| Research Abstract |
When starved for amino acids, Saccharomyces cerevlsiae accumulates uncharged tRNAS to activate its sole eukaryotic initiation factor (eIF) 2alpha kinase GCN2. Subsequent phospborylation of eIF2alpha impedes general translation, but translationally derepresses the transcription factor GCN4, which induces expression of various biosynthetic genes to elicit general amino acid control response (GCN pathway). By contrast, when supplied with enough nutrients, the yeast activates the target of rapamycin (TOR) signaling pathway to stimulate translation initiation by facilitating the assembly of eIF4F. A cross-talk was suggested between the two pathways by rapamycin-induced translation of GCN4 mRNA.
In this research, we showed that rapamycin, the specific inhibitor of TOR signaling pathway, causes an increase in phosphorylated eIF2alpha to translationally derepress GCN4. This increment is not observed in the cells expressing mammalian non-GCN2 eIF2alpha kinases in place of GCN2. It is thus suggested that rapamycin does not inhibit dephosphorylation of eIF2alpha but rather activates the kinase GCN2. This activation seems to require an interaction between the kinase and uncharged tRNAs, because rapamycin, similar to amino acid starvation, fails to induce eIF2alpha phosphorylation in the cells with GCN2 defective in tRNA binding. However, in contrast with amino acid starvation, rapamycin activates GCN2 without increasing the amount of uncharged tRNAs, but presumably by modifying the tRNA binding affinity of GCN2 via dephosphorylation of Ser-577. Indeed, mutants for this site failed to show rapamycin-induced activation of GCN2. On the other hand, we found that deletion of EAP1 encoding an eIF4E-binding protein regulated by TOR signaling pathway enhances rapamycin-induced, but not amino acid starvation-induced, translation of GCN4.
It thus seems that these two points mediate cross talk between pathways for general control response and TOR signaling.
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