CP12 is a small (~8.5 kDa), nuclear-encoded, conditionally disordered chloroplast protein that functions as a redox-sensitive linker/scaffold mediating the assembly of a supramolecular PRK/GAPDH/CP12 complex. In the dark, oxidized CP12 (with two intramolecular disulfide bonds) sequentially binds GAPDH then PRK, forming a ternary complex that inactivates both Calvin cycle enzymes. In the light, thioredoxin-mediated reduction of CP12 disulfide bonds causes the complex to dissociate, releasing active enzymes. CP12 also binds Cu2+ and Ni2+ ions in vitro; Cu2+ catalyzes re-oxidation of CP12 thiols, potentially linking metal homeostasis to Calvin cycle regulation.
Definition: A protein complex consisting of phosphoribulokinase (PRK), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and the linker protein CP12. The complex forms in the dark under oxidizing conditions and sequesters PRK and GAPDH in an inactive state, thereby negatively regulating the Calvin-Benson-Bassham cycle. Stoichiometry: 2 PRK dimers + 2 GAPDH tetramers + CP12.
Justification: No GO complex term currently exists for this well-characterized supramolecular complex, which is conserved across photosynthetic organisms from cyanobacteria to higher plants. The current annotation uses the generic GO:0032991 (protein-containing complex).
Parent term: protein-containing complex
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0009507
chloroplast
|
IEA
GO_REF:0000044 |
ACCEPT |
Summary: Chloroplast localization is well-supported by multiple lines of evidence. CP12 has a 27-residue chloroplast transit peptide confirmed by direct protein sequencing (PMID:12846565), and IDA evidence from CAFA also supports this localization. This IEA annotation from UniProt subcellular location mapping is consistent with all other evidence.
Reason: Although this is an IEA annotation, it is fully consistent with the IDA evidence (PMID:12846565) showing chloroplast localization via transit peptide identification and direct protein sequencing. The UniProt record confirms chloroplast transit peptide residues 1-27 with experimental evidence.
Supporting Evidence:
PMID:12846565
CP12 is an 8.5-kDa nuclear-encoded chloroplast protein
PMID:16259044
The small chloroplast protein CP12
|
|
GO:0005515
protein binding
|
IPI
PMID:24863370 Conformational modulation and hydrodynamic radii of CP12 pro... |
MODIFY |
Summary: This annotation records CP12 interaction with PRK (P19824) based on fluorescence correlation spectroscopy (FCS) experiments (PMID:24863370). While the interaction is real and well-documented, GO:0005515 (protein binding) is too generic per curation guidelines. CP12 binds the enzyme PRK as part of its core regulatory function; GO:0019899 (enzyme binding) is more informative and already annotated.
Reason: Per curation guidelines, GO:0005515 (protein binding) is uninformative. CP12's interaction with PRK is its core molecular function -- it acts as a scaffold/linker that binds PRK to form the regulatory PRK/GAPDH/CP12 complex. GO:0019899 (enzyme binding) is already annotated for the GAPDH interaction and is equally appropriate for the PRK interaction, as PRK is an enzyme (phosphoribulokinase, EC 2.7.1.19).
Proposed replacements:
enzyme binding
Supporting Evidence:
PMID:24863370
we characterize the diffusion dynamics and hydrodynamic radii of CP12 from Chlamydomonas reinhardtii upon binding to GAPDH and PRK using fluorescence correlation spectroscopy experiments
PMID:12846565
oxidized, but not reduced, CP12 acts as a linker in the assembly of the complex
|
|
GO:0005515
protein binding
|
IPI
PMID:24863370 Conformational modulation and hydrodynamic radii of CP12 pro... |
MODIFY |
Summary: This annotation records CP12 interaction with GAPDH (P50362) based on FCS experiments (PMID:24863370). Same issue as the PRK protein binding annotation -- GO:0005515 is too generic. The GAPDH interaction is already covered by the enzyme binding annotation from PMID:12846565.
Reason: GO:0005515 is uninformative per curation guidelines. CP12's interaction with GAPDH is its core function as a regulatory linker. GO:0019899 (enzyme binding) is more appropriate, as GAPDH is an enzyme (glyceraldehyde-3-phosphate dehydrogenase, EC 1.2.1.13). This interaction is already annotated as enzyme binding from PMID:12846565.
Proposed replacements:
enzyme binding
Supporting Evidence:
PMID:24863370
we characterize the diffusion dynamics and hydrodynamic radii of CP12 from Chlamydomonas reinhardtii upon binding to GAPDH and PRK using fluorescence correlation spectroscopy experiments
|
|
GO:0005515
protein binding
|
IPI
PMID:12846565 The small protein CP12: a protein linker for supramolecular ... |
MODIFY |
Summary: This annotation records CP12 interaction with both PRK (P19824) and GAPDH (P50362) based on reconstitution assays and SPR binding studies (PMID:12846565). As with the other protein binding annotations, GO:0005515 is too generic. The interactions are real but better captured by GO:0019899 (enzyme binding).
