PPP4R3A

PPP4R3A (SMEK1/SMK-1) is a substrate-targeting regulatory subunit of the Protein Phosphatase 4 (PP4) holoenzyme, in which it assembles into a stable heterotrimeric complex with the PP4 catalytic subunit (PP4C) and the regulatory subunit PP4R2; this complex dephosphorylates γH2AX, and the regulatory subunits tune the catalytic activity of PP4C (PMID:16085932, PMID:18715871). PPP4R3A directs the phosphatase to its targets through its EVH1 domain, which atypically recognizes FxxP and MxPP short linear motifs owing to a conserved leucine in place of the invariant EVH1 phenylalanine, with the Smk-1 domain also contributing to target binding (PMID:33352067). A recurring theme across species is that PPP4R3A governs PP4C subcellular targeting and substrate selection rather than bulk catalytic output — it recruits PP4C to replicating chromatin to silence the DNA damage checkpoint (PMID:17908915), and it controls PP4C nuclear localization to enable a developmental and stress-response transcriptional program (PMID:17353263). Through PP4-mediated dephosphorylation of the transcription elongation factor SPT-5/SUPT5H, PPP4R3A promotes RNA polymerase II recruitment and transcriptional initiation at FOXO/DAF-16 target genes, thereby specifying which stress-response and innate-immunity genes are activated (PMID:16530049, PMID:31919361). In neuroblast asymmetric division it directly binds Miranda and, in a cell-cycle-dependent manner dictated by its own nuclear/cytoplasmic shuttling, controls cortical localization of cell-fate determinants (PMID:19204120). In mammals, PPP4R3A positively regulates mTORC1 signaling to support synaptic protein synthesis, with loss of function producing depression- and anxiety-like phenotypes (PMID:35314861), and wild-type protein acts as a tumor suppressor through the Akt-mTOR-P70 S6K/4E-BP1 axis, a function lost by the Asp409Asn variant (PMID:38275415).

1. 2005 High

   Establishing PPP4R3A as a defined PP4 holoenzyme subunit answered whether it acts as a phosphatase-targeting protein and connected it to DNA damage repair across species.

   Evidence TAP-MS complex purification, yeast complementation, and cisplatin-sensitivity assays in yeast, Drosophila, and human cells

   PMID:16085932

   Open questions at the time

   • Did not define how PP4R3 selects substrates
   • Rad53/CHK2 interaction not mapped to a direct binding interface
2. 2006 High

   Genetic placement of SMK-1 downstream of DAF-16 nuclear entry showed it acts as a transcriptional-specificity determinant rather than a regulator of FOXO localization, defining its role in selecting target-gene subsets.

   Evidence Genetic epistasis with DAF-16::GFP localization and stress/immunity phenotypes in C. elegans loss-of-function mutants

   PMID:16530049

   Open questions at the time

   • Molecular mechanism of transcriptional specificity not resolved at this stage
   • No biochemical substrate identified
3. 2007 Medium

   Chromatin fractionation showed PPP4R3A controls PP4C recruitment to a site of action, establishing the subunit as a spatial targeting factor for the catalytic subunit.

   Evidence Genetic analysis plus SMK-1-dependent PPH-4.1 chromatin fractionation in C. elegans embryos; parallel Dictyostelium work showed SMEK controls PP4C nuclear localization without altering bulk activity

   PMID:17353263 PMID:17908915

   Open questions at the time

   • Direct chromatin-binding partner of SMK-1 not identified
   • Checkpoint substrate dephosphorylated on chromatin not defined
4. 2008 High

   Reconstitution of the PP4c·PP4R2·PP4R3 trimer with in vitro phosphatase assays answered whether regulatory subunits modulate catalysis, demonstrating activity toward γH2AX and subunit-dependent tuning of catalytic output.

   Evidence Affinity purification/MS, in vitro phosphatase activity toward fluorogenic substrate and γH2AX, and interface mutagenesis

   PMID:18715871

   Open questions at the time

   • Did not test physiological substrates beyond γH2AX
   • Structural basis of activity modulation not determined
5. 2009 High

   Identifying Miranda as a direct PP4R3 interactor and showing cell-cycle-dependent shuttling answered how the subunit links PP4 to asymmetric cell division.

