PPP2R5B

PPP2R5B (B56beta) is a substrate-directing regulatory subunit of the PP2A holoenzyme that targets the phosphatase to specific substrates to restrain AKT and oncogenic kinase signaling (PMID:17297438, PMID:27521959). It bridges the PP2A structural and catalytic subunits to substrates and binding partners, as shown when the intact PP2A trimer is recruited to the membrane protein CALEB/NGC specifically through B56beta, where it inhibits CALEB/NGC-induced Akt phosphorylation and Akt-dependent dendritic branching (PMID:18385213). As a negative regulator of AKT signaling, B56beta dephosphorylates and inactivates Akt in adipocytes, where its overexpression lowers p-Akt and glucose uptake while its knockdown enhances insulin sensitivity (PMID:27521959). Beyond AKT, B56beta selectively associates with Pim-1 kinase and promotes Pim-1 dephosphorylation, ubiquitination, and degradation, identifying a distinct substrate destabilized by this subunit (PMID:17297438). De novo heterozygous mutations clustering in a nine-amino-acid substrate-binding region of PPP2R5B cause human overgrowth with macrocephaly and intellectual disability, consistent with impaired PP2A-dependent dephosphorylation of PI3K/AKT pathway components (PMID:25972378).

1. 2007 Medium

   Established that B56beta is the specific PP2A regulatory subunit controlling a defined kinase substrate, answering whether PP2A targeting of Pim-1 is subunit-selective.

   Evidence Reciprocal co-immunoprecipitation against all B56 family members plus shRNA knockdown with protein half-life and ubiquitination readouts

   PMID:17297438

   Open questions at the time

   • No direct in vitro phosphatase assay on Pim-1 with reconstituted holoenzyme
   • Single lab; the Pim-1 phosphosite dephosphorylated by PP2A/B56beta not mapped
2. 2008 Medium

   Showed how B56beta directs the whole PP2A trimer to a membrane partner and a localized signaling output, answering whether the subunit acts as a substrate/partner-bridging adaptor.

   Evidence Yeast two-hybrid, Co-IP, affinity chromatography/mass spectrometry, and dendritic morphology readouts in neurons

   PMID:18385213

   Open questions at the time

   • Direct Akt dephosphorylation by the recruited trimer not assayed enzymatically
   • In vivo relevance of CALEB/NGC bridging not tested
3. 2015 Low

   Extended B56beta's role as an AKT phosphatase to hepatocyte stress responses, addressing whether it controls AKT-dependent cytoprotective output.

   Evidence Stable shRNA knockdown and retroviral overexpression with p-AKT and metallothionein Western blots and cytotoxicity assay

   PMID:26081707

   Open questions at the time

   • No direct phosphatase activity assay; AKT dephosphorylation inferred from p-AKT levels
   • Single lab, single primary method
   • Mechanism linking AKT to metallothionein not resolved
4. 2015 Low

   Linked PPP2R5B to a Mendelian overgrowth syndrome and localized causal mutations to a substrate-binding region, addressing the physiological consequence of perturbed substrate targeting.

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

   PMID:25972378

   Open questions at the time

   • Mechanistic effect of mutants on substrate dephosphorylation inferred from structural modeling, not functionally validated
   • Specific affected substrates not experimentally identified
5. 2016 Medium

   Established B56beta as a direct AKT-inactivating subunit in metabolic signaling and as the PP2A regulatory subunit required for erythroid differentiation.

   Evidence Reciprocal overexpression/knockdown in adipocytes (p-Akt, glucose uptake) and shRNA knockdown with hemin-induced differentiation in K562 cells

   PMID:27521959 PMID:27544028

   Open questions at the time

   • Direct phosphatase assay on Akt not performed
   • Substrate driving erythroid differentiation not identified
   • In vivo metabolic phenotype not established

• Whether the disease-causing substrate-binding mutations functionally alter PP2A/B56beta dephosphorylation of specific AKT/PI3K substrates remains unresolved.
• No functional assay of mutant B56beta on substrate dephosphorylation
• No reconstituted in vitro phosphatase characterization across substrates
• Substrate repertoire beyond Akt and Pim-1 uncharacterized