Reason: GO:0005515 is uninformative per curation guidelines. CP12 acts as a linker for two Calvin cycle enzymes; enzyme binding (GO:0019899) is the appropriate term. The original paper clearly demonstrates CP12 binding to both PRK and GAPDH through reconstitution assays and surface plasmon resonance.
Proposed replacements:
enzyme binding
Supporting Evidence:
PMID:12846565
oxidized, but not reduced, CP12 acts as a linker in the assembly of the complex, and we propose a model in which CP12 associates with GAPDH, causing its conformation to change. This GAPDH/CP12 complex binds PRK to form a half-complex (one unit).
|
|
GO:0009507
chloroplast
|
IDA
PMID:12846565 The small protein CP12: a protein linker for supramolecular ... |
ACCEPT |
Summary: Direct experimental evidence for chloroplast localization. The PMID:12846565 study determined the N-terminal sequence of the mature protein (residues 28-32), confirming cleavage of the chloroplast transit peptide and thereby demonstrating chloroplast import. UniProt lists this as transit peptide residues 1-27 with evidence from PMID:12846565.
Reason: Strong IDA evidence from direct protein sequencing confirming the transit peptide cleavage site. This is the primary experimental evidence for chloroplast localization and directly demonstrates that CP12 is imported into the chloroplast.
Supporting Evidence:
PMID:12846565
CP12 is an 8.5-kDa nuclear-encoded chloroplast protein
|
|
GO:0019899
enzyme binding
|
IPI
PMID:12846565 The small protein CP12: a protein linker for supramolecular ... |
ACCEPT |
Summary: Enzyme binding is CP12's core molecular function. CP12 acts as a regulatory linker that binds two Calvin cycle enzymes: GAPDH (P50362, glyceraldehyde-3-phosphate dehydrogenase) and PRK (P19824, phosphoribulokinase). The WITH column specifies GAPDH (P50362). This was demonstrated through reconstitution assays and SPR binding studies (PMID:12846565), and independently confirmed by FCS (PMID:24863370).
Reason: This is the most informative molecular function term for CP12's core activity. CP12 is defined by its ability to bind and regulate GAPDH and PRK through redox-dependent complex assembly. Enzyme binding accurately captures this scaffold/linker function. The evidence is strong, based on multiple in vitro reconstitution approaches.
Supporting Evidence:
PMID:12846565
oxidized, but not reduced, CP12 acts as a linker in the assembly of the complex, and we propose a model in which CP12 associates with GAPDH, causing its conformation to change
PMID:24863370
We quantify a hydrodynamic radius of 3.4 ± 0.2 nm for the CP12 protein with an increase up to 5.2 ± 0.3 nm upon complex formation with GAPDH and PRK
|
|
GO:0009507
chloroplast
|
ISS
GO_REF:0000024 |
ACCEPT |
Summary: ISS annotation based on sequence similarity to Arabidopsis thaliana CP12 (O22914). Consistent with the IDA evidence from PMID:12846565 and the IEA annotation. This is redundant with the IDA evidence but not incorrect.
Reason: Fully consistent with the IDA evidence (PMID:12846565). The ISS transfer from A. thaliana CP12 is appropriate given that chloroplast localization is a conserved feature of all CP12 family members, which universally possess chloroplast transit peptides.
Supporting Evidence:
PMID:12846565
CP12 is an 8.5-kDa nuclear-encoded chloroplast protein
|
|
GO:0032991
protein-containing complex
|
ISS
GO_REF:0000024 |
ACCEPT |
Summary: CP12 is part of the PRK/GAPDH/CP12 supramolecular complex. The annotation to the generic GO:0032991 (protein-containing complex) is correct but very broad. The full complex stoichiometry is 2 PRK dimers + 2 GAPDH tetramers + CP12 (PMID:12846565). A more specific GO complex term would be ideal, but no dedicated term for the PRK/GAPDH/CP12 complex currently exists in GO.
Reason: While GO:0032991 is generic, there is no more specific GO term available for the PRK/GAPDH/CP12 complex. The annotation is accurate -- CP12 is indeed a component of a well-characterized protein-containing complex. The ISS transfer from A. thaliana CP12 is appropriate since the PRK/GAPDH/CP12 complex is conserved across photosynthetic organisms. Proposing a new GO term for this complex could be considered.
Supporting Evidence:
PMID:12846565
It forms part of a core complex of two dimers of phosphoribulokinase (PRK), two tetramers of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and CP12
|
|
GO:0080153
negative regulation of reductive pentose-phosphate cycle
|
ISS
GO_REF:0000024 |
ACCEPT |
Summary: This is the core biological process annotation for CP12. CP12 negatively regulates the Calvin-Benson-Bassham (reductive pentose-phosphate) cycle by sequestering PRK and GAPDH into an inactive complex in the dark. The ISS transfer from A. thaliana CP12 (O22914) is well-justified, as this regulatory function is conserved across all characterized CP12 proteins. CRISPR-Cas9 knockout of CP12 in C. reinhardtii (PMID:35269851) provides direct IMP-grade evidence in the target organism — delta-CP12 cells show abolished DTT-dependent regulation of GAPDH activity and 3-fold reduced PRK specific activity, confirming that CP12 is required in vivo for dark regulation of the Calvin cycle. An upgraded experimental evidence annotation (e.g. IMP from PMID:35269851) should be considered by the curation source.
