Affinage

RPP21

Ribonuclease P protein subunit p21 · UniProt Q9H633

Length
154 aa
Mass
17.6 kDa
Annotated
2026-06-10
13 papers in source corpus 10 papers cited in narrative 10 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 5/5 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

RPP21 is an essential protein subunit of the human nuclear RNase P ribonucleoprotein that binds precursor tRNA and is required for endonucleolytic processing of pre-tRNA 5' leader sequences (PMID:11497433). It functions as an obligate heterodimer with RPP29, an interaction built through binding-coupled protein folding in which RPP21 contributes the primary RNA-contacting helices (PMID:18922021, PMID:22243443); the RPP21–RPP29 pair docks onto the specificity (S-domain) of the RNase P RNA—distinct from the catalytic C-domain engaged by other subunits—to position and activate the enzyme against pre-tRNA substrates (PMID:19733182, PMID:20139629). Structural work on archaeal homologs establishes RPP21 as an L-shaped protein with two N-terminal helices and a zinc-ribbon domain, where the zinc-coordinating cysteines and conserved basic residues are required for protein stability and RNase P activity (PMID:16142906), and where RPP21 acts as the primary S-domain RNA-binding element and scaffold for RPP29 (PMID:27810361). Beyond tRNA processing, human RPP21 is recruited to DNA double-strand breaks in a PARP1-dependent manner, binds poly-ADP-ribose, and is required for homology-directed repair but not non-homologous end joining (PMID:28432356). Within the nucleus its distribution between nucleoplasm, nucleoli, and Cajal bodies is governed by alternative splicing of its precursor mRNA (PMID:11497433).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 2001 Medium

    Established RPP21 as a bona fide protein subunit of human nuclear RNase P, answering whether it is part of the holoenzyme and contributes to substrate engagement.

    Evidence Co-purification with highly purified RNase P, precursor tRNA binding assay, and immunofluorescence/fractionation in human cells

    PMID:11497433

    Open questions at the time
    • Did not resolve the structural basis of tRNA binding
    • Functional consequence of splice-isoform localization differences not mechanistically dissected
  2. 2005 High

    Defined the RPP21 fold and identified the structural and chemical determinants of activity, showing zinc coordination and specific basic residues are required for RNase P function.

    Evidence X-ray crystallography of archaeal Ph1601p at 1.6 Å with site-directed mutagenesis and RNase P activity assays

    PMID:16142906

    Open questions at the time
    • Archaeal homolog rather than human protein
    • Did not map contacts to RNase P RNA or partner subunits
  3. 2008 High

    Located the RPP29-binding surface on RPP21 and showed the two N-terminal helices mediate heterodimerization, defining how the two subunits assemble.

    Evidence Solution NMR structure and chemical shift perturbation mapping of archaeal RPP21, plus crystal structure of the RPP21–RPP29 binary complex with mutagenesis and activity assays

    PMID:18922021 PMID:18929577

    Open questions at the time
    • Heterodimer characterized in archaea, not human complex
    • RNA-binding face inferred from charge distribution rather than direct RNA-bound structure
  4. 2009 High

    Showed the RPP21–RPP29 complex acts on the specificity (S-domain) of the RNase P RNA and forms through coupled folding at the interface, linking assembly thermodynamics to RNA functional targeting.

    Evidence Solution NMR structure of the binary complex with enzymatic footprinting of the RNase P RNA

    PMID:19733182

    Open questions at the time
    • Footprinting localizes binding but not residue-level RNA contacts
    • How S-domain binding mechanically activates catalysis unresolved
  5. 2010 Medium

    Established a division of labor among RNase P subunits, with the RPP21–RPP29 pair acting on the S-domain while other subunits act on the catalytic C-domain.

    Evidence Chimeric domain-swapped RNAs (E. coli M1 RNA / P. horikoshii pRNA) with in vitro RNase P activity assays

    PMID:20139629

    Open questions at the time
    • Domain assignment inferred from chimeras, not from human enzyme
    • Does not define catalytic contribution of S-domain binding
  6. 2012 High

    Quantified the thermodynamics of RPP21–RPP29 assembly, confirming binding-coupled folding contributes substantially to complex formation.

    Evidence Isothermal titration calorimetry across temperature, ionic strength, and pH, with a folding-deficient RPP21 point mutant and NMR structural support

    PMID:22243443

    Open questions at the time
    • Biophysics performed on archaeal proteins
    • Coupled-folding dynamics not connected to RNA-loaded holoenzyme state
  7. 2016 Medium

    Identified RPP21 as the primary S-domain RNA-binding element and scaffold for RPP29, pinpointing the N-terminal helix lysines required for RNA activation.

    Evidence Pull-down RNA-binding assays, site-directed mutagenesis, RNA substrate deletion analysis, and RNase P activity assays in archaea

    PMID:27810361

    Open questions at the time
    • Carried out in archaeal system
    • Single-stranded loop deletion effect not structurally resolved
  8. 2017 Medium

    Revealed a moonlighting role for human RPP21 in genome maintenance, showing PARP1-dependent recruitment to DNA breaks and a requirement for homology-directed repair.

