RPP25

RPP25 is a protein subunit of the human RNase P and RNase MRP endoribonuclease complexes, where it obligatorily heterodimerizes with RPP20 via their shared Alba-type core domains to form the functional RNA-binding module that recognizes the P3 domain of RNase MRP/P RNA with nanomolar affinity (PMID:12003489, PMID:20215441). Neither RPP25 nor RPP20 alone binds P3 RNA appreciably; heterodimerization is a prerequisite for RNA recognition, and the pre-formed heterodimer engages the P3 RNA distal stem and internal loop through a conserved positively charged surface, adopting a homodimer-of-heterodimers quaternary architecture evolutionarily derived from archaeal Alba chromatin proteins (PMID:20215441, PMID:33571640, PMID:29625199). RPP25 and RPP20 expression levels are codependent, and their heterodimerization controls nucleolar accumulation; knockdown of RPP25 reduces levels of other RNase P subunits, indicating coordinated subunit stability within the holoenzyme (PMID:17119099, PMID:15351636). Disease-related RMRP mutations in the P3 domain cluster at the RPP20–RPP25–RNA interface, linking disruption of this interaction to cartilage-hair hypoplasia pathogenesis (PMID:33571640).

1. 2002 High

   Establishing RPP25 as a bona fide RNase P subunit resolved its identity within the multi-protein holoenzyme and showed it possesses general RNA-binding activity.

   Evidence Immunoprecipitation of catalytically active RNase P from HeLa extracts using anti-Rpp25 antibodies, plus recombinant protein RNA-binding assays

   PMID:12003489

   Open questions at the time

   • Specific RNA target within the holoenzyme not identified
   • Relationship to other protein subunits not defined
   • No structural information
2. 2004 Medium

   Knockdown of RPP25 demonstrated that its depletion destabilizes other RNase P protein subunits, revealing interdependence among holoenzyme components.

   Evidence EGS-mediated RNA degradation and siRNA knockdown of Rpp25 in human cells with western blot readout

   PMID:15351636

   Open questions at the time

   • Mechanism of subunit co-stability (transcriptional vs. post-translational) not dissected
   • Direct vs. indirect effects of RPP25 loss on individual subunits not resolved
3. 2006 High

   Demonstrating that RPP20 and RPP25 form a tight, obligate heterodimer that is required for P3 RNA binding and nucleolar localization established the heterodimer as the functional unit within RNase MRP/P.

   Evidence Co-immunoprecipitation, siRNA and overexpression experiments, subcellular localization imaging in human cells

   PMID:17119099

   Open questions at the time

   • Quantitative binding affinity of the heterodimer for P3 RNA not yet measured
   • Structural basis of heterodimerization unknown
   • Mechanism by which heterodimerization drives nucleolar accumulation undefined
4. 2010 High

   Quantitative biophysical reconstitution showed that neither subunit alone binds P3 RNA and that the Alba-type core domain mediates both heterodimerization and nanomolar-affinity RNA recognition, mapping the minimal functional determinants.

   Evidence In vitro RNA-binding assays, deletion mutagenesis, analytical ultracentrifugation, and circular dichroism with recombinant proteins

   PMID:20215441

   Open questions at the time

   • Atomic-resolution structure of the heterodimer and RNA complex not yet available
   • Contributions of regions outside the Alba core remain uncharacterized
   • Role of the heterodimer in holoenzyme catalysis not tested
5. 2018 High

   The crystal structure of the RPP20–RPP25 heterodimer revealed how quaternary-level changes at the interface, relative to ancestral archaeal Alba homodimers, enabled the evolutionary switch to single-stranded RNA specificity in eukaryotic RNase P/MRP.

   Evidence X-ray crystallography with comparative structural analysis

   PMID:29625199

   Open questions at the time

   • Structure of the heterodimer bound to P3 RNA not yet solved
   • Functional validation of interface residues by mutagenesis not reported in this study
6. 2021 High

   The crystal structure of the RPP20–RPP25–P3 RNA ternary complex defined the molecular contacts at the protein–RNA interface and showed that disease-causing RMRP P3 mutations cluster at this interface, providing a structural rationale for cartilage-hair hypoplasia pathogenesis.

   Evidence X-ray crystallography of the ternary complex with structure-based mapping of RMRP disease mutations

   PMID:33571640

   Open questions at the time

   • Structure of the heterodimer within the complete RNase MRP or RNase P holoenzyme not determined
   • Functional consequences of specific disease mutations on holoenzyme activity not directly assayed
   • Whether the tetrameric organization observed in the crystal represents the physiological assembly state in vivo is unresolved

• How the RPP20–RPP25 heterodimer integrates into the fully assembled RNase P and RNase MRP holoenzymes and contributes to catalytic function remains structurally and mechanistically undefined.
• No structure of RPP20–RPP25 in the context of the complete human RNase P or RNase MRP holoenzyme
• Direct contribution of the heterodimer to substrate cleavage versus structural scaffolding not dissected
• Mechanism linking nucleolar targeting of the heterodimer to holoenzyme biogenesis unknown