Reason: GO:0080153 precisely captures CP12's core biological process function. CP12 coordinates the dark inactivation of GAPDH and PRK, two key Calvin cycle enzymes, by assembling them into an inactive supramolecular complex. This is the most specific and accurate BP term for CP12's role. The ISS annotation is consistent with strong direct experimental evidence in C. reinhardtii from both reconstitution experiments (PMID:12846565) and CRISPR knockout studies (PMID:35269851), so an IMP upgrade in the source GOA is warranted.
Supporting Evidence:
PMID:12846565
oxidized, but not reduced, CP12 acts as a linker in the assembly of the complex
PMID:16259044
a PRK/GAPDH/CP12 complex that is involved in CO2 assimilation in photosynthetic organisms. The redox state of CP12 regulates its role as a protein linker.
PMID:35269851
The chloroplast protein CP12 is involved in the dark/light regulation of the Calvin-Benson-Bassham cycle, in particular, in the dark inhibition of two enzymes
PMID:35269851
in the ∆CP12-Cr cell extracts, the NADPH-dependent activity of GAPDH displayed a linear curve regardless of DTT
|
|
GO:0005507
copper ion binding
|
IDA
PMID:16259044 Mass spectrometric analysis of the interactions between CP12... |
KEEP AS NON CORE |
Summary: IDA evidence from ESI-MS experiments demonstrating specific Cu2+ binding by CP12 with Kd = 26 +/- 1 uM (PMID:16259044). Importantly, Cu2+ also catalyzes re-formation of CP12 disulfide bonds, potentially linking copper to Calvin cycle regulation. The authors note sequence similarity between CP12 and copper chaperones from A. thaliana. However, the primary biological function of CP12 is as a regulatory scaffold, not as a dedicated copper-binding protein; the in vivo significance of copper binding remains uncertain.
Reason: The copper binding is real (IDA evidence, specific binding with measured Kd), but it represents a secondary/potential moonlighting function rather than CP12's core evolved function. CP12's primary role is as a redox-dependent linker for the PRK/GAPDH complex. The copper-catalyzed thiol oxidation may be physiologically relevant as it could promote CP12's active (oxidized) conformation, but this remains speculative. Retaining as non-core appropriately reflects the evidence.
Supporting Evidence:
PMID:16259044
The oxidized protein bound specifically Cu2+ and Ni2+ (Kd of 26+/-1 microM and 11+/-1 microM, respectively); other cations such as Fe2+ and Zn2+ did not bind
PMID:16259044
Cu2+ catalyzes the re-formation of the disulfide bonds of the reduced CP12, leading to recovery of the fully oxidized CP12 that is then able to bind a Cu2+ ion
|
|
GO:0016151
nickel cation binding
|
IDA
PMID:16259044 Mass spectrometric analysis of the interactions between CP12... |
KEEP AS NON CORE |
Summary: IDA evidence from ESI-MS experiments showing specific Ni2+ binding by CP12 with Kd = 11 +/- 1 uM (PMID:16259044). The binding is more specific than Cu2+ (lower Kd) but the biological relevance is even less clear. Nickel is not known to play a significant role in chloroplast metabolism in green algae. The His74 mutation had no impact on metal binding, suggesting the binding site may involve other residues.
Reason: The nickel binding is experimentally demonstrated (IDA) with good specificity (Kd = 11 uM, no binding of Fe2+ or Zn2+), but there is no evidence for an in vivo role. Unlike copper, nickel does not catalyze CP12 disulfide bond formation, so a regulatory connection to CP12's primary function is not established. This is a secondary characteristic best kept as non-core.
Supporting Evidence:
PMID:16259044
The oxidized protein bound specifically Cu2+ and Ni2+ (Kd of 26+/-1 microM and 11+/-1 microM, respectively); other cations such as Fe2+ and Zn2+ did not bind
PMID:16259044
the high similarity between CP12 and copper chaperones from Arabidopsis thaliana, as judged by hydrophobic cluster analysis, provides additional evidence for the relevance of metal binding for the in vivo situation
|
|
GO:0050821
protein stabilization
|
IMP
PMID:35269851 Reduction in Phosphoribulokinase Amount and Re-Routing Metab... |
NEW |
Summary: Proposed new annotation reflecting a redox-independent moonlighting function of CP12. PMID:35269851 demonstrates that CP12 stabilizes PRK against irreversible inactivation both in vitro (recombinant CP12 prevents irreversible loss of PRK activity in a redox-independent manner; DTT can restore activity only when CP12 is present) and in vivo (ΔCP12 cells show 6.5-fold reduction in PRK protein abundance with no change in mRNA, consistent with loss of post-translational stabilization). Site-directed mutagenesis identifies specific residues (D36, E39, E40, W35, H47) required for this protective function.