    Evidence Laser microirradiation with live-cell imaging, siRNA depletion, DR-GFP and NHEJ reporter assays, PAR-binding assay, and post-damage RNase P activity measurement in human cells

    PMID:28432356

    Open questions at the time
    • RPP21 and RPP29 not always separated, so individual contribution unclear
    • Mechanism by which RNase P activity at breaks promotes HDR is undefined
    • Direct PAR-binding residues on RPP21 not mapped
  9. 2025 Low

    Addressed how RPP21 is sorted exclusively into RNase P versus the related RNase MRP complex, distinguishing it from a structurally homologous MRP-specific protein.

    Evidence Structural homology analysis and interaction mapping between RPP21/RNase P and RMRPP1/RNase MRP (preprint)

    PMID:bio_10.1101_2025.01.28.635360

    Open questions at the time
    • Preprint, methodological detail limited to abstract-level description
    • Sequence/structural determinants of selective complex assignment not fully resolved

Open questions

Synthesis pass · forward-looking unresolved questions
  • How RPP21's S-domain binding mechanically activates pre-tRNA cleavage in the human holoenzyme, and how its DNA-repair function integrates with its catalytic role, remain unresolved.
  • No structure of the human RNase P holoenzyme bound to pre-tRNA in the timeline
  • Mechanistic link between PARP1-dependent recruitment and HDR outcome unknown
  • RNA targets, if any, processed at DNA damage sites unidentified

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0003723 RNA binding 3 GO:0005198 structural molecule activity 3 GO:0140098 catalytic activity, acting on RNA 2
Localization
GO:0005634 nucleus 2 GO:0005654 nucleoplasm 1 GO:0005730 nucleolus 1
Pathway
R-HSA-8953854 Metabolism of RNA 3 R-HSA-73894 DNA Repair 1
Partners
PARP1RPP29
Complex memberships
RNase P

Evidence

Reading pass · 10 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2001 Rpp21 is a protein subunit of human nuclear RNase P that binds precursor tRNA and is required for RNase P activity. It is predominantly localized in the nucleoplasm but also found in nucleoli and Cajal bodies. Intron retention and alternative splice-site selection in Rpp21 precursor mRNA regulate the intranuclear distribution of protein products and their association with the RNase P holoenzyme. Cloning by homology, co-purification with highly purified RNase P, precursor tRNA binding assay, immunofluorescence/subcellular fractionation, analysis of alternatively spliced isoforms RNA (New York, N.Y.) Medium 11497433
2005 The archaeal homolog of Rpp21 (Ph1601p from Pyrococcus horikoshii) adopts an L-shaped structure with an N-terminal two-helix domain and a C-terminal zinc ribbon domain stabilized by a zinc ion coordinated by four Cys residues. Mutation of the zinc-coordinating cysteines destabilizes the protein and inactivates RNase P activity. Positively charged residues (Lys69, Arg86, Arg105) are essential for RNase P functional activity, while additional basic residues play more modest roles. X-ray crystallography (MAD at 1.6 Å resolution), site-directed mutagenesis, RNase P activity assay Biochemistry High 16142906
2008 Archaeal RPP21 (Pfu) in solution consists of an unstructured N-terminus, two alpha-helices, a zinc-binding motif, and an unstructured C-terminus. The primary contact surface for RPP29 binding is localized to the two helices of RPP21, as identified by NMR chemical shift perturbations. Solution NMR structure determination, paramagnetic NMR, chemical shift perturbation mapping Biochemistry High 18922021
2008 Archaeal homologs of Rpp21 and Rpp29 (PhoRpp21 and PhoRpp29) form a heterodimer in which the two N-terminal helices of PhoRpp21 interact predominantly with the N-terminal extended structure, a beta-strand, and the C-terminal helix of PhoRpp29 via hydrogen bonds and salt bridges. The heterodimer presents a positively charged face as a putative RNA-binding surface, and heterodimerization is required for P. horikoshii RNase P function. X-ray crystallography of the binary complex, mutational analysis, RNase P activity assay Journal of molecular biology High 18929577
2009 The RPP21-RPP29 binary complex from Pyrococcus furiosus is formed with coupled folding of secondary structural elements at the interface. Enzymatic footprinting localized the RPP21-RPP29 complex to the specificity domain (S-domain) of the RNase P RNA. Conserved basic residue surfaces on the complex are implicated in recognition of the RPR and/or precursor tRNA. Solution NMR structure of the binary complex, enzymatic footprinting of RPR, chemical shift perturbation analysis Journal of molecular biology High 19733182
2010 PhoRpp21 and PhoRpp29 (archaeal homologs of human Rpp21 and Rpp29) function on the specificity domain (S-domain) of the RNase P RNA, in contrast to PhoPop5 and PhoRpp30 which act on the catalytic C-domain. This was established using chimeric RNAs in which the C- and S-domains of E. coli M1 RNA and P. horikoshii pRNA were exchanged. Genetic/biochemical epistasis using chimeric RNAs, in vitro RNase P activity assay with domain-swapped substrates Bioscience, biotechnology, and biochemistry Medium 20139629
2012 The RPP21-RPP29 interaction involves binding-coupled protein folding, with a large negative heat capacity change (ΔCp) approximately twice that predicted from surface accessibility calculations. ITC experiments revealed strong salt dependence and proton release at neutral pH. A folding-deficient RPP21 point mutant confirmed that coupled folding contributes significantly to the excess ΔCp. Isothermal titration calorimetry (ITC) over a range of temperatures, ionic strengths, pH values, and buffer ionization potentials; RPP21 point mutant analysis; NMR structural data Biochemistry High 22243443
2016 PhoRpp21 (archaeal Rpp21 homolog) can bind the RNase P RNA S-domain independently of PhoRpp29, while PhoRpp29 alone has reduced affinity. PhoRpp21 thus serves as the primary RNA-binding element and scaffold for PhoRpp29. Lys53, Lys54, and Lys56 in the N-terminal helix (α2) of PhoRpp21 and the 10 C-terminal residues of PhoRpp29 are essential for S-domain RNA activation. Deletion of the single-stranded loop linking P11 and P12 helices in the S-domain impaired complex binding. Pull-down assay for RNA binding, site-directed mutagenesis, deletion analysis of RNA substrate, RNase P activity assay Biochemical and biophysical research communications Medium 27810361
2017 Human Rpp21 (along with Rpp29) is recruited to laser-microirradiated DNA damage sites in a PARP1-dependent manner, binds poly ADP-ribose (PAR) moieties, and is required for homology-directed repair (HDR) of double-strand breaks but not for non-homologous end joining. Depletion of the catalytic H1 RNA subunit diminishes Rpp21/Rpp29 recruitment to damage sites. RNase P activity is augmented after DNA damage in a PARP1-dependent manner. Laser microirradiation with live-cell imaging, siRNA depletion, DR-GFP HDR reporter assay, NHEJ reporter assay, PAR-binding assay, RNase P activity assay post-damage Scientific reports Medium 28432356
2025 Rpp21 is unique to the RNase P complex and is not present in the related RNase MRP complex. Rpp21 and the newly identified RNase MRP-specific protein RMRPP1 display significant structural homology, but specific regions of each protein drive selective interactions with their respective complexes. Structural homology analysis, co-immunoprecipitation/interaction mapping between Rpp21 and RNase P vs. RMRPP1 and RNase MRP bioRxivpreprint Low bio_10.1101_2025.01.28.635360

