PWP2

PWP2 (Utp1) is an essential WD-repeat protein that scaffolds the UTP-B subcomplex of the SSU processome (90S pre-ribosome), the machinery that carries out the earliest steps of small ribosomal subunit biogenesis (PMID:12068309, PMID:15231838). It assembles UTP-B through two structurally independent regions: its N-terminal tandem WD-repeat domain associates with Utp21, Utp18, and Utp6 to form a core, while the C-terminal domains of PWP2 and Utp21 together recruit the Utp12:Utp13 heterodimer, an interaction required for stable incorporation of UTP-B into the processome (PMID:28600509). Depletion of PWP2 blocks formation of the 90S particle and abolishes the early endonucleolytic cleavages at sites A0, A1, and A2 that generate 18S rRNA, and prevents U3 snoRNP, Imp3, and Imp4 from associating with the 35S pre-rRNA; the PWP2 subcomplex can directly contact the 5' end of the 35S pre-rRNA independently of U3 snoRNP, placing it early in the assembly hierarchy upstream of factors such as Rrp36 (PMID:15231838, PMID:20038530). This nucleolar function is conserved: human PWP2 localizes to the dense fibrillar and granular components of nucleoli and binds tightly to large complexes, redistributing upon transcription arrest (PMID:24754225), and zebrafish loss of pwp2h reduces mature 18S rRNA, triggers Tp53 activation and compensatory autophagy, and arrests growth of proliferative tissues (PMID:23408911).

1. 1996 Medium

   Established PWP2 as an essential gene whose product resides in a stable proteinaceous complex, the first clue that it acts as part of a larger assembly rather than alone.

   Evidence Gene disruption, GAL1-conditional depletion, immunofluorescence, and cell fractionation in S. cerevisiae

   PMID:8804409

   Open questions at the time

   • Phenotypes (bud-site selection, cytokinesis) were interpreted before its ribosome biogenesis role was known
   • Complex composition and molecular function not yet defined
2. 2002 High

   Placed PWP2/Utp1 within the 80S U3 snoRNP (SSU processome) and showed its depletion impairs 18S rRNA production, defining its true molecular context as pre-rRNA processing.

   Evidence Biochemical purification of U3 snoRNP with MS identification and depletion analysis by rRNA northern blotting

   PMID:12068309

   Open questions at the time

   • Did not resolve which sub-complex PWP2 belongs to within the processome
   • Direct RNA contacts unknown
3. 2004 High

   Defined PWP2's pathway position by showing it is required for 90S particle formation and the A0/A1/A2 cleavages, and that it forms a discrete subcomplex able to contact 35S pre-rRNA independently of U3 snoRNP.

   Evidence Conditional depletion, particle sedimentation, immunoprecipitation, and northern blotting in yeast

   PMID:15231838

   Open questions at the time

   • Architecture of the subcomplex and how PWP2 organizes it not yet resolved
   • Direct vs indirect RNA binding not distinguished
4. 2008 Medium

   Showed the PWP2 subcomplex is functionally interconnected with Mpp10 and U3 snoRNP subcomplexes inside the assembled 90S particle, framing it as one interdependent module of a coordinated machine.

   Evidence GFP-fusion incorporation, partial loss-of-function allele, Miller spread EM, and northern blotting

   PMID:18586827

   Open questions at the time

   • Molecular nature of the inter-subcomplex contacts unresolved
   • Functional interaction defined genetically, not structurally
5. 2009 Medium

   Ordered the assembly hierarchy by demonstrating that UTP-B (PWP2) acts upstream of Rrp36 recruitment, establishing PWP2 as an early-acting scaffold in 90S maturation with conserved human function.

   Evidence Sequential depletion epistasis, sucrose gradient sedimentation, co-IP, and northern blotting in yeast and HeLa cells

   PMID:20038530

   Open questions at the time

   • Mechanism by which UTP-B licenses Rrp36 binding unknown
   • Human PWP2 not directly tested in this study
6. 2017 High

   Resolved how PWP2 builds UTP-B at the domain level, showing the tWD domain nucleates a Utp21/Utp18/Utp6 core while C-terminal domains recruit Utp12:Utp13 for stable processome incorporation.

   Evidence Structural analysis, domain deletion/mutagenesis, yeast two-hybrid, co-IP, and depletion with rRNA processing assays

   PMID:28600509

   Open questions at the time

   • Full atomic structure of PWP2 within the assembled processome not reported here
   • Dynamics of UTP-B assembly in vivo not captured
7. 2013 High

   Demonstrated organismal consequences of PWP2 loss, linking its 18S rRNA function to Tp53 activation, compensatory autophagy, and arrested growth of proliferative tissues.

   Evidence Forward genetic screen, zebrafish pwp2h mutant analysis, rRNA northern blotting, autophagy assays, and Tp53/Tor epistasis

   PMID:23408911

   Open questions at the time

   • Whether autophagy induction is a direct consequence of ribosome biogenesis stress is not mechanistically defined
   • No human disease linkage established
8. 2014 Medium

   Mapped human PWP2 to the dense fibrillar and granular nucleolar compartments and showed transcription-dependent dynamics, confirming conserved nucleolar engagement in rRNA-processing complexes.

   Evidence Live-cell fluorescence microscopy, FRAP, and rRNA transcription inhibition in HeLa cells

   PMID:24754225

   Open questions at the time

   • Identity of the human-specific binding partners not resolved
   • Functional rRNA processing role in human cells not directly assayed here

• How PWP2-dependent ribosome biogenesis stress is transduced into Tp53 activation and autophagy in metazoans, and whether human PWP2 variants cause disease, remain open.
• Molecular signal connecting impaired 18S production to Tp53 not identified
• No causative human disease mutation reported in the corpus