Reason: PMID:35269851 provides both in vivo (knockout) and in vitro (recombinant protein + mutagenesis) evidence that CP12 specifically protects PRK from irreversible inactivation in a redox-independent manner. This is mechanistically distinct from CP12's redox-dependent regulatory role in the PRK/GAPDH/CP12 complex and warrants a separate annotation. GO:0050821 (protein stabilization) accurately captures this function as the term covers maintenance of protein structure/integrity and prevention of aggregation/degradation.
Supporting Evidence:
PMID:35269851
Isolated PRK lost irreversibly its activity over-time in vitro, which was prevented in the presence of recombinant CP12 in a redox-independent manner
PMID:35269851
in the presence of CP12, the addition of DTT restored the initial PRK activity, indicating that CP12 prevented this irreversible inactivation
PMID:35269851
a significant difference was observed for PRK amount, with a 6.5-fold decrease of PRK amount in ΔCP12-Cr compared to the WT-Cr strain
|
Q: Is the copper ion binding by CP12 physiologically relevant in vivo, or is it an in vitro artifact? Does copper-catalyzed re-oxidation of CP12 thiols contribute to dark regulation of the Calvin cycle under normal conditions?
Suggested experts: Gontero B, Launay H
Q: Should CP12's role in protecting PRK from irreversible inactivation (redox-independent) be annotated separately from its role in dark/light regulation of the Calvin cycle (redox-dependent)?
Suggested experts: Gontero B
Experiment: Measure Calvin cycle enzyme activity and PRK/GAPDH/CP12 complex formation in C. reinhardtii cells grown under copper-depleted vs. copper-replete conditions, comparing wild-type and ΔCP12 strains. Use immunoprecipitation to assess complex formation kinetics during light-to-dark transitions.
Hypothesis: CP12 copper binding is relevant in vivo for Calvin cycle regulation
Experiment: Perform in vivo crosslinking mass spectrometry (XL-MS) or proximity labeling (BioID/TurboID) with tagged CP12 in C. reinhardtii to identify the full interactome, including reported interactions with aldolase and malate dehydrogenase.
Hypothesis: CP12 has additional interaction partners beyond GAPDH and PRK
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
The symbol CP12 is used across oxygenic phototrophs for “Calvin cycle protein 12,” a small chloroplast redox-regulatory protein. For the specific target provided (UniProt A6Q0K5, Chlamydomonas reinhardtii), the literature matches the UniProt description: CP12 is a small, nuclear-encoded chloroplast protein of the CP12 family that regulates the Calvin–Benson–Bassham cycle (CBBC) by forming a redox-controlled inhibitory complex with GAPDH and PRK. Primary Chlamydomonas genetic work explicitly states CP12 is encoded by a unique/single CP12 gene/isoform in C. reinhardtii and describes ΔCP12 mutants. (gerard2022reductioninphosphoribulokinase pages 1-2, gerard2022reductioninphosphoribulokinase pages 8-10, lopezcalcagno2014thecp12protein pages 2-3)
CP12 is best understood as a thioredoxin (Trx)-linked, light/dark-responsive metabolic switch that turns off key CBBC steps in the dark/oxidizing conditions by promoting formation of a GAPDH–CP12–PRK ternary complex that inhibits both enzymes; in the light/reducing conditions, Trx reduces regulatory disulfides and the complex dissociates, restoring enzymatic activity. (lopezcalcagno2014thecp12protein pages 2-3, gerard2022reductioninphosphoribulokinase pages 1-2)
A defining modern concept is that CP12 is a redox-dependent conditionally disordered protein: reduced CP12 is highly disordered, whereas oxidation (disulfide formation) partially orders CP12 but does not necessarily create a fully compact folded free state—partner binding completes functional structuring. This “disorder-to-partial-order” logic is central to how CP12 can rapidly engage/disengage partners. (giudice2023conformationaldisorderanalysis pages 1-2, mcfarlane2019structuralbasisof pages 1-2)
Across the green lineage, canonical CP12 proteins are ~80 aa and contain an N-terminal cysteine pair, a C-terminal cysteine pair, and a conserved central AWD_VEEL-like motif; the two cysteine pairs form two intramolecular disulfide bridges in oxidizing conditions. (giudice2023conformationaldisorderanalysis pages 1-2, lopezcalcagno2014thecp12protein pages 2-3)
High-resolution structural work (cyanobacterial homologs; mechanism conserved across algae/plants) describes CP12 as bipartite with:
- An N-terminal PRK-binding domain (two α-helices forming a hairpin/2-helix bundle) stabilized by an N-terminal disulfide (example: Cys19–Cys29), and
- A C-terminal GAPDH-binding domain,
connected by a flexible linker. Oxidation “preorders” the N-terminus for PRK binding. (mcfarlane2019structuralbasisof pages 2-3, mcfarlane2019structuralbasisof pages 1-2)
In the inhibited ternary complex, CP12 sterically blocks enzyme active sites: the CP12 N-terminal helical bundle plugs the PRK active-site cleft, and the CP12 C-terminus occupies/affects GAPDH active-site regions and alters substrate access. (mcfarlane2019structuralbasisof pages 2-3, mcfarlane2019structuralbasisof pages 1-2)
