PPP4R2

PPP4R2 is a regulatory subunit of protein phosphatase 4 (PPP4c) that directs the substrate specificity and subcellular targeting of the phosphatase across DNA repair, signaling, and circadian processes (PMID:10769191, PMID:29221109, PMID:34301769). It binds PPP4c directly and assembles into large native PPP4 complexes (450–600 kDa) that are catalytically inactive until activated by basic proteins, identifying PPP4R2 as a key modulator of holoenzyme activity (PMID:10769191). Through PPP4c, PPP4R2 promotes dephosphorylation of multiple substrates with distinct cellular consequences: it is required for dephosphorylation of the DNA damage response proteins KAP1, γH2AX, p53, and RPA2 and thereby for efficient double-strand break repair (PMID:29221109); it drives nuclear dephosphorylation of Suppressor of fused (Sufu), promoting Sufu degradation and enhancing Gli1-dependent Hedgehog signaling (PMID:32826873); and it counteracts BMAL1 phosphorylation to increase CLOCK/BMAL1 chromatin occupancy and set circadian period length, with depletion shortening and overexpression lengthening the period (PMID:34301769). PPP4R2 also functionally cooperates with SMN to support motor-neuronal differentiation and to protect against DNA damage-induced apoptosis (PMID:22559936).

1. 2000 High

   Established that PPP4R2 is a bona fide regulatory subunit of PPP4c and that the resulting complexes are activity-regulated, defining PPP4R2 as a control point for phosphatase activity rather than a passive scaffold.

   Evidence Co-sedimentation and recombinant co-purification of PPP4c with PPP4R2, plus native complex purification and activity measurements from skeletal muscle and testis

   PMID:10769191

   Open questions at the time

   • Mechanism by which basic proteins activate the inactive complex is not defined
   • No substrate identified in this study
   • Structural basis of the PPP4c–PPP4R2 interaction not resolved
2. 2000 Medium

   Provided the first hint of where PPP4R2 acts in the cell by localizing it to centrosomes, implying it targets PPP4c to microtubule organizing centres.

   Evidence Immunofluorescence microscopy localization in mammalian cells

   PMID:10769191

   Open questions at the time

   • Functional consequence of centrosomal localization not directly tested
   • No centrosomal substrate identified
   • Single-study localization without orthogonal validation
3. 2012 Medium

   Connected PPP4R2 to a defined cellular program by showing it is required for motor-neuronal differentiation and survival, and that it acts in cooperation with SMN.

   Evidence RNAi loss-of-function with SMN-overexpression rescue and apoptosis assays in NSC-34 cells

   PMID:22559936

   Open questions at the time

   • Molecular basis of PPP4R2–SMN cooperation not defined
   • No phosphatase substrate linked to the neuronal phenotype
   • In vivo relevance to neurodegeneration untested
4. 2017 Medium

   Defined a direct biochemical role in genome maintenance by identifying the DNA damage response phospho-proteins whose dephosphorylation depends on PPP4R2.

   Evidence RNAi knockdown and re-expression in hematopoietic/leukemic cells with phospho-protein western blotting of KAP1, γH2AX, p53, and RPA2

   PMID:29221109

   Open questions at the time

   • Direct vs indirect dephosphorylation of each substrate not distinguished
   • Recruitment of PPP4R2 to damage sites not characterized
   • Single-lab evidence
5. 2020 Medium

   Extended PPP4R2 function into signal transduction by establishing a Sufu binding and dephosphorylation event that controls Hedgehog pathway output.

   Evidence Co-IP/proteomic identification of Sufu interaction, subcellular fractionation, dephosphorylation and Gli1 reporter assays in medulloblastoma cells

   PMID:32826873

   Open questions at the time

   • Why Shh promotes the interaction specifically in the nucleus is unresolved
   • Direct catalytic action of PPP4c on Sufu vs scaffolding not separated
   • Single-lab evidence without reciprocal validation
6. 2021 High

   Placed PPP4R2 in the core circadian clock by showing the PPP4–PPP4R2 complex tunes BMAL1 phosphorylation, CLOCK/BMAL1 chromatin occupancy, and period length.

   Evidence Systematic RNAi screen, genetic depletion/overexpression in human cells and Drosophila, BMAL1 binding and phosphorylation assays, chromatin occupancy and period-length measurements

   PMID:34301769

   Open questions at the time

   • Specific BMAL1 phospho-sites controlled are not mapped
   • How elevated chromatin occupancy yields decreased transcriptional activity is mechanistically unexplained
   • Interplay with other clock kinases/phosphatases not resolved

• Whether PPP4R2's diverse substrate-targeting roles (DNA repair, Hedgehog, circadian) are governed by a shared recruitment logic or by context-specific cofactors remains unresolved.
• No structural model of substrate selection by PPP4R2
• Mechanism distinguishing nuclear vs centrosomal pools unknown
• No unifying determinant of substrate specificity identified