Architecture and function of the human endonucleases RNase P and RNase MRP

In the past decade, important advances have been made in our knowledge of t he composition of human RNase MRP and RNase P complexes. Both ribonucleopro tein particles function as endonucleases and contain RNA components that ar e structurally related. RNase MRP has been suggested to be involved in the processing of precursor rRNA; RNase P, in the maturation of tRNA. Here we g ive an overview of current data on the structure and function of human RNas e MRP and RNase P particles, with emphasis on their molecular composition. At present, seven protein subunits, probably all associated with both ribon ucleoprotein particles, have been isolated and their corresponding cDNAs cl oned. Although no known structural motifs can be identified in the amino ac id sequences of these proteins, the majority is clearly rich in basic resid ues. For two protein subunits, a cluster of basic amino acids have been sho wn to be involved in nucleolar accumulation, whereas another protein, which lacks such a region, probably enters the nucleolus by way of a piggyback m echanism. The binding regions for several of the protein subunits on the RN A have been identified, and the data have been used to create a putative st ructural model for the RNase MRP particle. The rather obscure situation con cerning the association of the autoantigenic Th-40 protein and its possible relationship with one of the subunits, Rpp38, is discussed.