PPP1CB

PPP1CB encodes PP1cβ, a serine/threonine protein phosphatase catalytic subunit whose activity is directed toward distinct substrates by regulatory partners across smooth muscle, cell cycle, and signaling contexts (PMID:30185619, PMID:8312365). In smooth muscle, PP1cβ is the functionally dominant PP1 isoform (>90% of total PP1c) and is essential for contractile force, dephosphorylating myosin regulatory light chain in both MYPT1-bound and MYPT1-unbound forms (PMID:30185619). The choice of regulatory subunit gates this activity: TIMAP competes with MYPT1 for PP1cβ, displacing it and occluding the catalytic active site to inhibit myosin phosphatase activity and raise MLC2 phosphorylation in endothelial cells (PMID:31315927), while Chk1-mediated phosphorylation of MYPT1 at Ser20 is required for MYPT1–PP1cβ assembly and the subsequent dephosphorylation and inactivation of Plk1 during mitotic damage (PMID:29262732). In the RAS/MAPK pathway, PP1cβ resides in a RAF1-containing complex bound by LZTR1 and contributes to dephosphorylation of RAF1 at Ser259 (PMID:30368668). PP1cβ catalytic activity also drives 3T3-L1 adipocyte differentiation, linking p38 activation to C/EBPδ induction, and is inhibited by chebulinic acid (PMID:26449462, PMID:35055051). Additional substrate-directed roles include dephosphorylation of STAT3 within an NSD3/PPP1CB/p-STAT3 trimeric complex that suppresses HK2-driven glycolysis (PMID:39119928), and its abundance is controlled by OIP5-recruited TRIP12-mediated ubiquitin-dependent degradation, which derepresses NF-κB signaling (PMID:39155295). De novo missense mutations in the PPP1CB catalytic domain cause a RASopathy resembling Noonan syndrome with loose anagen hair (PMID:27264673).

1. 1994 Medium

   Established the molecular identity of PPP1CB as a PP1 catalytic subunit, defining the gene, its chromosomal locus, and tissue expression before any functional role was assigned.

   Evidence cDNA cloning from a human library with Northern blotting, somatic cell hybrid analysis, and FISH mapping

   PMID:8312365

   Open questions at the time

   • No substrate or regulatory subunit identified at this stage
   • No structural or catalytic mechanism defined
2. 2013 Medium

   Linked PPP1CB phosphatase activity to growth control, showing that a truncated chimeric form has diminished activity and that PPP1CB silencing promotes proliferation.

   Evidence Transcriptome sequencing of a YPEL5/PPP1CB trans-splicing chimera, expression with phosphatase assay, and siRNA silencing in CLL-derived lines

   PMID:23382248

   Open questions at the time

   • Substrates mediating the proliferative effect not identified
   • Physiological relevance of the chimera unclear
3. 2015 Medium

   Placed PPP1CB in early adipogenesis as a node linking p38 activation to C/EBPδ induction and the downstream differentiation program.

   Evidence siRNA knockdown in 3T3-L1 cells with adipocyte marker expression, clonal expansion, and p38 pathway analysis

   PMID:26449462

   Open questions at the time

   • Direct phosphatase substrate in the p38–C/EBPδ axis not defined
   • Regulatory subunit directing this activity unknown
4. 2016 Low

   Connected PPP1CB to human disease, identifying de novo catalytic-domain mutations as a cause of a Noonan-syndrome-like RASopathy.

   Evidence Whole-exome sequencing of patients with conservation analysis and in silico impact prediction

   PMID:27264673

   Open questions at the time

   • No direct biochemical assay on mutant proteins in this study
   • Gain- vs loss-of-function effect on catalytic activity not measured
   • RAS/MAPK perturbation invoked but not demonstrated for these mutants
5. 2018 Medium

   Defined how regulatory subunits direct PP1cβ to specific substrates in two contexts: a RAF1 complex dephosphorylating Ser259, and a Chk1-phosphorylated MYPT1 complex inactivating Plk1.

   Evidence Endogenous Co-IP with siRNA and phospho-RAF1 readout (LZTR1); Co-IP, Ser20 mutagenesis, and Plk1 dephosphorylation assays (Chk1–MYPT1)

   PMID:29262732 PMID:30368668

   Open questions at the time

   • Direct catalysis of RAF1-Ser259 by PP1cβ not reconstituted in vitro
   • Whether LZTR1 modulates the phosphatase directly is unresolved
6. 2018 High

   Established PP1cβ as the functionally dominant PP1 isoform for smooth muscle contraction via RLC dephosphorylation.

   Evidence Isoform-specific immunoblotting, smooth-muscle conditional knockout mice, permeabilization, and ex vivo contractile force measurement

   PMID:30185619

   Open questions at the time

   • Relative contribution of MYPT1-bound vs soluble forms not quantified
   • Other smooth-muscle substrates beyond RLC not enumerated
7. 2019 High

   Revealed regulatory-subunit competition as a control mechanism, with TIMAP displacing MYPT1 and blocking the PP1cβ active site to inhibit myosin phosphatase.

   Evidence Endogenous Co-IP, GST-pulldown with recombinant proteins, microcystin-LR active-site binding assay, and TIMAP-deficient mouse lysates with MLC2 readout

   PMID:31315927

   Open questions at the time

   • Structural basis of active-site occlusion not solved
   • Signals controlling TIMAP vs MYPT1 occupancy in vivo unclear
8. 2019 Medium

   Identified a host–pathogen interaction between PP1cβ and CSFV glycoprotein E2, though PP1cβ knockdown did not alter viral replication.

   Evidence Yeast two-hybrid, Co-IP and PLA in infected cells, and pharmacological PP1 modulation with replication readout

   PMID:30934875

   Open questions at the time

   • Functional consequence of the E2 interaction undefined
   • Discrepancy between pharmacological and knockdown effects unresolved
9. 2024 Medium

   Extended PP1cβ substrate range to metabolic and survival signaling: dephosphorylating STAT3 to suppress glycolysis, and being subject to TRIP12-mediated degradation that activates NF-κB.

   Evidence Co-IP of NSD3/PPP1CB/p-STAT3 with glycolysis readouts; Co-IP of OIP5-TRIP12-PPP1CB with degradation and pathway assays and an in vivo gene circuit

   PMID:39119928 PMID:39155295

   Open questions at the time

   • Direct dephosphorylation of STAT3 by PP1cβ not reconstituted
   • Ubiquitination sites on PPP1CB not mapped

• How disease-associated catalytic-domain mutations alter PP1cβ activity toward specific substrates, and the structural basis for regulatory-subunit-directed substrate selection, remain unresolved.
• No biochemical characterization of RASopathy mutant phosphatase activity
• No structural model of the substrate-directing holoenzymes
• Substrate specificity rules across the many regulatory subunits undefined