Xenopus Ro Ribonucleoproteins: Members of an Evolutionarily Conserved Class of Cytoplasmic Ribonucleoproteins

Ro small ribonucleoproteins consist of a 60-kDa protein and possibly additional proteins complexed with several small RNA molecules. The RNA components of these particles, designated Y RNAs, are about 100 nt long. Although these small ribonucleoproteins are abundant components of a variety of vertebrate species and cell types, their subcellular location is controversial, and their function is completely unknown. We have identified and characterized the Ro RNPs of Xenopus laevis. Three of the four distinct Xenopus Y RNAs appear to be related to the previously sequenced human hY3, hY4, and hY5 RNAs. The fourth Xenopus Y RNA, xYα, does not appear to be a homologue of any of the human Y RNAs. Each of the human and Xenopus Y RNAs possesses a conserved stem that contains the binding site for the 60-kDa Ro protein. Xenopus and human 60-kDa Ro proteins are 78% identical in amino acid sequence, with the conservation extending throughout the entire protein. When human hY3 RNA is mixed with Xenopus egg extracts, the human RNA assembles with the Xenopus Ro protein to form chimeric Ro ribonucleoproteins. By analyzing RNA extracted from manually enucleated oocytes and germinal vesicles, we have determined that Y RNAs are located in the oocyte cytoplasm. By examining the distribution of mouse Ro ribonucleoproteins in cytoplast and karyoplast fractions derived from L-929 cells, we have determined that Ro ribonucleoprotein particles also primarily reside in the cytoplasm of mammalian cells.