Source papers

Stage 0 corpus · 13 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2001 Function and subnuclear distribution of Rpp21, a protein subunit of the human ribonucleoprotein ribonuclease P. RNA (New York, N.Y.) 57 11497433
2009 Solution structure of an archaeal RNase P binary protein complex: formation of the 30-kDa complex between Pyrococcus furiosus RPP21 and RPP29 is accompanied by coupled protein folding and highlights critical features for protein-protein and protein-RNA interactions. Journal of molecular biology 34 19733182
2008 Structure of an archaeal homolog of the human protein complex Rpp21-Rpp29 that is a key core component for the assembly of active ribonuclease P. Journal of molecular biology 34 18929577
2005 Crystal structure of a ribonuclease P protein Ph1601p from Pyrococcus horikoshii OT3: an archaeal homologue of human nuclear ribonuclease P protein Rpp21. Biochemistry 34 16142906
2017 A role of human RNase P subunits, Rpp29 and Rpp21, in homology directed-repair of double-strand breaks. Scientific reports 26 28432356
2008 Solution structure of Pyrococcus furiosus RPP21, a component of the archaeal RNase P holoenzyme, and interactions with its RPP29 protein partner. Biochemistry 19 18922021
2010 Archaeal homologs of human RNase P protein pairs Pop5 with Rpp30 and Rpp21 with Rpp29 work on distinct functional domains of the RNA subunit. Bioscience, biotechnology, and biochemistry 18 20139629
2004 Inhibition of the expression of the human RNase P protein subunits Rpp21, Rpp25, Rpp29 by external guide sequences (EGSs) and siRNA. Journal of molecular biology 16 15351636
2007 cGMP-dependent protein kinase I interacts with TRIM39R, a novel Rpp21 domain-containing TRIM protein. American journal of physiology. Lung cellular and molecular physiology 15 17601797
2012 Thermodynamics of coupled folding in the interaction of archaeal RNase P proteins RPP21 and RPP29. Biochemistry 9 22243443
2022 Sequence Variant in the TRIM39-RPP21 Gene Readthrough is Shared Across a Cohort of Arabian Foals Diagnosed with Juvenile Idiopathic Epilepsy. Journal of genetic mutation disorders 5 35465405
2019 TRIM39-RPP21 Variants (∆19InsCCC) Are Not Associated with Juvenile Idiopathic Epilepsy in Egyptian Arabian Horses. Genes 4 31623255
2016 Functional characterization of archaeal homologs of human nuclear RNase P proteins Rpp21 and Rpp29 provides insights into the molecular basis of their cooperativity in catalysis. Biochemical and biophysical research communications 2 27810361

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