INTS4

INTS4 is a HEAT-repeat scaffold protein that nucleates the heterotrimeric Integrator cleavage module (ICM) together with the endonuclease INTS11 and its catalytically impaired paralog INTS9, and is essential for snRNA 3′-end processing, Cajal body integrity, and transcription attenuation of protein-coding genes. Cryo-EM structures show that INTS4 stabilizes the INTS9/INTS11 nuclease dimer through N-terminal HEAT repeats and a C-terminal β-sheet domain, and that all three subunits create a composite electropositive groove for RNA engagement (PMID:33548203, PMID:29471365). Inositol hexakisphosphate (IP6) binds a pocket at the three-way interface of INTS4/9/11, locks the mature cleavage module conformation, and is required for both snRNA processing and mRNA transcription attenuation (PMID:36180473). Biogenesis of the ICM is an ordered process in which chaperones BRAT1 and WDR73 keep the INTS9–INTS11 dimer inactive and prevent premature INTS4 association; INTS4 joins only after nuclear import, and IP6-dependent conformational locking releases BRAT1 to yield the active complex (PMID:39032489).

1. 2012 High

   Establishing that INTS4 is functionally required for snRNA maturation and Cajal body homeostasis answered whether individual Integrator subunits beyond the catalytic core have non-redundant roles in vivo.

   Evidence siRNA knockdown of INTS4 in human cells with immunofluorescence and snRNA processing assays

   PMID:22250197

   Open questions at the time

   • Mechanism by which INTS4 contributes to snRNA processing was unknown
   • Whether INTS4 acts within a defined subcomplex or the holo-Integrator was unresolved
   • Direct versus indirect effects on Cajal body structure were not distinguished
2. 2018 High

   Identifying the INTS4/9/11 heterotrimer as a discrete cleavage module resolved how endonucleolytic activity is organized within Integrator and showed that INTS4 functions as the scaffold bridging the INTS9–INTS11 nuclease dimer.

   Evidence Co-immunoprecipitation, domain deletion/mutagenesis, and functional assays for snRNA processing and Cajal body integrity

   PMID:29471365

   Open questions at the time

   • No high-resolution structure of the ternary complex existed
   • The RNA-binding mode and path were unresolved
   • How the cleavage module is recruited to the holo-Integrator remained unclear
3. 2021 High

   The 3.5 Å cryo-EM structure of the INTS4/9/11 complex revealed how INTS4 stabilizes the INTS9–INTS11 nuclease domains and identified a composite electropositive groove as a putative RNA-binding path, providing the first atomic-level view of the cleavage module architecture.

   Evidence Cryo-EM at 3.5 Å resolution with biochemical subcomplex characterization

   PMID:33548203

   Open questions at the time

   • No RNA-bound structure was available to confirm the binding path
   • Regulatory cofactors or post-translational modifications were not captured
   • Mechanism of substrate specificity was unresolved
4. 2022 High

   Discovery that IP6 binds the three-subunit interface of the ICM and is required for snRNA processing and transcription attenuation revealed a metabolic cofactor essential for cleavage module function, answering how conformational maturity of the complex is achieved.

   Evidence Cryo-EM at 2.74 Å (Drosophila) with IP6-binding site mutagenesis and functional assays validated in human cells

   PMID:36180473

   Open questions at the time

   • Whether IP6 levels regulate Integrator activity physiologically was not tested
   • Structural basis of IP6-induced conformational change at atomic detail was not fully resolved
   • Relationship between IP6 binding and RNA engagement remained unclear
5. 2022 Medium

   Demonstrating that INTS11 SUMOylation is dispensable for INTS4/9/11 trimer formation but required for holo-Integrator assembly clarified the boundary between cleavage module integrity and full complex integration.

   Evidence SUMO isopeptidase overexpression, SUMOylation-site mutagenesis, co-IP, and snRNA cleavage assays

   PMID:36454007

   Open questions at the time

   • SUMOylation effects on INTS4 specifically (versus INTS11) were not deconvoluted
   • In vivo stoichiometry of SUMOylated versus unmodified complexes was not measured
   • Whether SUMOylation controls ICM activity or only localization/recruitment was unclear
6. 2024 High

   Identification of BRAT1 and WDR73 as biogenesis chaperones that enforce ordered ICM assembly — blocking premature INTS4 association and the INTS11 active site until nuclear import and IP6 locking — established that the cleavage module undergoes a regulated maturation pathway rather than spontaneous assembly.

   Evidence Biochemical reconstitution, co-IP, nuclear import assays, snRNA processing assays, and structural analysis

   PMID:39032489

   Open questions at the time

   • Signals triggering BRAT1/WDR73 release beyond IP6 locking were not defined
   • Whether biogenesis defects contribute to disease phenotypes was not explored
   • Structural basis of BRAT1 occlusion of the active site at atomic resolution was not reported
7. 2025 Medium

   Conditional depletion of INTS-4 in C. elegans showed that the cleavage module is essential during early larval development but dispensable after the L2/L3 stage, and that germline-specific loss causes snRNA misprocessing without overt developmental defects, revealing temporal and tissue-specific requirements.

   Evidence Auxin-inducible degron system for temporal and tissue-specific depletion with developmental phenotyping and snRNA processing assays

   PMID:40071568

   Open questions at the time

   • Which specific developmental programs depend on ICM-mediated transcription attenuation is unknown
   • Whether mammalian developmental timing requirements mirror the C. elegans findings has not been tested
   • Compensatory mechanisms after L2/L3 are uncharacterized

• Key open questions include the structural basis for RNA substrate recognition by the ICM, whether IP6 levels serve as a physiological regulator of Integrator activity, and the molecular details of how the cleavage module is recruited to and activated within the holo-Integrator on chromatin.
• No RNA-bound structure of the ICM exists
• Mechanism of substrate discrimination between snRNA and mRNA targets is not defined
• Structural view of ICM within the complete holo-Integrator is lacking