RPP25

RPP25 is a protein subunit of the human RNase P and RNase MRP endoribonucleases, contributing to recognition of their catalytic RNA scaffolds (PMID:12003489, PMID:20215441). It functions obligatorily as a heterodimer with RPP20: the two proteins form a salt- and detergent-resistant complex through their Alba-type core domains, and only the preformed 1:1 RPP20-RPP25 heterodimer binds the P3 domain of the RNase MRP/P RNA with nanomolar affinity, whereas RPP25 alone has negligible P3 affinity (PMID:17119099, PMID:20215441). Crystal structures of the heterodimer and of its ternary complex with the RMRP P3 RNA show that P3 engages a conserved, positively-charged surface via its distal stem and internal loop, and that quaternary rearrangement of the heterodimerization interface underlies the shift from archaeal Alba DNA binding to eukaryotic single-stranded RNA recognition (PMID:29625199, PMID:33571640). Heterodimerization also governs nucleolar accumulation of RPP20 and co-regulates the steady-state levels of both subunits, reflecting their mutual interdependence within the complex (PMID:17119099, PMID:15351636). Disease-associated RMRP P3 mutations map to the protein-RNA interface where they weaken P3 binding to the RPP20-RPP25 heterodimer (PMID:33571640).

1. 2002 Medium

   Established RPP25 as a bona fide subunit of the active RNase P holoenzyme rather than a loosely associated factor, anchoring it to tRNA processing biology.

   Evidence Immunoprecipitation of catalytically active holoenzyme with anti-Rpp25 antibodies plus RNA-binding assay with recombinant protein in HeLa cells

   PMID:12003489

   Open questions at the time

   • Did not define the specific RNA element or partner subunit RPP25 contacts
   • RNA-binding shown only as a general property, not sequence/structure specific
   • Single lab, single functional readout
2. 2004 Medium

   Showed that loss of RPP25 destabilizes other RNase P subunits, revealing that subunit expression within the complex is interdependent.

   Evidence EGS-mediated mRNA cleavage and siRNA knockdown in cell culture with protein-level Western readout

   PMID:15351636

   Open questions at the time

   • Did not identify which subunits are co-regulated or the mechanism (transcriptional vs. stability)
   • No direct enzymatic-activity readout of the consequence
   • Single lab
3. 2006 High

   Defined the obligate RPP25-RPP20 heterodimer, its stability, its enhanced P3 RNA binding, and its control of RPP20 nucleolar localization and reciprocal expression.

   Evidence Co-immunoprecipitation, salt/detergent-resistance assay, P3 RNA-binding assay, and overexpression/siRNA experiments with localization microscopy in HEp-2 cells

   PMID:17119099

   Open questions at the time

   • Did not quantify binding affinities of monomer vs. heterodimer
   • Stoichiometry of one copy per particle inferred biochemically, not structurally
   • Mechanism of co-dependent expression not resolved
4. 2010 High

   Demonstrated quantitatively that heterodimerization is a strict prerequisite for P3 RNA recognition and mapped the determinants to the Alba-type core domain.

   Evidence In vitro reconstitution with CD/fluorescence/ITC binding titrations on wild-type and deletion-mutant recombinant proteins

   PMID:20215441

   Open questions at the time

   • Did not provide atomic-resolution view of the heterodimer or RNA contacts
   • Did not address assembly into the full holoenzyme
5. 2018 High

   Provided the atomic structure of the RPP20/RPP25 heterodimer, explaining how the archaeal Alba scaffold was repurposed for eukaryotic RNase P/MRP via quaternary interface divergence.

   Evidence X-ray crystallography of the human heterodimer with comparative analysis against archaeal Alba proteins

   PMID:29625199

   Open questions at the time

   • Did not include bound RNA, leaving the protein-RNA interface uncharacterized
   • Single lab structural determination
6. 2021 High

   Resolved the ternary RPP20-RPP25-RMRP P3 complex, defining the positively-charged RNA-binding surface and placing disease-associated RMRP mutations at the protein-RNA interface.

   Evidence X-ray crystallography of the ternary complex with structural mapping of disease mutation positions

   PMID:33571640

   Open questions at the time

   • Functional consequence of disease mutations on binding inferred from structure, not measured
   • Proposed dimerization of RNase MRP in cells not validated in vivo
   • Does not capture the complete holoenzyme architecture

• How the RPP20-RPP25 heterodimer integrates into the full RNase P/MRP holoenzyme and contributes to catalytic substrate processing remains unresolved.
• No structure of RPP25 within the assembled holoenzyme
• Direct catalytic contribution beyond P3 RNA recognition not established
• In-cell relevance of the proposed RNase MRP dimerization untested