RPS27A

RPS27A is a structural protein of the small (40S) ribosomal subunit that is synthesized as a ubiquitin fusion precursor (Ub-RPS27a) and processed to release free ubiquitin and the 80-residue C2-C2 zinc-finger ribosomal protein (PMID:2538756, PMID:7488009). During late cytoplasmic 40S maturation the deubiquitinase USP16 removes ubiquitin from an internal lysine of RPS27a/eS31 in a translation-dependent manner, and loss of USP16 causes 18S rRNA processing defects and impaired recycling of biogenesis factors (PMID:32129764); RPS27A itself is required for rRNA production across cell lines (PMID:37371478). Beyond the ribosome, RPS27A is an extraribosomal regulator of the MDM2–p53 axis: it binds the MDM2 central acidic domain to suppress MDM2-mediated p53 ubiquitination and drive p53 activation and cell-cycle arrest, while MDM2 reciprocally ubiquitinates RPS27A for proteasomal degradation (PMID:21561866), and it directly binds RPL11 to tune the RPL11–MDM2 interaction during ribosomal stress (PMID:35073964). This regulation is coupled to nucleolar-to-nucleoplasm translocation of RPS27A that lowers MDM2 Ser166 phosphorylation and stabilizes p53 (PMID:28928040, PMID:34166715), and RPS27A is itself a direct p53 transcriptional target that amplifies the damage response (PMID:25592822). In the DNA-damage response RPS27A is recruited to lesions and, through its C-terminal ribosomal fragment, antagonizes RNF168-mediated H2A/H2AX K13/15 ubiquitination by masking the nucleosome acidic patch, impairing 53BP1 recruitment (PMID:37451480). RPS27A also controls NF-κB signaling, being targeted for degradation by the E3 ligase TRIM13 (PMID:39667820) and acting through a PSMD12/NF-κB axis in microglia (PMID:39069432). RPS27A is exploited by multiple pathogens, whose proteins bind it to stabilize viral oncoproteins or to shut off host translation (PMID:28735865, PMID:39069039).

1. 1989 High

   Established that the longer C-terminal extension of the ubiquitin fusion gene is a bona fide ribosomal protein, defining RPS27A's core identity as a 40S subunit component.

   Evidence Immunoblotting of rat 40S subunit protein fractions

   PMID:2538756

   Open questions at the time

   • Did not establish rRNA contacts or assembly role
   • Functional consequence of ribosomal incorporation not addressed
2. 1995 High

   Showed that the ubiquitin-S27a precursor is proteolytically processed to free ubiquitin plus an 80-aa zinc-finger protein, defining the precursor maturation step and RPS27A domain architecture.

   Evidence In vitro processing in reticulocyte lysate; recombinant cDNA characterization

   PMID:7488009

   Open questions at the time

   • Protease responsible not identified
   • Functional role of the C2-C2 zinc finger unresolved
3. 2005 Low

   First proposed a non-ribosomal partner by identifying RPS27A as a CK2alpha' interactor, raising the possibility of signaling roles.

   Evidence Yeast two-hybrid screen of HL-60 cDNA library

   PMID:15642198

   Open questions at the time

   • Single Y2H hit with no orthogonal validation
   • No functional consequence demonstrated
4. 2011 High

   Defined RPS27A as an extraribosomal regulator of p53 by mapping its binding to the MDM2 acidic domain and demonstrating a reciprocal MDM2-RPS27A regulatory loop.

   Evidence Co-IP, in vitro ubiquitination, shRNA knockdown, cell-cycle analysis under ribosomal stress

   PMID:21561866 PMID:22158882

   Open questions at the time

   • Structural basis of the acidic-domain interaction not resolved
   • Trigger for the nucleolar release of RPS27A not defined
5. 2015 Medium

   Placed RPS27A within a p53 feedback loop by showing it is a direct p53 transcriptional target required to amplify the p21 damage response.

