RNA-protein interactions in the human RNase MRP ribonucleoprotein complex

The eukaryotic nucleolus contains a large number of small RNA molecules tha t, in the form of small nucleolar ribonucleoprotein complexes (snoRNPs), ar e involved in the processing and modification of pre-rRNA. One of the snoRN Ps that has been shown to possess enzymatic activity is the RNase MRP, RNas e MRP is an endoribonuclease involved in the formation of the 5' end of 5.8 S rRNA. In this study the association of the hPop1 protein with the RNase M RP complex was investigated, The hPop1 protein seems not to be directly bou nd to the RNA component, but requires nt 1-86 and 116-176 of the MRP RNA to associate with the RNase MRP complex via protein-protein interactions, UV crosslinking followed by ribonuclease treatment and immunoprecipitation wit h anti-Th/To antibodies revealed three human proteins of about 20, 25, and 40 kDa that can associate with the RNase MRP complex, The 20- and 25-kDa pr oteins appear to bind to stem-loop I of the MRP RNA whereas the 40-kDa prot ein requires the central part of the MRP RNA (nt 86-176) for association wi th the RNase MRP complex. In addition, we show that the human RNase P prote ins Rpp30 and Rpp38 are also associated with the RNase MRP complex. Express ion of Vesicular Stomatitis Virus- (VSV) tagged versions of these proteins in HeLa cells followed by anti-VSV immunoprecipitation resulted in coprecip itation of both RNase P and RNase MRP complexes, Furthermore, UV crosslinki ng followed by anti-Th/To and anti-Rpp38 immunoprecipitation revealed that the 40-kDa protein we detected in UV crosslinking is probably identical to Rpp38.