Intracellular localization of histidyl-tRNA synthetase/Jo-1 antigen in T24 cells and some other cells.

Anti-Jo-1 antibody is characteristic of patients suffering from autoimmune-disease myositis. Since the antigen has been identified to be a histidyl-tRNA synthetase (HisRS), it is reasonable to suppose that it localizes mainly in the cytoplasm. However, contradictory results (localization in the nucleus, cytoplasm, or both) have been reported on this point. This study examined whether or not HisRS tagged with a green fluorescent protein (GFP) localizes, even if partially, in particular regions of the nucleus. The cDNA of human HisRS was ligated into either pEGFP-N1 or pEGFP-C1, and transfected into T24 cells. Transfectants expressed either HisRS-GFP or GFP-HisRS, both with the expected LDS-PAGE mobility. Observations with a confocal fluorescence microscope revealed that, in most cells, both GFP-tagged HisRSs were present solely in the cytoplasm. Occasional fluorescent spots seen in the nuclear region coincided with the immunofluorescent stain of the nuclear pore complex, indicating that they represent GFP-tagged HisRS in the cytoplasm that had invaginated deeply into the nucleus. Transient transfection into HeLa and L6 cells also resulted in the cytoplasmic localization of GFP-tagged HisRSs. These results indicate that HisRS would localize predominantly in the cytoplasm. The possible nuclear antigens other than HisRS that might be detected by anti-Jo-1 antisera are discussed.