Structural evidence for the ternary complex is visually summarized in cropped figure regions from McFarlane et al. 2019 (PNAS), showing CP12 bridging PRK and GAPDH and the disulfide-stabilized PRK-binding hairpin. (mcfarlane2019structuralbasisof media e719c995, mcfarlane2019structuralbasisof media c7aae535, mcfarlane2019structuralbasisof media 44ce9d30)
Multiple sources support a sequential assembly model in which CP12 first binds GAPDH to form a GAPDH–CP12 binary complex, which then recruits PRK to form the ternary complex. (gerard2022reductioninphosphoribulokinase pages 1-2, mcfarlane2019structuralbasisof pages 1-2)
CP12 also integrates nicotinamide dinucleotide availability:
- In one described binary complex, each GAPDH tetramer typically binds two CP12 molecules physiologically (though higher occupancy can be obtained in vitro under CP12 excess). (mcfarlane2019structuralbasisof pages 2-3)
- A plant-type inhibited complex architecture is described as GAPDH2–CP124–PRK2, and complex stability can depend on metabolites: NAD(H) stabilizes inhibited assemblies, while NADP(H) and BPGA (1,3-bisphosphoglycerate) destabilize; reduced thioredoxins also destabilize the complex. (giudice2023conformationaldisorderanalysis pages 1-2)
- Structural contacts suggest CP12 can be incompatible with NADP(H) at certain sites (e.g., CP12 Glu69 disfavors NADP(H) binding). (mcfarlane2019structuralbasisof pages 2-3)
In C. reinhardtii, CP12 is described as a nuclear-encoded chloroplast protein. Functionally it acts in the chloroplast where the CBBC enzymes GAPDH and PRK operate (classically in the chloroplast stroma). (gerard2022reductioninphosphoribulokinase pages 1-2)
CP12 does not catalyze a reaction. Its primary function is regulatory/scaffolding: to control CBBC flux by inhibiting (and in some cases stabilizing) two key enzymes:
- GAPDH (the NADPH-dependent reductive step producing glyceraldehyde-3-phosphate), and
- PRK (ATP-dependent phosphorylation producing RuBP from Ru5P).
The literature explicitly frames CP12 as a coordinator (“conductor”) of these two key steps that represent the unique reduction and a key phosphorylation step in the CBBC. (gerard2022reductioninphosphoribulokinase pages 8-10)
Unlike land plants that often encode multiple CP12 isoforms, C. reinhardtii is reported to possess a single/unique canonical CP12 isoform, enabling relatively clean functional genetics. (gerard2022reductioninphosphoribulokinase pages 8-10, lopezcalcagno2014thecp12protein pages 2-3)
A CRISPR-Cas9 knockout of CP12 (ΔCP12) showed:
- No major growth defect under tested conditions (including continuous light and a light/dark cycle in the referenced study),
- Loss of the typical redox-dependent GAPDH regulation signature in extracts (WT extracts show behavior consistent with an inhibited ternary complex that can be relieved by reductants; ΔCP12 extracts show linear NADPH-dependent GAPDH activity),
- PRK activity reduction and lower PRK abundance. (gerard2022reductioninphosphoribulokinase pages 2-4, gerard2022reductioninphosphoribulokinase pages 1-2)
A separate dissertation source reports a diurnal growth phenotype (slower growth under 12 h light/12 h dark but not under continuous light) and proposes that in CP12 knockouts PRK/GAPDH protein levels fluctuate (accumulation–degradation) across the cycle, delaying response to changing conditions. (teh2021regulationofcp12mediated pages 63-65)
A 2023 Frontiers in Plant Science study engineered C. reinhardtii strains with PRK levels spanning ~16% to 250% of WT, showing that ~86% PRK content is sufficient to fully restore photoautotrophic growth and that PRK is present in moderate excess under optimal conditions. This work explicitly notes PRK is regulated by thioredoxin and also by dark formation of the inhibitory GAPDH–CP12–PRK complex, emphasizing that regulation (not only abundance) matters for CBBC control and synthetic design. (boisset2023phosphoribulokinaseabundanceis pages 1-2, boisset2023phosphoribulokinaseabundanceis pages 2-3)
A 2023 SAXS-focused study (Arabidopsis CP12) provides contemporary experimental support for CP12 as a redox-dependent conditionally disordered protein, and it compiles mechanistic consensus that the inhibited GAPDH–CP12–PRK assembly is sensitive to thioredoxin, NAD(H)/NADP(H) and BPGA. While not Chlamydomonas-specific, these are conserved principles directly relevant to interpreting the CP12 family protein A6Q0K5. (giudice2023conformationaldisorderanalysis pages 1-2)
A 2024 expert review on cyanobacterial primary carbon metabolism highlights that “new results” from metabolomic and redox analyses on strains with Cp12 variants extend CP12’s role toward acclimation to external glucose under diurnal conditions and to CO2 fluctuations in the light. The review explicitly frames such regulatory discoveries as enabling targeted redirection of carbon flow and helping establish cyanobacteria as green cell factories powered by sunlight and CO2. (lucius2024theprimarycarbon pages 1-2)
