AIMP3/p18 Controls Translational Initiation by Mediating the Delivery of Charged Initiator tRNA to Initiation Complex

Aminoacyl-tRNA synthetase-interacting multifunctional proteins (AIMPs) are nonenzymatic scaffolding proteins that comprise multisynthetase complex (MSC) with nine aminoacyl-tRNA synthetases in higher eukaryotes. Among the three AIMPs, AIMP3/p18 is strongly anchored to methionyl-tRNA synthetase (MRS) in the MSC. MRS attaches methionine (Met) to initiator tRNA (tRNAiMet) and plays an important role in translation initiation. It is known that AIMP3 is dispatched to nucleus or nuclear membrane to induce DNA damage response or senescence; however, the role of AIMP3 in translation as a component of MSC and the meaning of its interaction with MRS are still unclear. Herein, we observed that AIMP3 specifically interacted with Met-tRNAiMet in vitro, while it showed little or reduced interaction with unacylated or lysine-charged tRNAiMet. In addition, AIMP3 discriminates Met-tRNAiMet from Met-charged elongator tRNA based on filter-binding assay. Pull‐down assay revealed that AIMP3 and MRS had noncompetitive interaction with eukaryotic initiation factor 2 (eIF2) γ subunit (eIF2γ), which is in charge of binding with Met-tRNAiMet for the delivery of Met-tRNAiMet to ribosome. AIMP3 recruited active eIF2γ to the MRS–AIMP3 complex, and the level of Met-tRNAiMet bound to eIF2 complex was reduced by AIMP3 knockdown resulting in reduced protein synthesis. All these results suggested the novel function of AIMP3 as a critical mediator of Met-tRNAiMet transfer from MRS to eIF2 complex for the accurate and efficient translation initiation.