Background: Mig1 is a transcriptional repressor responsible for glucose repression of many genes in the budding yeast Saccharomyces cerevisiae. Glucose regulates Mig1 function by affecting its phosphorylation, which is catalyzed by the Snf1 protein kinase. Phosphorylation alters the subcellular localization of Mig1, causing it to be nuclear when glucose is present, and cytoplasmic when glucose is absent.
Results: Here, we report that Msn5, a member of the importin beta family of nuclear transport receptors, is required to export Mig1 from the nucleus when glucose is removed. Mig1 and Msn5 interacted in a yeast two-hybrid assay. Within the portion of Mig1 that regulates its nuclear transport, we found a region that directed its nuclear export. Within this region, two leucine-rich sequences similar to known nuclear export signals were not required for Mig1 export. The corresponding domain of the yeast Kluyveromyces lactis Mig1 conferred glucose-regulated Msn5-dependent protein export from the nucleus in S. cerevisiae. Sequence comparison with S. cerevisiae Mig1 revealed short patches of homology in K. lactis and K. marxianus Mig1 that might be Msn5-interaction domains. These regions overlapped with the serine residues predicted to be Snf1 phosphorylation sites, suggesting that Msn5 and Snf1 recognize similar sequences in Mig1. Altering these serines abolished glucose-dependent phosphorylation of Mig1 and caused it to be a constitutive repressor that was retained in the nucleus.
Conclusions: Mig1 contains a new nuclear export signal that is phosphorylated by Snf1 upon glucose removal, causing it to be recognized by the nuclear exportin Msn5 and carried out of the nucleus into the cytoplasm where it contributes to derepression of glucose-repressed genes.