The 2023 PRK engineering work in C. reinhardtii provides a concrete example of real-world implementation: modular redesign of transcriptional units (promoters/introns) to tune enzyme abundance across a wide range (16–250% WT) and identify sufficiency thresholds for growth (≈86% WT PRK). Because PRK is a CP12-regulated enzyme (complex formation in the dark; thioredoxin-dependent dissociation), this engineering framework is directly relevant to designing strains where regulatory dynamics are optimized rather than only boosting protein levels. (boisset2023phosphoribulokinaseabundanceis pages 1-2, boisset2023phosphoribulokinaseabundanceis pages 2-3)
Structural and mechanistic insights highlight CP12 as a rational engineering target: because CP12 integrates redox state and dinucleotide availability, altering CP12–partner interfaces (or CP12 cysteines) could tune the on/off kinetics of carbon fixation across light regimes, with implications for productivity under fluctuating light. The PNAS structural study explicitly notes that understanding this regulation provides a starting point for modulating redox regulation with potential applications in improving productivity. (mcfarlane2019structuralbasisof pages 1-2)
The 2024 review explicitly links CP12-mediated regulation to the broader goal of redirecting carbon allocation toward desired compounds, i.e., a design goal in industrial phototrophic biotechnology. CP12’s position at a major control point (PRK + GAPDH inhibition) makes it a plausible “valve” for balancing storage vs growth vs product formation under diurnal or mixotrophic regimes. (lucius2024theprimarycarbon pages 1-2)
| Annotation aspect | Specific claim | Key evidence sources (year, DOI URL) |
|---|---|---|
| Role (GAPDH/PRK complex) | CP12 assembles an inhibitory GAPDH–CP12–PRK ternary complex in the dark, sequentially binding GAPDH then PRK; complex dissociates in the light via thioredoxin, activating both enzymes. | López-Calcagno 2014 — https://doi.org/10.3389/fpls.2014.00009 (lopezcalcagno2014thecp12protein pages 2-3); McFarlane 2019 — https://doi.org/10.1073/pnas.1906722116 (mcfarlane2019structuralbasisof pages 1-2); Gérard 2022 — https://doi.org/10.3390/ijms23052710 (gerard2022reductioninphosphoribulokinase pages 8-10) |
| Redox regulation | CP12 is a conditionally disordered protein: reduced CP12 is highly disordered; oxidation forms two intramolecular disulfides that partially order domains and enable binding, with thioredoxin and pyridine nucleotides modulating complex stability. | Del Giudice 2023 — https://doi.org/10.3390/ijms24119308 (giudice2023conformationaldisorderanalysis pages 1-2); McFarlane 2019 — https://doi.org/10.1073/pnas.1906722116 (mcfarlane2019structuralbasisof pages 2-3); López-Calcagno 2014 — https://doi.org/10.3389/fpls.2014.00009 (lopezcalcagno2014thecp12protein pages 2-3) |
| Localization (chloroplast stroma) | CP12 is a nuclear-encoded chloroplast protein; the PRK/CP12/GAPDH supracomplex (~550 kDa) forms in the chloroplast stroma. | Teh 2021 — https://doi.org/10.5282/edoc.29016 (teh2021regulationofcp12mediated pages 18-20); Gérard 2022 — https://doi.org/10.3390/ijms23052710 (gerard2022reductioninphosphoribulokinase pages 1-2) |
| Genetics (copy number; mutant) | Chlamydomonas reinhardtii possesses a single/unique canonical CP12 gene; a ΔCP12 knockout was generated for functional analysis. | Gérard 2022 — https://doi.org/10.3390/ijms23052710 (gerard2022reductioninphosphoribulokinase pages 1-2, gerard2022reductioninphosphoribulokinase pages 8-10); López-Calcagno 2014 — https://doi.org/10.3389/fpls.2014.00009 (lopezcalcagno2014thecp12protein pages 2-3) |
| Quantitative stats — PRK decrease in ΔCP12 | In ΔCP12, PRK protein amount is reduced ~6-fold and PRK activity declines; CP12 protects PRK from irreversible inactivation in vitro in a redox-independent manner. | Gérard 2022 — https://doi.org/10.3390/ijms23052710 (gerard2022reductioninphosphoribulokinase pages 10-11) |
| Quantitative stats — PRK abundance window | Engineered PRK levels spanning ~16–250% of WT show that ~86% of WT PRK suffices to fully restore photoautotrophic growth; PRK is not limiting under optimal conditions. | Boisset 2023 — https://doi.org/10.3389/fpls.2023.1230723 (boisset2023phosphoribulokinaseabundanceis pages 1-2, boisset2023phosphoribulokinaseabundanceis pages 2-3) |
| Structural notes | CP12 carries two conserved cysteine pairs (N-terminal disulfide preorders a helix-hairpin for PRK binding; C-terminal region binds GAPDH); CP12 remains largely disordered with a small preformed helical fraction (<20%). | McFarlane 2019 — https://doi.org/10.1073/pnas.1906722116 (mcfarlane2019structuralbasisof pages 2-3); Gérard 2022 — https://doi.org/10.3390/ijms23052710 (gerard2022reductioninphosphoribulokinase pages 10-11); Del Giudice 2023 — https://doi.org/10.3390/ijms24119308 (giudice2023conformationaldisorderanalysis pages 1-2) |
Table: Concise, evidence-backed annotations for CP12 (UniProt A6Q0K5) in Chlamydomonas reinhardtii, covering function, redox control, localization, genetics, quantitative impacts, and structural features. Useful for rapid verification and for guiding functional studies or engineering of Calvin cycle regulation.
