THE MEP2 AMMONIUM PERMEASE REGULATES PSEUDOHYPHAL DIFFERENTIATION IN SACCHAROMYCES-CEREVISIAE

In response to nitrogen starvation, diploid cells of the budding yeast Saccharomyces cerevisiae differentiate into a filamentous, pseudohyph al growth form. This dimorphic transition is regulated by the G alpha protein GPA2, by RAS2, and by elements of the pheromone-responsive MAP kinase cascade, yet the mechanisms by which nitrogen starvation is se nsed remain unclear, We have found that: MEP2, a high affinity ammoniu m permease, is required for pseudohyphal differentiation in response t o ammonium limitation,In contrast, MEP1 and MEP3, which are lower affi nity ammonium permeases, are not required for filamentous growth. Delt a mep2 mutant strains had no defects in growth fates or ammonium uptak e, even at limiting ammonium concentrations, The pseudohyphal defect o f Delta mep2/Delta mep2 strains was suppressed by dominant active GPA2 or RAS2 mutations and by addition of exogenous cAMP, but was not supp ressed by activated alleles of the MAP kinase pathway, Analysis of MEP 1/MEP2 hybrid proteins identified a small intracellular loop of MEP2 i nvolved in the pseudohyphal regulatory function. In addition, mutation s in GLN3, URE2 and NPR1, which abrogate MEP2 expression or stability, also conferred pseudohyphal growth defects, We propose that MEP2 is a n ammonium sensor, generating a signal to regulate filamentous growth in response to ammonium starvation.