The Initiation Factor eIF4A Is Involved in the Response to Lithium Stress in Saccharomyces cerevisiae

Abstract

A gene, TIF2, was identified as corresponding to the translation initiation factor eIF4A and when overexpressed it confers lithium tolerance in galactose medium to Saccharomyces cerevisiae. Incubation of yeast with 6 mM LiCl in galactose medium leads to inhibition of [ 35 S]methionine incorporation. By polysome analysis we show that translation is inhibited by lithium at the initiation step, accumulating 80 S monosomes. We further show by immunoblot analysis that when cells are incubated with lithium eIF4A does not sediment with ribosomal subunits. Overexpression of TIF2 overcomes inhibition of protein synthesis and restores its sedimentation with the initiation complex. In vivo, eIF4A is induced by lithium stress. We have shown previously that lithium is highly toxic to yeast when grown in galactose medium mainly due to inhibition of phosphoglucomutase, an enzyme responsible for the entry of galactose into glycolysis. We show that conditions that revert inhibition of phosphoglucomutase also revert inhibition of protein synthesis. Interestingly, glucose starvation leads to loss of polysomes but not to dissociation of eIF4A from the preinitiation complexes. Overexpression of SIT4, a protein phosphatase related to the TOR kinase pathway, reverts inhibition of protein synthesis by lithium and association of eIF4A with the initiation complex.

Transcriptional, Proteomic, and Metabolic Responses to Lithium in Galactose-grown Yeast Cells

“…In addition, repression at the post-transcriptional level is seen in the uncorrelated mRNA and protein levels for some genes. These results fit a previous study showing that translation initiation is inhibited by lithium (6). Furthermore, genes containing at least two STRE elements were found to be up-regulated already 20 min after addition of LiCl, indicating a global stress response, which was further supported by the increased accumulation of acetate, glycerol, and glycogen, and the up-regulation of genes related to these pathways.…”

“…up-regulation of the genes involved in the synthesis of glycogen and trehalose. Lithium Down-regulates Genes in Protein BiosynthesisLithium has previously been shown to inhibit protein synthesis at the initiation step in galactose grown cells (6). Among the down-regulated genes we observed an over-representation of genes encoding proteins in protein biosynthesis such as ribosomal proteins as well as proteins in ribosome biogenesis and …”

“…Both inhibition of glycolysis by lithium and glucose deprivation lead to inhibition of protein synthesis (6,9), but only the first seems to be mediated by Sit4 as overexpression leads to recovery of protein synthesis when cells are grown in galactose in the presence of lithium but not when cells are deprived of glucose (6). These results suggest that each phenomenon could induce a different mechanism.…”

“…During the course of our studies involving the response of cells to lithium in galactose medium we have cloned SIT4, a putative Ser-Thr phosphatase, that when overexpressed confers lithium tolerance in galactose medium (4). SIT4 overexpression also protects translation initiation from inhibition by lithium, however, it does not alter the fermentation rate (6). We have previously observed that SIT4 acts in a pathway not involving induction of transcription of the Na ϩ transporter ENA1.…”

“…In response to inhibition of fermentation, protein synthesis is inhibited at the translation initiation step (6). During the course of our studies involving the response of cells to lithium in galactose medium we have cloned SIT4, a putative Ser-Thr phosphatase, that when overexpressed confers lithium tolerance in galactose medium (4).…”