Genetic and carbon source regulation of phosphorylation of Sip1p, a Snf1p-associated protein involved in carbon response in Saccharomyces cerevisiae

She3p possesses a novel activity required for ASH1 mRNA localization in Saccharomyces cerevisiae

Intracellular and intercellular polarity requires that specific proteins be sorted to discreet locations within and between cells. One mechanism for sorting proteins is through RNA localization. In Saccharomyces cerevisiae, ASH1 mRNA localizes to the distal tip of the bud, resulting in the asymmetric sorting of the transcriptional repressor Ash1p. ASH1 mRNA localization requires four cis-acting localization elements and the trans-acting factors Myo4p, She3p, and She2p. Myo4p is a type V myosin motor that functions to directly transport ASH1 mRNA to the bud. She2p is an RNA-binding protein that directly interacts with the ASH1 mRNA cis-acting elements. Currently, the role for She3p in ASH1 mRNA localization is as an adaptor protein, since it can simultaneously associate with Myo4p and She2p. Here, we present data for two novel mutants of She3p, S348E and the double mutant S343E S361E, that are defective for ASH1 mRNA localization, and yet both of these mutants retain the ability to associate with Myo4p and She2p. These observations suggest that She3p possesses a novel activity required for ASH1 mRNA localization, and our data imply that this function is related to the ability of She3p to associate with ASH1 mRNA. Interestingly, we determined that She3p is phosphorylated, and global mass spectrometry approaches have determined that Ser 343, 348, and 361 are sites of phosphorylation, suggesting that the novel function for She3p could be negatively regulated by phosphorylation. The present study reveals that the current accepted model for ASH1 mRNA localization does not fully account for the function of She3p in ASH1 mRNA localization.

Novel Ree1 regulates the expression of ENO1 via the Snf1 complex pathway in Saccharomyces cerevisiae

Using cDNA microarray analysis, we found that the mRNA of YJL217W and several other genes related to cell wall organization and biogenesis were up-regulated by galactose in Saccharomyces cerevisiae early during the induction process. YJL217W is also known as REE1 (Regulation of Enolase I). Both the Gal4 regulatory region and the Mac1 binding domain were found on the upstream region of REE1, and the expression of REE1 was up-regulated by galactose but not by glucose. The up-regulation of REE1 by galactose was not observed in the Deltagal4 strain. From the two-hybrid analysis, we found that Ree1 physically interacted with Gal83. Furthermore, from 2-D gel electrophoresis we found that the deletion of REE1 resulted in the up-regulation of Eno1. From Western blotting, we learned that the expression of Eno1 in the Deltaree1 strain was different from that in wild-type strains and that Eno1 expression was not changed by glucose stimulation. Taken together, these results suggest that Ree1p functions in the galactose metabolic pathway via the Gal83 protein and that it may control the level of Eno1p, which is also affected by the Snf1 complex, in S. cerevisiae.

Cyclic AMP-dependent protein kinase regulates the subcellular localization of Snf1-Sip1 protein kinase

The Snf1/AMP-activated protein kinase family has diverse roles in cellular responses to metabolic stress. In Saccharomyces cerevisiae, Snf1 protein kinase has three isoforms of the beta subunit that confer versatility on the kinase and that exhibit distinct patterns of subcellular localization. The Sip1 beta subunit resides in the cytosol in glucose-grown cells and relocalizes to the vacuolar membrane in response to carbon stress. We show that translation of Sip1 initiates at the second ATG of the open reading frame, yielding a potential site for N myristoylation, and that mutation of the critical glycine abolishes relocalization. We further show that the cyclic AMP-dependent protein kinase (protein kinase A [PKA]) pathway maintains the cytoplasmic localization of Sip1 in glucose-grown cells. The Snf1 catalytic subunit also exhibits aberrant localization to the vacuolar membrane in PKA-deficient cells, indicating that PKA regulates the localization of Snf1-Sip1 protein kinase. These findings establish a novel mechanism of regulation of Snf1 protein kinase by the PKA pathway.