RPP21

RPP21 is a core protein subunit of the nuclear RNase P holoenzyme that functions as the primary RNA-binding scaffold within the RPP21–RPP29 heterodimer, enabling endonucleolytic 5′-leader cleavage of precursor tRNAs. RPP21 adopts an L-shaped fold with an N-terminal helical domain and a C-terminal zinc ribbon; its two α-helices mediate heterodimerization with RPP29 through a coupled folding mechanism, and the resulting binary complex binds the specificity (S-)domain of the RNase P RNA (H1 RNA), positioning RPP29 for catalytic co-activation (PMID:16142906, PMID:19733182, PMID:27810361). RPP21 directly binds precursor tRNA and localizes predominantly to the nucleoplasm, with additional presence in nucleoli and Cajal bodies (PMID:11497433). RPP21 and RPP29 are also recruited to DNA double-strand break sites in a PARP1- and poly(ADP-ribose)-dependent manner, where they promote homology-directed repair independently of non-homologous end joining (PMID:28432356).

1. 2001 High

   Establishing RPP21 as a bona fide RNase P subunit that directly binds pre-tRNA resolved whether this protein participates in substrate recognition and placed it in specific nuclear compartments.

   Evidence Biochemical co-purification with RNase P, pre-tRNA binding assay, and immunofluorescence in human cells

   PMID:11497433

   Open questions at the time

   • Binding mode to pre-tRNA undefined
   • No structure available
   • Functional contribution to catalysis not tested
2. 2005 High

   Determining the atomic structure of an RPP21 homolog revealed an L-shaped architecture with a catalytically essential zinc ribbon, identifying specific basic residues required for RNase P activity and implicating RPP21 as an RNA-binding module.

   Evidence Crystal structure at 1.6 Å of archaeal Ph1601p with site-directed mutagenesis and activity assays

   PMID:16142906

   Open questions at the time

   • No structure of RPP21 in complex with RNA or RPP29
   • Human RPP21 structure not determined
3. 2008 High

   Crystal and NMR structures of the RPP21–RPP29 binary complex defined the heterodimerization interface — centered on RPP21's two α-helices contacting RPP29's N-terminal region, β-strand, and C-terminal helix — establishing that this obligate heterodimer presents a positively charged surface for RNA binding.

   Evidence Crystal structure of PhoRpp21–PhoRpp29 complex and solution NMR of Pfu RPP21 with chemical shift perturbation mapping

   PMID:18922021 PMID:18929577

   Open questions at the time

   • RNA-bound structure of the dimer not yet solved
   • Contribution of individual interface residues to human RNase P activity untested
4. 2009 High

   Demonstrating that RPP21–RPP29 assembly involves coupled folding (disorder-to-order transition) and that the dimer localizes to the S-domain of RNase P RNA explained how protein binding specifies catalytic RNA architecture.

   Evidence NMR structure of Pfu RPP21–RPP29 complex combined with enzymatic footprinting of RNA contacts

   PMID:19733182

   Open questions at the time

   • Thermodynamic basis of coupled folding not quantified
   • Whether coupled folding occurs in human ortholog unknown
5. 2010 Medium

   Chimeric RNA assays confirmed that the RPP21–RPP29 pair acts exclusively on the S-domain rather than the catalytic domain, delineating a modular architecture in which different protein pairs serve distinct RNA domains.

   Evidence Chimeric RNA exchange between bacterial and archaeal RNase P RNAs with reconstitution activity assays

   PMID:20139629

   Open questions at the time

   • Applicability to human RNase P not directly tested
   • Structural basis of domain selectivity unresolved
6. 2012 High

   Biophysical dissection of RPP21–RPP29 binding thermodynamics showed that coupled folding contributes a large negative ΔCp and that electrostatic interactions and proton release drive assembly specificity, explaining how an intrinsically disordered interface achieves high-affinity, selective complex formation.

   Evidence ITC across temperatures, ionic strengths, and pH; NMR of free versus complexed states; folding-deficient RPP21 mutant

   PMID:22243443

   Open questions at the time

   • Kinetics of coupled folding not measured
   • Effect of RNA binding on thermodynamic parameters unknown
7. 2016 Medium

   Dissecting individual roles within the heterodimer established that RPP21 is the primary RNA-binding element that scaffolds RPP29 into a catalytically productive conformation, with specific lysine residues in the α2 helix essential for RNA activation.

   Evidence Pull-down binding assays, mutational analysis, and deletion analysis of RNA loop in archaeal system

   PMID:27810361

   Open questions at the time

   • Direct structural visualization of RPP21-RNA contacts lacking
   • Whether scaffolding role is conserved in human complex not shown
8. 2017 High

   Discovery that RPP21 is recruited to DNA double-strand breaks via PARP1-generated poly(ADP-ribose) and is required for homology-directed repair expanded RPP21's functional repertoire beyond tRNA processing to the DNA damage response.

   Evidence siRNA depletion with HDR/NHEJ reporters, laser microirradiation live imaging, PAR-binding assay, PARP inhibitor treatment in human cells

   PMID:28432356

   Open questions at the time

   • Molecular mechanism by which RNase P promotes HDR not defined
   • Whether RPP21 acts at DSBs independently of its tRNA processing role unclear
   • Structural basis of PAR recognition by RPP21 unknown

• A high-resolution structure of the human RNase P holoenzyme showing RPP21 contacts with H1 RNA and pre-tRNA substrate, and the precise mechanism by which RPP21/RNase P facilitates homology-directed DNA repair, remain to be established.
• No cryo-EM or crystal structure of human RNase P holoenzyme with RPP21 resolved
• Mechanism linking RNase P catalytic activity to HDR promotion undefined
• Whether RPP21 PAR-binding is direct or mediated through H1 RNA not distinguished