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 (tRNA_i ^Met) 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-tRNA _i^Met in vitro, while it showed little or reduced interaction with unacylated or lysine-charged tRNA_i^Met. In addition, AIMP3 discriminates Met-tRNA_i^Met 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-tRNA_i^Met for the delivery of Met-tRNA_i^Met to ribosome. AIMP3 recruited active eIF2γ to the MRS-AIMP3 complex, and the level of Met-tRNA_i ^Met 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-tRNA_i^Met transfer from MRS to eIF2 complex for the accurate and efficient translation initiation.