Actin cytoskeleton is required for nuclear accumulation of Gln3 in response to nitrogen limitation but not rapamycin treatment in Saccharomyces cerevisiae.

Saccharomyces cerevisiae selectively utilizes good nitrogen sources in preference to poor ones by down-regulating transcription of genes encoding proteins that transport and degrade poor nitrogen sources when excess nitrogen is available. This regulation is designated nitrogen catabolite repression (NCR). When cells are transferred from a good to a poor nitrogen source (glutamine to proline) or treated with rapamycin, an inhibitor of the protein kinases Tor1/2, Gln3 (NCR-sensitive transcription activator) moves from the cytoplasm into the nucleus. Gln3 re-accumulates in the cytoplasm when cells are returned to a good nitrogen source. However, Gln3 is not uniformly distributed in the cytoplasm. Such non-uniform distribution could result from a variety of interactions including association with a cytoplasmic vesicular system or components of the cytoskeleton. We used latrunculin, a drug that disrupts the actin cytoskeleton by inhibiting actin polymerization, to determine whether the actin cytoskeleton participates in intracellular Gln3 movement. Latrunculin-treatment prevents nuclear accumulation of Gln3 and NCR-sensitive transcription in cells transferred from ammonia to proline medium but does not prevent its accumulation in the cytoplasm of cells transferred from proline to glutamine medium. In contrast, rapamycin-induced nuclear accumulation of Gln3 is not demonstrably affected by latrunculin treatment. These data indicate the actin cytoskeleton is required for nuclear localization of Gln3 in response to limiting nitrogen but not rapamycin-treatment. Therefore, the actin cytoskeleton either participates in the response of Gln3 intracellular localization to nitrogen limitation before Tor1/2, or Tor1/2 inhibition only mimics the outcome of nitrogen limitation rather than directly regulating it.