Lysyl tRNA Synthetase (KARS)

Lysyl-tRNA synthetase is a homodimer localized to the cytoplasm which belongs to the class II family of tRNA synthetases. The 625-amino acid mitochondrial enzyme and the 597-amino acid cytoplasmic enzyme are identical over the last 576 amino acids, but the mitochondrial enzyme has a different 49-amino acid N terminus containing a mitochondrial targeting sequence. Transfection of both fluorescence-tagged isoforms into an osteosarcoma cell line showed that the cytoplasmic isoform produced a diffuse, cellwide fluorescence, while the mitochondrial isoform resulted in a punctate pattern that colocalized with mitochondrial markers. Ribonuclease protection analysis indicated that the mRNA encoding the cytoplasmic isoform makes up approximately 70%, and the mitochondrial isoform approximately 30%, of mature KARS transcripts.