RPP40

RPP40 is a core protein subunit of the human RNase P ribonucleoprotein, co-purifying with catalytically active holoenzyme and being precipitated together with active enzyme by anti-RPP40 antibodies, which established it as one of at least eight protein subunits of the tRNA-processing complex (PMID:9630247, PMID:10024167). RPP40 is shared between RNase P and the related RNase MRP complex: it sediments in both small (12S) and large (60–80S) particles and co-immunoprecipitates with both holoenzymes, and unlike subunits such as Rpp20 and Rpp25 it associates with essentially all RNase MRP particles (PMID:16723659). Within RNase MRP it makes direct protein-protein and protein-RNA contacts that contribute to a structural model of complex assembly (PMID:15096576). Beyond these assembly and processing roles, RPP40 has been linked in hepatocellular carcinoma cells to coordination of pre-rRNA transcription and ribosomal protein gene expression promoting proliferation and invasion (PMID:40517827).

1. 1998 Medium

   Established whether RPP40 is a genuine constituent of the tRNA-processing machinery rather than a co-incidental copurifying protein, by tying it directly to catalytic activity.

   Evidence Co-purification and immunoprecipitation of active RNase P holoenzyme using antibodies against recombinant Rpp40

   PMID:9630247

   Open questions at the time

   • Does not define RPP40's specific molecular contribution to catalysis or substrate binding
   • No structural localization within the complex
2. 1999 Medium

   Confirmed RPP40 as a stable subunit of the multi-protein human RNase P enzyme across independent preparations.

   Evidence Immunoprecipitation of catalytically active RNase P from HeLa extracts

   PMID:10024167

   Open questions at the time

   • Subunit stoichiometry not resolved
   • Role in enzyme function not addressed
3. 2001 Low

   Mapped RPP40 into the holoenzyme interaction network, defining which other subunits it contacts during assembly.

   Evidence Yeast two-hybrid analysis of human nuclear RNase P subunits

   PMID:11158571

   Open questions at the time

   • Single yeast two-hybrid screen with no orthogonal validation of RPP40-specific interactions
   • Interactions described as weak; in vivo relevance unconfirmed
4. 2004 Medium

   Extended RPP40 beyond RNase P by mapping its direct protein and RNA contacts within RNase MRP, contributing to an assembly model.

   Evidence GST pull-down assays and analysis of mutant RNase MRP RNA

   PMID:15096576

   Open questions at the time

   • Precise RNA-binding determinants on RPP40 not defined at residue level
   • No high-resolution structure of the assembled complex
5. 2006 Medium

   Distinguished RPP40's distribution across particle populations, showing it is present in essentially all RNase MRP particles in contrast to substoichiometric subunits.

   Evidence Glycerol gradient sedimentation and co-immunoprecipitation with VSV-tagged subunits

   PMID:16723659

   Open questions at the time

   • Functional consequence of pan-particle association not tested
   • Does not address pre-rRNA processing activity directly
6. 2025 Low

   Probed a cellular-phenotype role for RPP40, linking it to pre-rRNA transcription and ribosomal protein gene expression that drive cancer-cell proliferation and invasion.

   Evidence Loss- and gain-of-function proliferation/migration/invasion assays and transcriptional analysis in HCC cell lines

   PMID:40517827

   Open questions at the time

   • Single lab, no mechanistic reconstitution or structural validation
   • Direct molecular link between RNase P/MRP function and rRNA transcription program not established
   • Causality versus correlation in transcriptional changes unresolved

• How RPP40's specific subunit contacts contribute to RNase P/MRP catalysis and substrate selection, and the molecular basis of its reported influence on rRNA transcription, remain unresolved.
• No high-resolution structure assigning RPP40 a defined catalytic or substrate-positioning role
• Mechanism connecting RNase MRP subunit identity to pre-rRNA transcriptional output not defined