Cytoplasmic accumulation of the 52 kDa RO/SS-A nuclear autoantigen in transfected cell lines

The 60 kDa Ro/SS-A (Ro60) autoantigen is thought to reside predominantly in the nucleus; however, the intracellular localization of 52 kDa Ro/SS-A (Ro52) in normal cells is controversial, probably due to its low abundance. Therefore, we studied the intracellular expression and localization of the human Ro52 following transfection of a human Ro52 cDNA into cultured cell lines. Immunofluorescence staining of human (HEp-2) and mouse (LTA-5) cell transfectants with affinity-purified anti-Ro52 antibodies revealed that Ro52 antigen was most abundant in the cytoplasm and present to a lesser extent in the nucleus. This relative localization was supported by a preponderance of the Ro52 antigen in the cytoplasmic rather than nuclear fraction of enucleated cell lines detected by immunoblotting. In contrast to the Ro52 autoantigen, Ro60 and La autoantigens were mainly expressed in the nucleus of transfected cells under similar circumstances, indicating distinct localization of the intracellular pools of these autoantigens. The findings indicate that the Ro52 autoantigen lacks intrinsic signals required for nuclear localization and suggest that a significant pool of this autoantigen resides in the cytoplasm. Ro52 may therefore rely upon an association with other molecules for any specific nuclear transport.