PYCR2

PYCR2 is a mitochondrial NAD(P)H-dependent reductase that catalyzes the terminal step of proline biosynthesis, converting L-Δ1-pyrroline-5-carboxylate (L-P5C) to L-proline through an ordered sequential mechanism in which L-P5C binds before NAD(P)H and NAD(P)+ is released before L-proline (PMID:32330411, PMID:33771508, PMID:33734376). The enzyme is active as an oligomer—a crystal structure of the apo-enzyme places the disease-associated p.Gly249Val substitution at the dimer interface where it lowers catalytic activity, and patient missense mutations impair multimeric assembly (PMID:27130255, PMID:32330411). PYCR2 functions largely redundantly with PYCR1 as a P5C reductase, both localizing to mitochondria and both complementing loss of the yeast P5C-to-proline reductase, with double-mutant mice more severely affected than either single mutant (PMID:33734376). Loss of PYCR2 causes neurodegeneration: it upregulates SHMT2 and elevates cerebral glycine, depletes PYCR1 in neural lineages, and impairs neuronal morphology, while SHMT2 knockdown partially rescues the axonal and neurite defects (PMID:32330411). Biallelic PYCR2 variants such as p.Arg119Cys and p.Arg251Cys—which retain mitochondrial targeting but are catalytically impaired and, for R251C, also misfolded—underlie hypomyelinating leukodystrophy, with mutant proteins driving abnormal mitochondrial fusion/fission balance and blocking oligodendroglial differentiation (PMID:25865492, PMID:33771508, PMID:36548190). PYCR2 abundance is controlled post-translationally by E4B-mediated K48-linked polyubiquitination targeting it for proteasomal degradation and by PFDN2, which binds PYCR2 and protects it from degradation (PMID:35669517, PMID:41656306). Functionally, PYCR2 collaborates with RRM2B to confer resistance to oxidative stress, and its loss lowers mitochondrial membrane potential and sensitizes cells to apoptosis (PMID:26733354, PMID:25865492).

1. 2015 Medium

   Establishing that PYCR2 missense variants disrupt protein stability and cellular fitness rather than mislocalization linked the gene to disease through a loss-of-function mechanism.

   Evidence CRISPR-Cas9 knockout cell line with mitochondrial membrane potential and apoptosis assays, plus mutant cDNA localization imaging

   PMID:25865492

   Open questions at the time

   • Did not resolve whether instability reflects catalytic or folding defects
   • No structural basis for variant destabilization
2. 2016 Low

   Identifying impaired multimerization in patient mutants connected oligomeric assembly to PYCR2 function, framing assembly as a disease-relevant property.

   Evidence Biochemical multimerization assay of patient-derived mutations

   PMID:27130255

   Open questions at the time

   • Limited methodological detail on the multimerization assay
   • Oligomeric state not linked to catalytic output here
   • No structural model
3. 2016 Medium

   Placing PYCR1/PYCR2 within RRM2B complexes and showing their joint requirement for RRM2B anti-oxidation activity revealed a role beyond proline synthesis in oxidative stress defense.

   Evidence Flag-RRM2B affinity purification with mass spectrometry plus shRNA double-knockdown under oxidative stress

   PMID:26733354

   Open questions at the time

   • Direct physical contact between PYCR2 and RRM2B not demonstrated
   • Mechanism of anti-oxidation contribution unresolved
4. 2020 High

   A crystal structure plus a knockout mouse defined the catalytic architecture and the in vivo neurodegenerative cascade, showing loss of PYCR2 elevates glycine via SHMT2 and depletes PYCR1.

   Evidence Apo-enzyme crystal structure, Pycr2 knockout mouse phenotyping, brain neurotransmitter quantification, and SHMT2-knockdown rescue in cultured neurons

   PMID:32330411

   Open questions at the time

   • Mechanism linking SHMT2 upregulation to PYCR2 loss not defined
   • How PYCR1 depletion is triggered unclear
   • No holo/substrate-bound structure
5. 2021 High

   Steady-state kinetics established the ordered catalytic mechanism and quantified how disease variants impair catalysis versus folding, distinguishing two failure modes.

