hPop5, a protein subunit of the human RNase AMP and RNase P endoribonucleases

The RNase MRP and RNase P particles both function as endoribonucleases. RNa se AMP has been implicated in the processing of precursor-rRNA, whereas RNa se P has been shown to function in the processing of pre-tRNA. Both ribonuc leoprotein particles have an RNA component that can be folded into a simila r secondary structure and share several protein components. We have identif ied human, rat, mouse, cow, and Drosophila homologues of the Pop5p protein subunit of the yeast RNase MRP and RNase P complexes. The human Pop5 cDNA e ncodes a protein of 163 amino acids with a predicted molecular mass of 18.8 kDa. Polyclonal antibodies raised against recombinant hPop5 identified a 1 9-kDa polypeptide in HeLa cells and showed that hPop5 is associated with bo th RNase MRP and RNase P. Using affinity-purified anti-hPop5 antibodies, we demonstrated that the endogenous hPop5 protein is localized in the nucleus and accumulates in the nucleolus, which is consistent with its association with RNase MRP and RNase P. Catalytically active RNase P was partially pur ified from HeLa cells, and hPop5 was shown to be associated with it. Finall y, the evolutionarily conserved acidic C-terminal tail of hPop5 appeared to be required neither for complex formation nor for RNase P activity.