The yeast Mep2 ammonium transceptor physically interacts with the 14-3-3 protein Bmh1

In response to limiting nitrogen levels diploid yeast undergo a dimorphic switch from yeast like growth to pseudohyphal growth. During this morphological change yeast grow as elongated chains of cells away from the colony to search for nutrients. Studies by a number of groups over many years have established that signal transduction pathways that regulate pseudohyphal growth include the MAP kinase and PKA pathways. An essential but poorly understood component of the regulation of pseudohyphal growth is the Mep2 ammonium importer. Two models of Mep2 function during pseudohyphal growth have been proposed. First, the Mep2 substrate (either ammonium ion, ammonia gas or ammonia gas plus proton) cause changes in cytosolic pH that is sensed by a relevant signal transduction pathway. Second, Mep2 acts as a transceptor that physically interacts with a signalling partner to control pseudohyphal growth. In the transceptor model, Mep2 acts in a way analogous to G protein-coupled receptors undergoing a conformational change during substrate translocation that initiates signalling. We have undertaken a genetic screen to identify potential Mep2 signalling partners and have identified an interaction between Mep2 and the 14-3-3 protein Bmh1. We have confirmed this interaction using western analysis of membrane fractions and demonstrated that this interaction is lost when analysing signalling deficient Mep2 mutants. Importantly, we have identified the 14-3-3 protein binding site in Mep2 which is required for the Mep2 dependent activation of the MAP Kinase pathway.