   Evidence Steady-state kinetic measurements, thermostability and circular dichroism on purified recombinant wild-type and R119C/R251C proteins

   PMID:33771508

   Open questions at the time

   • Substrate-bound structural states not captured
   • Physiological cofactor preference (NADPH vs NADH) in vivo unresolved
6. 2021 Medium

   Yeast complementation and double-mutant mice confirmed PYCR2 as a bona fide P5C reductase that is largely functionally redundant with PYCR1.

   Evidence Yeast Pro3 complementation, mitochondrial localization, Pycr1;Pycr2 double-mutant mouse genetics and tissue proline measurements

   PMID:33734376

   Open questions at the time

   • Non-redundant, tissue-specific roles of the two enzymes not delineated
   • Quantitative contribution of each isozyme to proline pools unresolved
7. 2021 Low

   A viral interactor was found to downregulate PYCR2 and trigger autophagy, hinting at PYCR2 levels influencing autophagic signaling.

   Evidence Co-IP/MS of ASFV E199L with PYCR2, western blot and autophagy assays in Vero and HEK-293T cells

   PMID:33830435

   Open questions at the time

   • Single Co-IP/MS without reciprocal validation
   • Direct causal link between PYCR2 loss and autophagy not isolated
8. 2022 Medium

   Identifying E4B-mediated K48 polyubiquitination defined a proteasomal route controlling PYCR2 abundance.

   Evidence In vitro ubiquitination assay, co-IP in HEK293 cells, K48 linkage typing, and E4B variable-region domain mapping

   PMID:35669517

   Open questions at the time

   • Physiological contexts regulating E4B–PYCR2 not defined
   • Deubiquitinase counterpart unknown
9. 2022 Medium

   Showing that R119C/R251C drive abnormal mitochondrial fusion/fission and block oligodendroglial differentiation connected variant biochemistry to the cellular pathology of hypomyelinating leukodystrophy.

   Evidence Transfection of wild-type/mutant PYCR2 into FBD-102b oligodendroglial cells with mitochondrial morphology, fusion/fission and differentiation-marker readouts

   PMID:36548190

   Open questions at the time

   • Molecular link between reductase deficiency and fission/fusion machinery unclear
   • Overexpression system may not reflect endogenous stoichiometry
10. 2023 Low

    Transcriptional and m6A-associated regulators (c-Myc, ALKBH5) were placed upstream of PYCR2 in cancer, embedding it in proline-driven oncogenic circuits.

    Evidence Luciferase promoter reporter and western blot (c-Myc); knockdown, proline rescue and AMPK/mTOR analysis (ALKBH5 feedback loop) in breast and glioblastoma cells

    PMID:37325047 PMID:38101533

    Open questions at the time

    • Single-method transcriptional evidence per study
    • Direct vs indirect regulation not fully separated
    • Generality across tumor types untested
11. 2025 Medium

    PFDN2 was shown to bind and stabilize PYCR2 against proteasomal turnover, with PYCR2 activity required for Wnt/β-catenin signaling, defining a stabilizing partner opposing E4B-driven degradation.

    Evidence Reciprocal co-IP, immunofluorescence colocalization, cycloheximide chase, MG132 rescue, and TOP/FOPflash reporter with PYCR2 re-expression in colorectal cancer cells

    PMID:41656306

    Open questions at the time

    • How PFDN2 protects PYCR2 mechanistically (shielding vs competition with E4B) unknown
    • Link between proline metabolism and Wnt activation not mechanistically resolved

• How PYCR2's enzymatic proline output is mechanistically coupled to its downstream non-metabolic phenotypes—mitochondrial dynamics, oligodendrocyte differentiation, oxidative-stress resistance and Wnt/AMPK-mTOR signaling—remains unresolved.
• No substrate-bound structure to explain variant-specific defects
• Causal chain from proline depletion to mitochondrial morphology changes undefined
• In vivo relevance of cancer-associated regulatory loops untested