Involvement of the stress-responsive transcription factor gene MSN2 in the control of amino acid uptake in Saccharomyces cerevisiae.

The transcriptional factor Msn2 plays a pivotal role in response to environmental stresses by activating the transcription of stress-responsive genes in Saccharomyces cerevisiae. Our previous studies demonstrate that intracellular proline acts as a key protectant against various stresses. It is unknown, however, whether Msn2 is involved in proline homeostasis in S. cerevisiae cells. We here found that MSN2-overexpressing (MSN2-OE) cells showed higher sensitivity to a toxic analogue of proline, l-azetidine-2-carboxylic acid (AZC), as well as to the other amino acid toxic analogues, than wild-type cells. Overexpression of MSN2 increased the intracellular content of AZC, suggesting that Msn2 positively regulates the uptake of proline. Among the known proline permease genes, GNP1 was shown to play a predominant role in the AZC toxicity. Based on quantitative real-time PCR and western blot analyses, the overexpression of MSN2 did not induce any increases in the transcript levels of GNP1 or the other proline permease genes, while the amount of the Gnp1 protein was markedly increased in MSN2-OE cells. Microscopic observation suggested that the endocytic degradation of Gnp1 was impaired in MSN2-OE cells. Thus, this study sheds light on a novel link between the Msn2-mediated global stress response and the amino acid homeostasis in S. cerevisiae.