Retractile lysyl-tRNA synthetase-AIMP2 assembly in the human multi-aminoacyl-tRNA synthetase complex

Multi-aminoacyl-tRNA synthetase complex (MSC) is the second largest machinery for protein synthesis in human cells and also regulates multiple nontranslational functions through its components. Previous studies have shown that the MSC can respond to external signals by releasing its components to function outside it. The internal assembly is fundamental to MSC regulation. Here, using crystal structural analyses (at 1.88 resolution) along with molecular modeling, gel-filtration chromatography, and co-immunoprecipitation, we report that human lysyl-tRNA synthetase (LysRS) forms a tighter assembly with the scaffold protein aminoacyl-tRNA synthetase complex-interacting multifunctional protein 2 (AIMP2) than previously observed. We found that two AIMP2 N-terminal peptides form an antiparallel scaffold and hold two LysRS dimers through four binding motifs and additional interactions. Of note, the four catalytic subunits of LysRS in the tightly assembled complex were all accessible for tRNA recognition. We further noted that two recently reported human disease-associated mutations conflict with this tighter assembly, cause LysRS release from the MSC, and inactivate the enzyme. These findings reveal a previously unknown dimension of MSC subcomplex assembly and suggest that the retractility of this complex may be critical for its physiological functions.