STRN3

STRN3 (SG2NA) is a striatin-family scaffolding protein that assembles PP2A phosphatase complexes to regulate Hippo signaling, cell cycle progression, and cell survival under stress. As a PP2A regulatory subunit, STRN3 recruits MST1/2 kinases for dephosphorylation, thereby suppressing Hippo pathway activity and activating YAP; this association with PP2A is methylation-independent and is actually enhanced when the catalytic subunit is unmethylated (PMID:32589942, PMID:11160832). STRN3 also scaffolds a DJ-1–Akt trimeric complex at mitochondria and plasma membrane that protects cells from oxidative stress–induced apoptosis, with STRN3 stabilizing DJ-1 against proteasomal degradation (PMID:25035075, PMID:26022125). STRN3 protein stability is controlled by GSK3β-mediated phosphorylation opposed by ERK signaling, and its levels peak at G2/M to regulate cell cycle transitions (PMID:28790387).

1. 2000 Medium

   Establishing STRN3 as a calmodulin-binding protein with dual cytosolic/membrane distribution revealed it as a Ca²⁺-responsive signaling scaffold rather than a simple cytoplasmic factor.

   Evidence Ca²⁺-dependent calmodulin-binding assay, subcellular fractionation, and immunostaining in rat brain

   PMID:10748158

   Open questions at the time

   • Functional consequence of calmodulin binding on STRN3 scaffold activity not determined
   • Whether Ca²⁺-dependent regulation operates in non-neuronal cells unknown
2. 2001 High

   Demonstrating that STRN3 binds the PP2A catalytic subunit independently of Leu309 methylation — unlike the canonical Bα subunit — established a distinct mode of PP2A holoenzyme assembly through striatin-family proteins.

   Evidence Co-immunoprecipitation with methylation-selective antibodies, 13 C-subunit point mutants, and enzymatic demethylation in cell lysates

   PMID:11160832

   Open questions at the time

   • Structural basis of methylation-independent binding not resolved
   • Whether other striatin-family members share the same methylation-independent mechanism not tested in parallel
3. 2001 Medium

   Identifying the N-terminal domain as a transcriptional activator and the C-terminal WD-40 repeats as an inhibitory module revealed an internal regulatory architecture within STRN3.

   Evidence GAL4-VP16 reporter assays in yeast and mammalian cells with domain-swap experiments

   PMID:11570823

   Open questions at the time

   • Endogenous transcriptional targets of STRN3 not identified
   • Whether transcriptional activity operates in physiological contexts or is an artifact of the artificial reporter system not established
4. 2014 Medium

   Discovery that STRN3 scaffolds DJ-1 and Akt into a trimeric complex at mitochondria and plasma membrane revealed a new pro-survival mechanism linking STRN3 to oxidative stress protection, and showed that Parkinson's disease DJ-1 mutants fail to assemble this complex.

   Evidence Reciprocal co-immunoprecipitation, domain-deletion mapping, confocal colocalization, and apoptosis assays upon ROS manipulation

   PMID:25035075

   Open questions at the time

   • Direct phosphorylation targets of Akt downstream of this complex not mapped
   • Physiological relevance in neuronal systems (where DJ-1 mutations cause disease) not tested
5. 2015 Medium

   Showing that STRN3 stabilizes DJ-1 against proteasomal degradation and that loss of STRN3 reduces cell growth extended the scaffold model to cancer cell proliferation and survival.

   Evidence shRNA knockdown with proteasome inhibitor rescue, colony formation assays, and ROS modulation with NAC

   PMID:26022125

   Open questions at the time

   • Ubiquitin ligase responsible for DJ-1 degradation upon STRN3 loss not identified
   • In vivo tumor growth effects of STRN3 depletion not tested in this context
6. 2017 Medium

   Establishing that GSK3β phosphorylation stabilizes STRN3 while ERK opposes this through GSK3β inhibition, with STRN3 levels peaking at G2/M, placed STRN3 within a kinase signaling circuit controlling cell cycle progression.

   Evidence Kinase inhibitors (LiCl, PD98059), okadaic acid, proteasome inhibitor, and flow cytometry cell cycle analysis

   PMID:28790387

   Open questions at the time

   • Specific GSK3β phosphorylation sites on STRN3 not mapped
   • Whether cell cycle phenotype is direct or mediated through PP2A substrate changes not resolved
7. 2017 Medium

   Finding that STRN3 depletion triggers ER stress and that ER stressors redirect STRN3 to mitochondria and microsomes connected STRN3 to the unfolded protein response and organelle-specific stress adaptation.

   Evidence shRNA knockdown with proteome analysis, thapsigargin/tunicamycin treatment, subcellular fractionation, and flow cytometry

   PMID:28634818

   Open questions at the time

   • Mechanism by which STRN3 loss induces ER stress not defined
   • Whether ER stress relocalization of STRN3 requires PP2A association unknown
8. 2020 High

   Demonstrating that STRN3 functions as the PP2A subunit that recruits MST1/2 for dephosphorylation — and that a structure-guided peptide (SHAP) disrupting STRN3–PP2Aα interaction reactivates Hippo signaling in vivo — provided the key mechanistic link between STRN3 and Hippo/YAP oncogenic signaling.

   Evidence Co-IP, in vitro phosphatase assay, SHAP peptide inhibitor design, knockdown/overexpression with phospho-MST1/2 and YAP readouts, and xenograft models

   PMID:32589942

   Open questions at the time

   • Whether STRN3 is the sole striatin-family member mediating MST1/2 dephosphorylation or acts redundantly with striatin/STRN4 not fully resolved
   • Structural basis of STRN3–MST1/2 interaction at atomic resolution not available
9. 2023 Medium

   Characterization of the STRN3–PDGFRB fusion as a cytoplasmic oncoprotein that transforms hematopoietic cells and causes MDS/MPN-like disease in mice demonstrated that the STRN3 coiled-coil domain can serve as an oligomerization platform for constitutive kinase activation.

   Evidence RT-PCR/Sanger sequencing of patient fusion, Ba/F3 transformation assay, mouse transplantation model, and TKI sensitivity testing

   PMID:37550570

   Open questions at the time

   • Whether endogenous STRN3 coiled-coil domain oligomerizes PP2A/STRIPAK complexes in the same manner not addressed
   • Clinical response data for this fusion limited to a single case

• The structural basis of STRN3's selective recruitment of MST1/2 to PP2A, the identity of endogenous transcriptional targets, and the functional interplay between its roles as a Hippo regulator and a DJ-1–Akt scaffold remain unresolved.
• No high-resolution structure of STRN3 in complex with PP2A and MST1/2
• Endogenous gene targets of STRN3's transcriptional activator domain not identified
• Whether DJ-1–Akt scaffolding and MST1/2–PP2A dephosphorylation are coordinated or independent functions unknown