Basic domains target protein subunits of the RNase MRP complex to the nucleolus independently of complex association

The RNase MRP and RNase P ribonucleoprotein particles both function as endo ribonucleases, have a similar RNA component, and share several protein subu nits. RNase MRP has been implicated in pre-rRNA processing and mitochondria l DNA replication, whereas RNase P functions in pre-tRNA processing. Both R Nase MRP and RNase P accumulate in the nucleolus of eukaryotic cells. In th is report we show that for three protein subunits of the RNase MRP complex (hPop1, hPop4, and Rpp38) basic domains are responsible for their nucleolar accumulation and that they are able to accumulate in the nucleolus indepen dently of their association with the RNase MRP and RNase P complexes. We al so show. that certain mutants of hPop4 accumulate in the Cajal bodies, sugg esting that hPop4 traverses through these bodies to the nucleolus. Furtherm ore, we characterized a deletion. mutant of Rpp38 that preferentially assoc iates with the RNase MRP complex, giving a first clue about the difference in protein composition of the human RNase MRP and RNase P complexes. On the basis of all available data on nucleolar localization sequences, we hypoth esize that nucleolar accumulation of proteins containing basic domains proc eeds by diffusion and retention rather than by an active transport process. The existence of nucleolar localization sequences is discussed.