Proteolytic signal sequences (PEST) in the mammalian aminoacyl-tRNA synthetase complex

Eight aminoacyl-tRNA synthetases together with three unidentified proteins are associated as a multi-enzyme complex in mammalian cells. Partial peptide sequences for lysyl- and aspartyl-tRNA synthetases are determined and no highly hydrophobic peptides are found. The partial amino acid sequences for two of the unidentified proteins in the complex are shown to have substantial homology and each has a number of unique sequences. The results suggest that the two unidentified proteins are fragments of synthetases. The partial sequences revealed the presence of PEST sequences in at least three proteins. Inasmuch as PEST sequences are signals for intracellular degradation, the mammalian synthetase complex may have evolved to protect these synthetases against intracellular proteolysis.