Cytoplasmic compartmentation of Gln3 during nitrogen catabolite repression and the mechanism of its nuclear localization during carbon starvation in Saccharomyces cerevisiae.

Regulated intracellular localization of Gln3, the transcriptional activator responsible for nitrogen catabolite repression (NCR)-sensitive transcription, permits Saccharomyces cerevisiae to utilize good nitrogen sources (e.g. glutamine and ammonia) in preference to poor ones (e.g. proline). During nitrogen starvation or growth in medium containing a poor nitrogen source, Gln3 is nuclear and NCR-sensitive transcription is high. However, when cells are grown in excess nitrogen, Gln3 is localized to the cytoplasm with a concomitant decrease in gene expression. Treating cells with the Tor protein inhibitor, rapamycin, mimics nitrogen starvation. Recently, carbon starvation has been reported to cause nuclear localization of Gln3 and increased NCR-sensitive transcription. Here we show that nuclear localization of Gln3 during carbon starvation derives from its indirect effects on nitrogen metabolism, i.e. Gln3 does not move into the nucleus of carbon-starved cells if glutamine rather than ammonia is provided as the nitrogen source. In addition, these studies have clearly shown Gln3 is not uniformly distributed in the cytoplasm, but rather localizes to punctate or tubular structures. Analysis of these images by deconvolution microscopy suggests that Gln3 is concentrated in or associated with a highly structured system in the cytosol, one that is possibly vesicular in nature. This finding may impact significantly on how we view (i) the mechanism by which Tor regulates the intracellular localization of Gln3 and (ii) how proteins move into and out of the nucleus.