References
(gerard2022reductioninphosphoribulokinase pages 1-2): Cassy Gérard, Régine Lebrun, Erwan Lemesle, Luisana Avilan, Kwang Suk Chang, EonSeon Jin, Frédéric Carrière, Brigitte Gontero, and Hélène Launay. Reduction in phosphoribulokinase amount and re-routing metabolism in chlamydomonas reinhardtii cp12 mutants. International Journal of Molecular Sciences, 23:2710, Feb 2022. URL: https://doi.org/10.3390/ijms23052710, doi:10.3390/ijms23052710. This article has 13 citations.
(gerard2022reductioninphosphoribulokinase pages 8-10): Cassy Gérard, Régine Lebrun, Erwan Lemesle, Luisana Avilan, Kwang Suk Chang, EonSeon Jin, Frédéric Carrière, Brigitte Gontero, and Hélène Launay. Reduction in phosphoribulokinase amount and re-routing metabolism in chlamydomonas reinhardtii cp12 mutants. International Journal of Molecular Sciences, 23:2710, Feb 2022. URL: https://doi.org/10.3390/ijms23052710, doi:10.3390/ijms23052710. This article has 13 citations.
(lopezcalcagno2014thecp12protein pages 2-3): Patricia E. López-Calcagno, Thomas P. Howard, and Christine A. Raines. The cp12 protein family: a thioredoxin-mediated metabolic switch? Frontiers in Plant Science, Jan 2014. URL: https://doi.org/10.3389/fpls.2014.00009, doi:10.3389/fpls.2014.00009. This article has 111 citations.
(giudice2023conformationaldisorderanalysis pages 1-2): Alessandra Del Giudice, Libero Gurrieri, Luciano Galantini, Silvia Fanti, Paolo Trost, Francesca Sparla, and Simona Fermani. Conformational disorder analysis of the conditionally disordered protein cp12 from arabidopsis thaliana in its different redox states. International Journal of Molecular Sciences, 24:9308, May 2023. URL: https://doi.org/10.3390/ijms24119308, doi:10.3390/ijms24119308. This article has 3 citations.
(mcfarlane2019structuralbasisof pages 1-2): Ciaran R. McFarlane, Nita R. Shah, Burak V. Kabasakal, Blanca Echeverria, Charles A. R. Cotton, Doryen Bubeck, and James W. Murray. Structural basis of light-induced redox regulation in the calvin–benson cycle in cyanobacteria. Proceedings of the National Academy of Sciences of the United States of America, 116:20984-20990, Sep 2019. URL: https://doi.org/10.1073/pnas.1906722116, doi:10.1073/pnas.1906722116. This article has 99 citations and is from a highest quality peer-reviewed journal.
(mcfarlane2019structuralbasisof pages 2-3): Ciaran R. McFarlane, Nita R. Shah, Burak V. Kabasakal, Blanca Echeverria, Charles A. R. Cotton, Doryen Bubeck, and James W. Murray. Structural basis of light-induced redox regulation in the calvin–benson cycle in cyanobacteria. Proceedings of the National Academy of Sciences of the United States of America, 116:20984-20990, Sep 2019. URL: https://doi.org/10.1073/pnas.1906722116, doi:10.1073/pnas.1906722116. This article has 99 citations and is from a highest quality peer-reviewed journal.
(mcfarlane2019structuralbasisof media e719c995): Ciaran R. McFarlane, Nita R. Shah, Burak V. Kabasakal, Blanca Echeverria, Charles A. R. Cotton, Doryen Bubeck, and James W. Murray. Structural basis of light-induced redox regulation in the calvin–benson cycle in cyanobacteria. Proceedings of the National Academy of Sciences of the United States of America, 116:20984-20990, Sep 2019. URL: https://doi.org/10.1073/pnas.1906722116, doi:10.1073/pnas.1906722116. This article has 99 citations and is from a highest quality peer-reviewed journal.
(mcfarlane2019structuralbasisof media c7aae535): Ciaran R. McFarlane, Nita R. Shah, Burak V. Kabasakal, Blanca Echeverria, Charles A. R. Cotton, Doryen Bubeck, and James W. Murray. Structural basis of light-induced redox regulation in the calvin–benson cycle in cyanobacteria. Proceedings of the National Academy of Sciences of the United States of America, 116:20984-20990, Sep 2019. URL: https://doi.org/10.1073/pnas.1906722116, doi:10.1073/pnas.1906722116. This article has 99 citations and is from a highest quality peer-reviewed journal.
