammonium Compounds; D029701:Saccharomyces cerevisiae Proteins; C066958:NPR1 protein, S cerevisiae; D011494:Protein Kinases; D009584:Nitrogen