PPP2R5B

PPP2R5B (B56β) is a regulatory B subunit of the protein phosphatase 2A (PP2A) holoenzyme that directs substrate specificity toward growth- and survival-related signaling pathways. It recruits PP2A to dephosphorylate and inactivate AKT, thereby suppressing insulin-stimulated glucose uptake in adipocytes, regulating metallothionein expression and cadmium sensitivity, and inhibiting dendritic branching via CALEB/NGC in neurons (PMID:27521959, PMID:26081707, PMID:18385213). B56β also selectively binds Pim-1 kinase, promoting its dephosphorylation, ubiquitination, and proteasomal degradation (PMID:17297438). De novo missense mutations clustering in the substrate-binding region of B56β cause a human overgrowth syndrome with macrocephaly and intellectual disability, consistent with impaired PP2A-mediated dephosphorylation of growth-regulatory substrates (PMID:25972378).

1. 1996 Medium

   Identification of PPP2R5B as a brain-enriched PP2A regulatory subunit mapped to chromosome 11q12 established the gene's identity and suggested tissue-specific functions.

   Evidence Northern blotting and FISH chromosomal mapping

   PMID:8812429

   Open questions at the time

   • No substrates or signaling pathways identified
   • No functional assay performed beyond expression profiling
2. 2007 High

   Demonstrating that B56β selectively binds Pim-1 kinase and promotes its dephosphorylation-dependent ubiquitination and degradation revealed the first specific substrate of the B56β-PP2A holoenzyme and linked it to oncogenic kinase regulation.

   Evidence Reciprocal co-IP, shRNA knockdown with pulse-chase half-life and ubiquitination assays in cell lines

   PMID:17297438

   Open questions at the time

   • Specific phospho-site on Pim-1 targeted by B56β-PP2A not identified
   • In vivo relevance to tumorigenesis not tested
3. 2008 High

   Discovery that B56β interacts with CALEB/NGC to recruit the full PP2A holoenzyme and suppress Akt phosphorylation in dendrites established a neuronal-specific substrate-targeting mechanism controlling dendritic branching.

   Evidence Yeast two-hybrid, affinity chromatography/mass spectrometry, co-IP, and functional overexpression in primary neurons

   PMID:18385213

   Open questions at the time

   • Direct dephosphorylation of AKT by CALEB/NGC-recruited PP2A not reconstituted in vitro
   • Whether this mechanism operates in vivo in brain development not demonstrated
4. 2015 Medium

   Showing that B56β-PP2A dephosphorylates AKT to upregulate metallothionein expression and modulate cadmium cytotoxicity broadened the functional repertoire of B56β-directed AKT dephosphorylation to stress-response pathways.

   Evidence Retroviral overexpression and shRNA knockdown with pharmacological epistasis (wortmannin) in cell lines

   PMID:26081707

   Open questions at the time

   • Direct in vitro dephosphorylation of AKT by purified B56β-PP2A not reconstituted
   • Transcriptional mechanism linking AKT inactivation to MT gene induction not defined
5. 2015 Medium

   Identification of de novo PPP2R5B mutations in patients with overgrowth, macrocephaly, and intellectual disability linked impaired PP2A substrate engagement to a human developmental disorder, connecting the AKT-regulatory function to growth control.

   Evidence Trio-based exome sequencing with mutation mapping onto PP2A holoenzyme crystal structure

   PMID:25972378

   Open questions at the time

   • Functional impact of mutations on phosphatase activity not biochemically validated
   • Specific substrates affected by disease mutations not determined experimentally
   • Animal model of disease mutations not generated
6. 2016 Medium

   Demonstrating that B56β mediates AKT dephosphorylation in adipocytes under chronic hyperinsulinemia established a metabolic role for B56β-PP2A in insulin resistance.

   Evidence Overexpression and shRNA knockdown in adipocytes with glucose uptake assay, corroborated in high-fat diet mouse model

   PMID:27521959

   Open questions at the time

   • Direct reconstitution of AKT dephosphorylation by B56β-PP2A not performed
   • Adipocyte-specific knockout model not generated
7. 2016 Medium

   Finding that B56β is specifically induced during erythroid differentiation and is required for hemin-induced erythroid maturation suggested a lineage-specific developmental function beyond AKT regulation.

   Evidence Expression analysis and siRNA knockdown in K562 cells treated with hemin

   PMID:27544028

   Open questions at the time

   • Downstream PP2A substrates mediating erythroid differentiation not identified
   • Not validated in primary erythroid progenitors or in vivo

• Key unresolved questions include the full substrate repertoire of B56β-PP2A, the biochemical mechanism by which disease-associated mutations impair phosphatase function, and whether B56β-PP2A plays non-redundant roles versus other B56 family members in vivo.
• No crystal structure of disease-mutant B56β-PP2A holoenzyme available
• No genome-wide substrate identification (e.g., phosphoproteomics after acute B56β depletion) performed
• Genetic knockout or conditional deletion models in mammals not reported