Heterogeneity of human Ro ribonucleoproteins (RNPS): nuclear retention of Ro RNPS containing the human hY5 RNA in human and mouse cells

Abstract

Summary Ro ribonucleoproteins (RNPs) are autoantigens that result from the association of a 60-kDa protein (Ro60) with a small RNA (hY1, hY3, hY4 or hY5 in humans, mY1 or mY3 in mice). Previous studies localized Ro RNPs to the cytoplasm. Because Ro RNPs containing hY5 RNA (RohY5 RNPs) have unique biochemical and immunological properties, their intracellular localization was reassessed. Subcellular distribution of mouse and human Ro RNPs in intact and hY-RNA transfected cells was assessed by immunoprecipitation and Northern hybridization. Human RohY1−4 RNPs as well as murine RomY1, mY3 RNPs are exclusively cytoplasmic. Ro RNPs containing an intact hY5 RNA, but not those containing a mutated form of hY5 RNA, are found in the nuclear fractions of human and mouse cells. RohY5 RNPs are stably associated with transcriptionally active La protein and are known to associate with RoBPI, a nuclear autoantigen. Our results demonstrate that RohY5 RNPs are specifically present in the nucleus of cultured human and murine cells. The signal for nuclear localization of RohY5 RNPs appears to reside within the hY5 sequence itself. In conclusion, we suggest that the unique localization and interactions of primate-specific RohY5 RNPs reflect functions that are distinct from the predicted cytoplasmic function(s) of more conserved Ro RNPs.