Dynamic Organization of Aminoacyl-tRNA Synthetase Complexes in the Cytoplasm of Human Cells

The localization in space and in time of proteins within the cytoplasm of eukaryotic cells is a central question of the cellular compartmentalization of metabolic pathways. The assembly of proteins within stable or transient complexes plays an essential role in this process. Here, we examined the subcellular localization of the multi-aminoacyl-tRNA synthetase complex in human cells. The sequestration of its components within the cytoplasm rests on the presence of the eukaryotic-specific polypeptide extensions that characterize the human enzymes, as compared with their prokaryotic counterparts. The cellular mobility of several synthetases, assessed by measuring fluorescence recovery after photobleaching, suggested that they are not freely diffusible within the cytoplasm. Several of these enzymes, isolated by tandem affinity purification, were copurified with ribosomal proteins and actin. The capacity of aminoacyl-tRNA synthetases to interact with polyribosomes and with the actin cytoskeleton impacts their subcellular localization and mobility. Our observations have conceptual implications for understanding how translation machinery is organized in vivo.