THE ULTRASTRUCTURAL-LOCALIZATION OF 60-KDA RO PROTEIN AND HUMAN CYTOPLASMIC RNAS - ASSOCIATION WITH NOVEL ELECTRON-DENSE BODIES

The 60-kDa Ro ribonucleoprotein is an important target of humoral auto immune responses, However. the ultrastructural locations of the 60-kDa Ro protein and its associated small cytoplasmic RNAs (Y RNAs) have no t been previously determined, and the functions of the Ro protein and RNAs are not known. In this study, the cellular locations of the 60-kD a Ro protein and the Ro Y1 and Y4 RNAs are determined by immunoelectro n microscopy and in situ hybridization electron microscopy, respective ly. Both Ro protein and Y RNAs are concentrated in discrete areas of t he nucleoplasm, nucleolus, and cytoplasm of cultured cells and human s kin sections, The 60-kDa Ro protein and Y RNAs are also present diffus ely in the cytoplasm. where they occur in ribosome-rich regions, and i n the nucleus, The presence of Ro ribonucleoprotein components in nucl eoli and in ribosome-rich cytoplasmic areas suggests a potential for t he involvement of Y RNAs and/or 60-kDa Ro protein in ribosome synthesi s, assembly, or transport. Double labeling experiments show that Ro pr otein and Y RNAs colocalize in the nucleoplasm, nucleolus, and cytopla sm. In addition, aggregates of Y RNA occur unassociated with 60-kDa Ro protein, and aggregates of 60-kDa Ro protein occur unassociated with Y RNA aggregates of both Ro protein and Y RNAs label previously unrepo rted nuclear and cytoplasmic electron-dense bodies, We propose that th ese distinctive Re-associated electron-dense bodies may represent stru cture(s) important for cellular transport and/or Ro function.