Figure 1. The non-translational functions of aaRSs.

To a first approximation, aaRSs are organized in two groups in the cytoplasm of higher eukaryotes. Some are in a free form, while others are part of a high molecular weight multi-tRNA synthetase complex (MSC), which includes 3 scaffold proteins designated as MSC^p43, MSC^p38, and MSC^p18. These proteins not only are an essential part of the translation apparatus, but also have a myriad of cytoplasmic, nuclear and extra-cellular functions. For simplicity, the various synthetases are designated by single letters, using the standard abbreviations for amino acids.