Evidence for similar structural organization of the multienzyme aminoacyl- tRNA synthetase complex in vivo and in vitro

Although aminoacyl-tRNA synthetases from higher eukaryotic cells are routinely isolated as components of a multienzyme complex, it has remained unclear how closely the isolated complex reflects a structure that exists within the cell. To answer this question, we have used chemical cross-linking and immunological detection to identify the nearest neighbor(s) of arginyl-tRNA synthetase both in the isolated, purified complex and in saponin-permeabilized cells, which retain much of the structural organization of intact cells. Our results show that arginyl-tRNA synthetase is cross-linked primarily both in vitro and in vivo to a single protein, an as yet uncharacterized 38-kDa polypeptide known to be present in synthetase complexes from many sources. These data demonstrate that the isolated, multienzyme aminoacyl-tRNA synthetase complex reflects a defined structure that also pre-exists in the cell and that the 38-kDa polypeptide is an integral component of this complex.