Use of chemical modification and mass spectrometry to identify substrate-contacting sites in proteinaceous RNase P, a tRNA processing enzyme.

Among all enzymes in nature, RNase P is unique in that it can use either an RNA- or a protein-based active site for its function: catalyzing cleavage of the 5'-leader from precursor tRNAs (pre-tRNAs). The well-studied catalytic RNase P RNA uses a specificity module to recognize the pre-tRNA and a catalytic module to perform cleavage. Similarly, the recently discovered proteinaceous RNase P (PRORP) possesses two domains - pentatricopeptide repeat (PPR) and metallonuclease (NYN) - that are present in some other RNA processing factors. Here, we combined chemical modification of lysines and multiple-reaction monitoring mass spectrometry to identify putative substrate-contacting residues in Arabidopsis thaliana PRORP1 (AtPRORP1), and subsequently validated these candidate sites by site-directed mutagenesis. Using biochemical studies to characterize the wild-type (WT) and mutant derivatives, we found that AtPRORP1 exploits specific lysines strategically positioned at the tips of it's V-shaped arms, in the first PPR motif and in the NYN domain proximal to the catalytic center, to bind and cleave pre-tRNA. Our results confirm that the protein- and RNA-based forms of RNase P have distinct modules for substrate recognition and cleavage, an unanticipated parallel in their mode of action.