Physiological role of an alternative initiator tRNA carrier eIF2A

Abstract

Under various stress conditions, translations of most eukaryotic mRNAs are repressed. Inactivation of translation factor eukaryotic translation initiation factor 2 (eIF2), which is the major carrier of initiator tRNA (Met- tRNAi) to the 40S ribosome in normal condition, is the underlying mechanism of translational inhibition under stress conditions. However, some mRNAs are refractory to the translational inhibition by stress. c-Src, a notable non-receptor protein tyrosine kinase, regulates cell proliferation, cell motility and programmed cell death. And the elevated level of activated c-Src protein is related with solid tumor generation. Translation of c-Src mRNA is directed by an IRES element which mediates persistent translation under stress conditions when translation of most mRNAs is inhibited by a phosphorylation of the alpha subunit of eIF2. The molecular basis of the stress-resistant translation of c-Src mRNA remained to be elucidated. Previously, we revealed that an alternative carrier of Met-tRNAi named eIF2A facilitates the binding of initiator tRNA to 40S ribosome that is associated with hepatitis c viral (HCV) mRNA, which results in stress resistance of HCV mRNA translation. Here I report that eIF2A, an alternative tRNAi carrier, is responsible for the stress-resistant translation of c-Src mRNA. eIF2A facilitates tRNAi loading onto the 40S ribosomal subunit in a c-Src mRNA-dependent manner. And a direct interaction between eIF2A and a stem-loop structure (SL I) in the c-Src IRES is required for the c-Src IRES-dependent translation under stress conditions but not under normal conditions. Furthermore, I proved that the eIF2A-dependent translation of c-Src mRNA plays a pivotal role in cell proliferation under stress conditions. Previously, our group revealed that eIF2A is utilized by c-Src and HCV IRES only under stress conditions. Therefore, certain modulation on eIF2A under stress conditions should be exist. However, underlying mechanism for regulation of eIF2A under stress conditions are not well understood. Kinases for eIF2 are well-known factors that are activated specifically under stress conditions. Therefore, eIF2 kinases might also phosphorylates eIF2A under stress conditions and phosphorylation of eIF2A could control the activity of eIF2A related with translation. Here, I report that eIF2A interacts with PKR in RNA independent manner, and activated PKR efficiently phosphorylates eIF2A. This is the first report showing that eIF2 kinase performs dual role under stress conditions. Furthermore, this is the first report which identify the modification of eIF2A under stress conditions.