The role of Mep2 in yeast pseudohyphal growth

Abstract

In response to limiting levels of nitrogen in the environment, the diploid yeast Saccharomyces cerevisiae undergoes a dimorphic switch from yeast like growth to filamentous pseudohyphal growth. During this morphological change yeast cells grow as elongated chains of cells attached to each other away from the colony to forage for nutrients. Earlier studies have established the two major signalling pathways that regulate pseudohyphal growth include the MAP Kinase and cAMP-PKA pathways. The Mep2 ammonium transporter is an indispensable but poorly understood element of the pseudohyphal pathway. Although the role of Mep2 in this dimorphic switch has been recognized, the precise molecular mechanisms that link ammonium transport to this dimorphic switch is still unclear. Two distinct models of Mep2 function have been proposed. In the first, pH model, import of substrate during ammonium transport (either ammonium ion, ammonia gas or ammonia gas plus proton) would result in localised cytosolic pH changes which is sensed by an appropriate signal transduction pathway. In the second, transceptor model, Mep2 behaves like a transceptor by undergoing a conformational change during ammonium transport allowing it to physically engage a downstream signalling partner to initiate pseudohyphal growth. The pH model was tested which demonstrates that Mep2 signalling is independent of intracellular pH changes. The genetic screen to identify potential interaction partners of Mep2 identified an interaction between Mep2 and the 14-3-3 protein Bmh1. This interaction has been confirmed using western analysis of membrane fractions and demonstrated that this interaction is lost in signalling deficient Mep2 mutants. The 14-3-3 protein binding site in Mep2 has been identified which is required for the Mep2 dependent activation of the MAP Kinase pathway during pseudohyphal growth. A model for Mep2 sensing is proposed where Mep2 recruits signalling components to the membrane enabling cells to establish polarity where Mep2 is most active.