Ammonia-specific regulation of Gln3 localization in Saccharomyces cerevisiae by protein kinase Npr1

Events directly regulating Gln3 intracellular localization and nitrogen catabolite repression (NCR)-sensitive transcription in Saccharomyces cerevisiae are interconnected with many cellular processes that influence the utilization of environmental metabolites. Among them are intracellular trafficking of the permeases that transport nitrogenous compounds and their control by the Tor1,2 signal transduction pathway. Npr1 is a kinase that phosphorylates and thereby stabilizes NCR-sensitive permeases, e. g. Gap1 and Mep2. It is also a phosphoprotein for which phosphorylation and kinase activity are regulated by Tor1,2 via Tap42 and Sit4. Npr1 has been reported to negatively regulate nuclear localization of Gln3 in SD (ammonia)-grown cells. Thus we sought to distinguish whether Npr1: (i) functions directly as a component of NCR control; or (ii) influences Gln3 localization indirectly, possibly as a consequence of participating in protein trafficking. If Npr1 functions directly, then the ability of all good nitrogen sources to restrict Gln3 to the cytoplasm should be lost in an npr1 Delta just as occurs when URE2 ( encoding this well studied negative Gln3 regulator) is deleted. We show that nuclear localization of Gln3-Myc(13) in an npr1 Delta occurred only with ammonia as the nitrogen source. Other good nitrogen sources, e. g. glutamine, serine, or asparagine, restricted Gln3-Myc(13) to the cytoplasm of both wild type and npr1 Delta cells. In other words, the npr1 Delta did not possess the uniform phenotype for all repressive nitrogen sources characteristic of ure2 Delta. This suggests that the connection between Gln3 localization and Npr1 is indirect, arising from the influence of Npr1 on the ability of cells to utilize ammonia as a repressive nitrogen source.