טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentBavli-Kertselli Ira
SubjectTranslation Regulation upon Glucose Depletion and eIF5
Involvement in this Response
DepartmentDepartment of Biology
Supervisor Professor Yoav Arava
Full Thesis textFull thesis text - English Version


Abstract

Cells respond to various intra- and extra- cellular stimuli by changing their protein repertoire. This ability is essential for cells' survival and in most cases it is mediated by signal transduction pathways. These pathways were shown to change the cell's proteome by affecting the transcription of many gene targets. It is known that signaling pathways can regulate protein synthesis also by affecting the translation machinery. However, the molecules that are involved in this post-transcriptional regulation are largely unknown.

Glucose is one of the major carbon sources for S. cerevisiae and mediates many signaling cascades. It was shown that glucose removal leads to translation inhibition in yeast cells and is the fastest and strongest of all tested cellular responses. This translation inhibition does not occur due to extensive mRNA degradation and does not require the established regulatory pathways, neither the TOR nor the eIF2α. Translational arrest upon glucose depletion, however, does not occur in tpk1w, a PKA kinase mutant.  We suggest that this mutant expresses altered levels of a translation factor and thereby change the cell's ability to sense the stress. Using genome-wide expression analysis we identified several translation factors with modified expression profiles in mutant cells. Directed overexpression of one of these factors (eIF5) appeared to rescue this phenotype in a stress-resistant strain. Restoring ribosomal dissociation by eIF5 necessitated an active GAP domain and multiple regions throughout this protein. Furthermore, eIF5 undergoes phosphorylation upon glucose depletion in wild-type cells at Threonine 191. Mutating this residue and introducing it into tpk1w abolished its ability to rescue the translational defect. Intriguingly, introducing this mutation into the parental strain did not hamper its translational response. We further show that T191 is phosphorylated in vitro by CKII, and yeast cells with a mutated CKII have a reduced response to glucose depletion. These results implicate phosphorylation of eIF5 at T191 by CKII as one of the pathways for regulating translation upon glucose depletion.