   Evidence Clonal loss-of-function, live imaging of Flfl localization, nuclear-excluded/membrane-targeted constructs, direct Miranda binding, and lgl epistasis in Drosophila neuroblasts

   PMID:19204120

   Open questions at the time

   • Whether Miranda is a direct dephosphorylation substrate not shown
   • Mechanism controlling Flfl shuttling not defined
6. 2015 Medium

   Bidirectional genetic perturbation identified PPP4R3A as a negative regulator of JNK-dependent cell death, extending its function to stress-induced apoptotic signaling.

   Evidence Gain- and loss-of-function with JNK pathway epistasis and cell death assays in Drosophila eye and wing

   PMID:26583122

   Open questions at the time

   • JNK pathway substrate of PP4-Flfl not identified
   • Single organism, no biochemical mechanism
7. 2020 High

   Phosphoproteomics identified SPT-5/SUPT5H as a PP4^SMK-1 substrate and linked its dephosphorylation to RNA Pol II recruitment, providing the molecular mechanism for transcriptional specificity at FOXO target genes; parallel work defined the age-dependent immune role.

   Evidence Phosphoproteomics, RNA Pol II promoter recruitment, RNAi epistasis, and pathogen survival assays in C. elegans

   PMID:31919361 PMID:32161087 PMID:33315870

   Open questions at the time

   • SPT-5 dephosphorylation site/consequence not structurally resolved
   • Mammalian conservation of the SPT-5 mechanism not tested
8. 2020 Medium

   Mapping EVH1 specificity to FxxP/MxPP motifs via a conserved leucine substitution explained how PPP4R3A engages substrates and distinguished it from canonical EVH1 domains.

   Evidence Binding partner identification, mutagenesis of the critical leucine, and Smk-1 domain mapping in Drosophila Falafel

   PMID:33352067

   Open questions at the time

   • Full substrate repertoire bearing these motifs not enumerated
   • No structure of the EVH1-motif complex
9. 2022 Medium

   Conditional knockout and rescue placed mammalian PPP4R3A upstream of mTORC1 in neurons, linking it to synaptic protein synthesis and affective behavior.

   Evidence Mouse cortex/hippocampus conditional KO, viral overexpression, rapamycin blockade, and behavioral and biochemical assays

   PMID:35314861

   Open questions at the time

   • Direct phosphatase substrate connecting PPP4R3A to mTORC1 not identified
   • Whether the effect requires PP4 catalytic activity not tested
10. 2024 Medium

    Comparing wild-type and Asp409Asn PPP4R3A in cancer cells defined a tumor-suppressor function through the Akt-mTOR axis and a gain-of-function consequence of the variant.

    Evidence Wild-type vs. mutant overexpression in thyroid cancer lines with Akt-mTOR-P70 S6K/4E-BP1 readouts and proliferation/migration/invasion assays

    PMID:38275415

    Open questions at the time

    • No in vitro reconstitution of the variant's effect on PP4 activity
    • Reconciliation with neuronal positive regulation of mTORC1 not addressed
11. 2024 Medium

    Linking cathepsin B (CPR-6) to SMK-1 protein levels positioned SMK-1 as a downstream effector in Aβ proteotoxicity regulation.

    Evidence cpr-6 RNAi, activity-based protease probes, SMK-1 protein quantification, and epistasis in a C. elegans Aβ model

    PMID:39362844

    Open questions at the time

    • Mechanism of SMK-1 level regulation by CPR-6 not defined
    • Whether PP4 catalytic activity mediates the protective effect untested

• How PPP4R3A's role as a positive regulator of neuronal mTORC1 is reconciled with its tumor-suppressive restraint of the Akt-mTOR axis, and whether both depend on PP4 phosphatase activity toward shared substrates, remains unresolved.
• No mammalian phosphatase substrate bridging PPP4R3A to mTORC1 identified
• Tissue-specific differences in mTOR-axis regulation not mechanistically explained