   Evidence Reporter/runoff assay with dominant-negative p53 and RNAi, Western blot

   PMID:25592822

   Open questions at the time

   • p53 response element not finely mapped
   • Cell-type generality not tested
6. 2016 Medium

   Linked oncogenic signaling to RPS27A expression by demonstrating phospho-STAT3 transactivation through defined promoter elements.

   Evidence ChIP, promoter reporter, STAT3 inhibitor, Western blot in CML cells

   PMID:26942564

   Open questions at the time

   • Downstream consequence of elevated RPS27A in CML not mechanistically dissected
   • Single cell context
7. 2017 Medium

   Connected drug-induced microtubule perturbation to p53 activation through nucleolar-to-nucleoplasm translocation of RPS27A and reduced MDM2 Ser166 phosphorylation.

   Evidence Immunofluorescence localization, Western blot, half-life assays; viral oncoprotein stabilization assays

   PMID:28735865 PMID:28928040

   Open questions at the time

   • Molecular driver of translocation incompletely defined
   • How RPS27A alters MDM2 phosphorylation not established
8. 2020 High

   Established the deubiquitination step in 40S maturation (USP16 removing ubiquitin from RPS27a) and showed DNA breaks deplete RPS27a from ribosomes to remodel translation.

   Evidence MS of trapped pre-40S, USP16 deletion with rRNA processing assay; ribosome profiling with Cas9/dCas9 controls and proteasome inhibition

   PMID:32129764 PMID:34914197

   Open questions at the time

   • E3 ligase adding the internal ubiquitin not identified
   • Translational targets remodeled after RPS27a loss not fully cataloged
9. 2022 High

   Refined the p53 mechanism by identifying direct RPS27A-RPL11 binding that gates RPL11-MDM2 interaction, and revealed cell-type-dependent p53 dependence on RPS27A.

   Evidence GST pull-down, Co-IP, in vitro ubiquitination, xenograft; systematic knockdown across multiple cell lines

   PMID:35073964 PMID:37371478

   Open questions at the time

   • Basis for cell-type variability in p53 response unexplained
   • Stoichiometry of the RPS27A-RPL11-MDM2 assembly unknown
10. 2023 High

    Defined a chromatin-level function in which the RPS27A C-terminal fragment masks the nucleosome acidic patch to antagonize RNF168 H2A ubiquitination and 53BP1 recruitment.

    Evidence Co-IP, laser micro-irradiation imaging, domain mapping, H2A ubiquitination and 53BP1 foci assays

    PMID:37451480

    Open questions at the time

    • Regulation of RPS27A recruitment to damage sites not defined
    • Relationship between this role and its ribosomal pool unclear
11. 2024 Medium

    Connected RPS27A to NF-κB control through TRIM13-mediated degradation and a PSMD12/NF-κB axis, and demonstrated pathogen hijacking for host translational shutoff.

    Evidence Co-IP and ubiquitination assays with NF-κB reporters and tumor models; ASFV pCP312R crystal structure, Co-IP, RPS27A knockout/complementation, translation assays

    PMID:39069039 PMID:39069432 PMID:39667820

    Open questions at the time

    • Direct mechanism by which RPS27A modulates NF-κB not resolved
    • Whether translational and signaling pools are distinct unclear
12. 2025 Medium

    Showed RPS27A can act in trans, delivered by extracellular vesicles, binding the MDM2 promoter to promote p53 degradation and confer neuroprotection.

    Evidence EV co-culture, ChIP for MDM2 promoter binding, MDM2-overexpression rescue, PD models

    PMID:40338442

    Open questions at the time

    • Direct DNA-binding activity of RPS27A at the MDM2 promoter not biochemically characterized
    • Reconciliation with RPS27A's p53-stabilizing roles in other contexts unaddressed

• How RPS27A partitions between its ribosomal, p53-regulatory, chromatin, and NF-κB functions, and what governs its release from the nucleolus and ribosome, remains unresolved.
• No unified model linking ribosomal incorporation to extraribosomal pools
• Signals controlling subcellular redistribution not defined
• Structural basis of MDM2/RPL11/nucleosome interactions incomplete