UTP6

UTP6 (HCA66) is a dual-function protein that operates both in nucleolar ribosome biogenesis and at the centrosome during the cell cycle (PMID:19299467, PMID:22434888). In the nucleolus it is a stable component of the 90S pre-ribosome/SSU processome, partitioning into a subcomplex with Pwp2, Dip2, Utp13, Utp18, and Utp21 that engages the 5' end of the pre-rRNA independently of the U3 snoRNP (PMID:15231838); within this assembly its HAT (half-a-tetratricopeptide repeat) domain binds a specific peptide ligand in Utp21 (Kd ~10 µM) while its N-terminal domain contacts Utp18, and an intact HAT domain is required for efficient pre-rRNA processing and growth (PMID:18725399). This role is conserved in mammals, where UTP6 is required for nucleolar steps of 40S ribosomal subunit maturation (PMID:22434888). Independently, HCA66 localizes to the centrosome from S-phase through mitosis, where it stabilizes the gamma-tubulin small complex (gamma-tubulin/GCP2/GCP3) and is required for centrosome duplication and bipolar spindle assembly (PMID:19299467); a dominant-negative that accumulates at centrosomes but not nucleoli dissociates the ribosomal and centrosomal functions (PMID:22434888). HCA66 directly binds the CED4 domain of Apaf-1, which controls its recruitment to the centrosome and positively regulates Apaf-1-dependent apoptosis by promoting caspase-9 recruitment to the apoptosome and caspase-3 activation downstream of cytochrome c release (PMID:17380155, PMID:21984814).

1. 2004 High

   Established that the Utp6 ortholog is an integral, structured component of the early ribosome assembly machinery rather than a transient factor, by placing it in a defined pre-rRNA-binding subcomplex.

   Evidence Immunoprecipitation, gradient sedimentation, and conditional depletion in yeast

   PMID:15231838

   Open questions at the time

   • Did not resolve which domain of Utp6 mediates subcomplex assembly
   • No direct demonstration of Utp6's individual contribution to pre-rRNA cleavage
2. 2008 High

   Defined the molecular interactions anchoring Utp6 within the subcomplex, showing its HAT domain is a peptide-binding module essential for pre-rRNA processing.

   Evidence Yeast two-hybrid mapping, mutagenesis, biophysical Kd measurement, and growth assays

   PMID:18725399

   Open questions at the time

   • No high-resolution structure of the HAT domain bound to the Utp21 peptide
   • Functional consequence of the Utp18 contact not separately tested
3. 2009 Medium

   Mapped the functionally critical surface of the HAT domain to interpret loss-of-function residues structurally.

   Evidence Homology modeling combined with random and directed mutagenesis and growth assays in yeast

   PMID:19515729

   Open questions at the time

   • Model is computational, not an experimental structure
   • Functional surface not validated by direct binding measurements
4. 2007 High

   Revealed an unexpected role for the human protein in apoptosis through direct binding to the Apaf-1 CED4 domain, linking it to apoptosome function.

   Evidence Co-IP, gain/loss-of-function in cells, and cell-free apoptosome and caspase activity assays

   PMID:17380155

   Open questions at the time

   • Did not define how HCA66 promotes caspase-9 recruitment mechanistically
   • Did not connect this role to its nucleolar/ribosomal function
5. 2009 High

   Identified a centrosomal function distinct from ribosome biogenesis, showing HCA66 stabilizes the gamma-tubulin small complex required for spindle assembly.

   Evidence Mass spectrometry of pericentriolar material, siRNA silencing, immunofluorescence, and immunoblotting

   PMID:19299467

   Open questions at the time

   • Molecular mechanism by which HCA66 stabilizes gamma-TuSC not defined
   • Whether centrosomal and nucleolar pools are the same molecules unresolved
6. 2011 Medium

   Connected the Apaf-1 interaction to the centrosomal function, showing Apaf-1 controls HCA66 recruitment to the centrosome.

   Evidence Co-IP, Apaf1 depletion, immunofluorescence, and centrosome functional assays

   PMID:21984814

   Open questions at the time

   • Single-lab loss-of-function without reciprocal structural mapping of the recruitment interface
   • Apoptotic versus centrosomal roles of the Apaf-1 interaction not cleanly separated
7. 2012 High

   Demonstrated genetically that the ribosome biogenesis and centriole duplication functions are mechanistically independent, using a localization-restricted dominant negative.

   Evidence siRNA depletion, dominant-negative overexpression, pre-rRNA processing assays, and immunofluorescence in HeLa cells

   PMID:22434888

   Open questions at the time

   • Does not explain how a single protein partitions between two organelles
   • Regulatory cues controlling pool distribution unknown
8. 2026 Low

   Extended the conserved SSU processome role to trypanosomatids, confirming UTP6 as a component of early pre-small-subunit complexes across divergent eukaryotes.

   Evidence Affinity purification-mass spectrometry in T. brucei (bait pulldown)

   PMID:42150138

   Open questions at the time

   • Single AP-MS experiment with no functional follow-up on UTP6 in T. brucei
   • No confirmation that the trypanosome complex mirrors the yeast/human subcomplex composition

• How a single protein is partitioned and regulated between its nucleolar ribosome-assembly role and its Apaf-1-coupled centrosomal/apoptotic roles remains unresolved.
• No structural model of the human protein in either complex
• Signals governing centrosome versus nucleolus localization unknown
• Direct mechanistic link between HAT-domain peptide binding and gamma-TuSC stabilization not established