Localization in the nucleolus and coiled bodies of protein subunits of theribonucleoprotein ribonuclease P

The precise location of the tRNA processing ribonucleoprotein ribonuclease P (RNase P) and the mechanism of its intranuclear distribution have not bee n completely delineated. We show that three protein subunits of human RNase P (Rpp), Rpp14, Rpp29 and Rpp38, are found in the nucleolus and that each can localize a reporter protein to nucleoli of cells in tissue culture. In contrast to Rpp38, which is uniformly distributed in nucleoli, Rpp14 and Rp p29 are confined to the dense fibrillar component. Rpp29 and Rpp35 possess functional, yet distinct domains required for subnucleolar localization. Th e subunit Rpp14 lacks such a domain and appears to be dependent on a piggyb ack process to reach the nucleolus. Biochemical analysis suggests that cata lytically active RNase P exists in the nucleolus. We also provide evidence that Rpp29 and Rpp38 reside in coiled bodies, organelles that are implicate d in the biogenesis of several other small nuclear ribonucleoproteins requi red for processing of precursor mRNA. Because some protein subunits of RNas e P are shared by the ribosomal RNA processing ribonucleoprotein RNase MRP, these two evolutionary related holoenzymes may share common intranuclear l ocalization and assembly pathways to coordinate the processing of tRNA and rRNA precursors.