NOB1

NOB1 is a conserved PIN-domain endonuclease essential for the final step of small ribosomal subunit biogenesis, catalyzing cleavage of 20S pre-rRNA at site D to generate the mature 3'-end of 18S rRNA. Its PIN domain carries the Mn²⁺-dependent catalytic active site and mediates substrate binding at the single-stranded cleavage site, while a zinc ribbon domain anchors the enzyme to helix 40 of small subunit rRNA, as demonstrated by NMR structure and mutagenesis of the archaeal ortholog (PMID:15388878, PMID:19706509, PMID:22156373). In yeast, Nob1 catalytic activity at site D is coordinated with the DEAH-box helicase Prp43 and its cofactor Pfa1 through a genetically defined epistatic pathway (PMID:19801658), and Nob1 separately functions as a chaperone in 26S proteasome assembly by facilitating joining of the 19S regulatory particle with the 20S core, after which it is internalized and degraded (PMID:12502737). In human cells, NOB1 proximity interactors are enriched in ribosome assembly and translation factors including EIF4B and EIF4G2 (PMID:40157618), and NOB1 silencing in multiple cancer cell types suppresses proliferation, induces apoptosis, and modulates MAPK/JNK signaling (PMID:23911301, PMID:27035645).

1. 2000 Medium

   Identifying Nob1 as a proteasome-associated protein established its initial functional context: Nob1 physically interacts with the 19S regulatory particle subunit Rpn12 and co-fractionates with proteasomal complexes, and is itself a proteasome substrate.

   Evidence Two-hybrid screen, co-immunoprecipitation with Rpt1, glycerol gradient fractionation in yeast

   PMID:10675611

   Open questions at the time

   • No reciprocal Co-IP at this stage
   • Mechanism of Nob1 degradation by proteasome not defined
   • Relationship to ribosome biogenesis unknown
2. 2002 High

   Demonstrating that Nob1 acts as a chaperone for 26S proteasome assembly — facilitating 19S–20S joining and being degraded upon completion — resolved its mechanistic role in proteasome biogenesis distinct from its later-discovered rRNA processing function.

   Evidence Temperature-sensitive mutants, overexpression suppression, reciprocal Co-IP, glycerol gradient fractionation, immunofluorescence in yeast

   PMID:12502737

   Open questions at the time

   • Whether this proteasome chaperone role is conserved in metazoans is unknown
   • Structural basis of 19S–20S bridging by Nob1 not resolved
3. 2004 High

   Linking Nob1's PIN domain to D-site cleavage of 20S pre-rRNA revealed its second and now primary known function — as the endonuclease that generates the mature 3'-end of 18S rRNA.

   Evidence Homology modeling of PIN domain, active-site point mutagenesis, in vivo pre-rRNA processing assay in yeast

   PMID:15388878

   Open questions at the time

   • No in vitro cleavage demonstrated yet
   • Domain architecture beyond PIN not characterized
   • Cofactor requirements unknown
4. 2009 High

   Reconstitution of D-site cleavage in vitro with recombinant Nob1, combined with RNA footprinting, proved Nob1 is the direct endonuclease and mapped its binding site to the single-stranded region at the 3'-end of 18S rRNA; genetic epistasis simultaneously placed Nob1 catalytic activity downstream of the helicase Prp43/Pfa1 in the pre-40S maturation pathway.

   Evidence In vitro cleavage with recombinant yeast Nob1, RNA footprinting, analytical ultracentrifugation; genetic suppressor dosage assays with Prp43/Pfa1/Ltv1 mutants

   PMID:19706509 PMID:19801658

   Open questions at the time

   • Oligomeric state in vivo (tetramer vs. monomer on pre-40S) unresolved
   • How Prp43 helicase activity licenses Nob1 cleavage mechanistically unclear
   • Metal ion identity for yeast Nob1 not defined
5. 2011 High

   The NMR structure of archaeal Nob1 revealed a two-domain architecture — PIN domain plus zinc ribbon domain — and showed that the zinc ribbon anchors Nob1 to helix 40 of small subunit rRNA while the PIN domain performs Mn²⁺-dependent cleavage, explaining how the enzyme is positioned on the pre-40S particle.

   Evidence NMR structure determination of Pyrococcus horikoshii Nob1, in vitro RNA cleavage assays with manganese, domain deletion and mutagenesis, RNA binding assays

   PMID:22156373

   Open questions at the time

   • No high-resolution structure of eukaryotic Nob1 on the pre-40S particle at this time
   • How cleavage timing is regulated structurally remains open
6. 2005 Medium

   Cloning and localization of human NOB1 confirmed conservation of the PIN and zinc ribbon domains and established its predominantly nuclear localization in mammalian cells.

   Evidence cDNA cloning, domain analysis, fluorescence localization and fractionation in transfected mammalian cells

   PMID:16172919

   Open questions at the time

   • Functional conservation of endonuclease activity not tested in human cells
   • Nuclear vs. cytoplasmic distribution during ribosome maturation not resolved
7. 2013 Medium

   Functional loss-of-function studies in human cancer cells first established that NOB1 is required for proliferation, colony formation, and migration, and that its depletion induces cell cycle arrest and apoptosis — extending its relevance beyond ribosome biogenesis to cancer cell biology.

   Evidence Lentiviral shRNA knockdown in glioma lines U251/U87-MG, MTT, colony formation, flow cytometry, Transwell assay

   PMID:23911301

   Open questions at the time

   • Whether proliferative effects are secondary to ribosome biogenesis defects or represent independent functions is unclear
   • Downstream signaling pathways not identified in this study
8. 2014 Medium

   Multiple cancer-type knockdown studies converged on MAPK pathway modulation (p38, JNK, ERK) as a downstream consequence of NOB1 depletion, providing a signaling framework for its pro-survival role.

   Evidence shRNA/siRNA knockdown in thyroid carcinoma, laryngeal cancer, and colorectal cancer cells; phospho-Western blot for MAPK pathway components; xenograft models

   PMID:25231838 PMID:27035645 PMID:31906747

   Open questions at the time

   • Causal chain from NOB1 loss to MAPK activation not defined — could be indirect via ribosome stress
   • No rescue with catalytic-dead NOB1 to distinguish endonuclease-dependent vs. -independent effects
   • Studies are from single labs per cancer type
9. 2025 Medium

   Proximity proteomics of human NOB1 revealed its interactome is overwhelmingly enriched in ribosome assembly and translation factors, with validated interactions with EIF4B and EIF4G2, expanding its functional network beyond pre-rRNA processing.

   Evidence TurboID proximity labeling, mass spectrometry, co-immunoprecipitation in human cells

   PMID:40157618

   Open questions at the time

   • Proximity interactions need validation as direct physical contacts
   • Functional significance of EIF4B/EIF4G2 association for translation regulation not tested
   • Whether these interactions depend on Nob1 endonuclease activity is unknown

• Key open questions include: (1) whether the cancer-proliferative phenotype of NOB1 depletion is entirely secondary to ribosome biogenesis defects or involves endonuclease-independent functions; (2) the structural basis of human NOB1 positioning on the pre-40S particle; and (3) the mechanistic link between NOB1 and translation initiation factor interactions.
• No catalytic-dead rescue experiment in human cancer models
• No cryo-EM or crystal structure of human NOB1 on pre-40S
• Functional role of NOB1–EIF4B/EIF4G2 interaction uncharacterized