PPDPF

PPDPF (EXDPF/C20orf149) is a small multifunctional adaptor and enzyme that couples developmental, metabolic, and oncogenic signaling, and was first defined as a Ptf1a-regulated, retinoic-acid-dependent driver of exocrine pancreas cell proliferation in zebrafish, where its loss triggers cell-cycle arrest through induction of p21, p27, and cyclin G1 (PMID:19067490). Its best-characterized biochemical activities are direct: PPDPF binds GTP and transfers it to SOS1 to enhance SOS1 GEF activity and KRAS activation, with GTP-binding and SOS1-interaction mutants abolishing this function (PMID:36453576), and it acts as a thiol-disulfide oxidoreductase that supports NMNAT activity to maintain NAD+ and mitochondrial homeostasis (PMID:40106551). A recurring mechanistic theme is that PPDPF modulates signaling by disrupting or scaffolding specific protein-protein interactions: it interferes with the CK1α–Axin interaction to dismantle the β-catenin destruction complex and activate Wnt/β-catenin (PMID:37477088), blocks STAT3–PTPN1 binding to hyperactivate STAT3 (PMID:35906391), disrupts c-Myc–GSK3β association to stabilize c-Myc and drive CD24-mediated immune escape (PMID:40774693), and prevents assembly of the metabolic carboxylase pairs MCCA–MCCB and PCCA–PCCB to block leucine and methionine catabolism and feed mTORC1 and SAM pools (PMID:40025229, PMID:39694223). PPDPF also scaffolds ubiquitination events, recruiting TRIM21 to catalyze K63-linked ubiquitination of RIPK1 at K140 to activate NF-κB (PMID:37027301) and blocking MDM2-mediated ubiquitination of BABAM2 to confer radioresistance (PMID:34975328). Its own abundance is controlled post-translationally, being stabilized by IL6/JAK2-mediated phosphorylation at Tyr16/Tyr17 (PMID:37477088) and by the opposing actions of the E3 ligase ARIH2 and deubiquitinase OTUD4 under amino acid starvation (PMID:40025229). Notably, the developmental role is not conserved: PPDPF knockout in human embryonic stem cells has only a modest effect on pancreatic progenitor development and does not alter lineage specification in vivo (PMID:40193385).

1. 2008 High

   Established the first biological role and upstream regulation of PPDPF, placing its zebrafish ortholog as a retinoic-acid/Ptf1a-controlled effector of exocrine pancreas proliferation.

   Evidence Promoter binding-site analysis, luciferase reporter, morpholino knockdown, mRNA rescue, and cell-cycle gene profiling in zebrafish

   PMID:19067490

   Open questions at the time

   • Molecular activity of the protein itself was not defined
   • Whether the proliferation role generalizes beyond exocrine pancreas was untested
2. 2022 High

   Defined PPDPF as a direct GTP-binding factor that potentiates SOS1 GEF activity, providing a concrete biochemical mechanism for its RAS-pathway and tumor-promoting effects.

   Evidence GTP-binding and GEF activity assays, active-site mutagenesis, co-IP, and KRASG12D mouse models in pancreatic ductal adenocarcinoma

   PMID:36453576

   Open questions at the time

   • Structural basis of GTP transfer to SOS1 not resolved
   • Whether this activity operates outside pancreatic cancer is unclear
3. 2022 Medium

   Showed PPDPF acts as an interaction modulator and ubiquitination regulator in lung cancer, interfering with STAT3-PTPN1 and protecting BABAM2 from MDM2.

   Evidence Co-IP, ubiquitination assays, knockout/overexpression, and lung cancer mouse models

   PMID:34975328 PMID:35906391

   Open questions at the time

   • Direct vs indirect nature of the disruption not fully resolved
   • Single-lab mechanistic placement
4. 2023 High

   Generalized the protein-interaction-disruption and scaffolding model across cancers, linking PPDPF to Wnt/β-catenin activation and to TRIM21-dependent K63-ubiquitination of RIPK1 for NF-κB signaling, and identified JAK2 phosphorylation as a stabilizing input.

   Evidence Co-IP, site-specific ubiquitination assays (RIPK1 K140), phosphorylation-site mapping, organoid and tissue-specific KO mouse models

   PMID:37027301 PMID:37477088

   Open questions at the time

   • How one small protein selects among many distinct partners is unexplained
   • Stoichiometry and structural mode of complex disruption unknown
5. 2024 Medium

   Extended the interaction-disruption mechanism into amino acid metabolism, with PPDPF blocking PCCA-PCCB assembly to elevate methionine and SAM.

   Evidence Mass spectrometry, co-IP, and metabolite measurements in esophageal squamous cell carcinoma

   PMID:39694223

   Open questions at the time

   • Direct binding interface with PCCA not mapped
   • Single-lab finding
6. 2025 High

   Defined an intrinsic enzymatic activity (thiol-disulfide oxidoreductase) supporting NMNAT and NAD+ homeostasis, and added further metabolic and immune-evasion mechanisms plus a writer/eraser pair controlling PPDPF stability.

   Evidence Enzymatic and NMNAT activity assays, SILAC and metabolic flux analysis, GST-pulldown, ubiquitination assays, and multiple KO mouse models

   PMID:40025229 PMID:40106551 PMID:40774693

   Open questions at the time

   • How thiol-disulfide oxidoreductase activity reconciles with the GTP-binding and adaptor activities is unknown
   • Whether a single domain mediates all these functions is unresolved
7. 2025 Medium

   Revealed that the developmental role is not conserved in humans, with PPDPF dispensable for human pancreatic lineage specification, sharpening interpretation of the original zebrafish phenotype.

   Evidence CRISPR/Cas9 knockout in hESCs with pancreatic differentiation and orthotopic transplantation

   PMID:40193385

   Open questions at the time

   • Functional compensation in human cells not ruled out
   • Does not address PPDPF roles in adult/disease tissues

• How a single small protein integrates a thiol-disulfide oxidoreductase activity, GTP binding, and a broad set of protein-interaction-disruption events into a coherent structural and regulatory logic remains unresolved.
• No structure of PPDPF or its complexes reported in the corpus
• Determinants of partner selectivity unknown
• Relationship between enzymatic and adaptor functions unexplained