(mcfarlane2019structuralbasisof media 44ce9d30): Ciaran R. McFarlane, Nita R. Shah, Burak V. Kabasakal, Blanca Echeverria, Charles A. R. Cotton, Doryen Bubeck, and James W. Murray. Structural basis of light-induced redox regulation in the calvin–benson cycle in cyanobacteria. Proceedings of the National Academy of Sciences of the United States of America, 116:20984-20990, Sep 2019. URL: https://doi.org/10.1073/pnas.1906722116, doi:10.1073/pnas.1906722116. This article has 99 citations and is from a highest quality peer-reviewed journal.
(gerard2022reductioninphosphoribulokinase pages 2-4): Cassy Gérard, Régine Lebrun, Erwan Lemesle, Luisana Avilan, Kwang Suk Chang, EonSeon Jin, Frédéric Carrière, Brigitte Gontero, and Hélène Launay. Reduction in phosphoribulokinase amount and re-routing metabolism in chlamydomonas reinhardtii cp12 mutants. International Journal of Molecular Sciences, 23:2710, Feb 2022. URL: https://doi.org/10.3390/ijms23052710, doi:10.3390/ijms23052710. This article has 13 citations.
(teh2021regulationofcp12mediated pages 63-65): Jing Tsong Teh. Regulation of cp12-mediated carbon metabolism by ntrc during cold acclimation. Dissertation, Jan 2021. URL: https://doi.org/10.5282/edoc.29016, doi:10.5282/edoc.29016. This article has 0 citations.
(gerard2022reductioninphosphoribulokinase pages 10-11): Cassy Gérard, Régine Lebrun, Erwan Lemesle, Luisana Avilan, Kwang Suk Chang, EonSeon Jin, Frédéric Carrière, Brigitte Gontero, and Hélène Launay. Reduction in phosphoribulokinase amount and re-routing metabolism in chlamydomonas reinhardtii cp12 mutants. International Journal of Molecular Sciences, 23:2710, Feb 2022. URL: https://doi.org/10.3390/ijms23052710, doi:10.3390/ijms23052710. This article has 13 citations.
(boisset2023phosphoribulokinaseabundanceis pages 1-2): Nicolas D. Boisset, Giusi Favoino, Maria Meloni, Lucile Jomat, Corinne Cassier-Chauvat, Mirko Zaffagnini, Stéphane D. Lemaire, and Pierre Crozet. Phosphoribulokinase abundance is not limiting the calvin-benson-bassham cycle in chlamydomonas reinhardtii. Frontiers in Plant Science, Aug 2023. URL: https://doi.org/10.3389/fpls.2023.1230723, doi:10.3389/fpls.2023.1230723. This article has 8 citations.
(boisset2023phosphoribulokinaseabundanceis pages 2-3): Nicolas D. Boisset, Giusi Favoino, Maria Meloni, Lucile Jomat, Corinne Cassier-Chauvat, Mirko Zaffagnini, Stéphane D. Lemaire, and Pierre Crozet. Phosphoribulokinase abundance is not limiting the calvin-benson-bassham cycle in chlamydomonas reinhardtii. Frontiers in Plant Science, Aug 2023. URL: https://doi.org/10.3389/fpls.2023.1230723, doi:10.3389/fpls.2023.1230723. This article has 8 citations.
(lucius2024theprimarycarbon pages 1-2): Stefan Lucius and Martin Hagemann. The primary carbon metabolism in cyanobacteria and its regulation. Frontiers in Plant Science, Jul 2024. URL: https://doi.org/10.3389/fpls.2024.1417680, doi:10.3389/fpls.2024.1417680. This article has 66 citations.
(lucius2024theprimarycarbon pages 9-10): Stefan Lucius and Martin Hagemann. The primary carbon metabolism in cyanobacteria and its regulation. Frontiers in Plant Science, Jul 2024. URL: https://doi.org/10.3389/fpls.2024.1417680, doi:10.3389/fpls.2024.1417680. This article has 66 citations.
(teh2021regulationofcp12mediated pages 18-20): Jing Tsong Teh. Regulation of cp12-mediated carbon metabolism by ntrc during cold acclimation. Dissertation, Jan 2021. URL: https://doi.org/10.5282/edoc.29016, doi:10.5282/edoc.29016. This article has 0 citations.
CP12 (UniProt: A6Q0K5) is a small (~8.5 kDa, 107 aa with transit peptide, 80 aa mature), nuclear-encoded chloroplast protein in Chlamydomonas reinhardtii. It is a conditionally disordered protein (CDP) that acts as a linker/scaffold mediating the formation of a supramolecular PRK/GAPDH/CP12 complex, thereby regulating the Calvin-Benson-Bassham (CBB) cycle in response to light/dark transitions.
CP12 coordinates the reversible inactivation of two CBB cycle enzymes during darkness:
- Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (A4 form, EC 1.2.1.13)
- Phosphoribulokinase (PRK) (EC 2.7.1.19)
The mechanism is redox-dependent:
- In the dark (oxidizing conditions): CP12's four cysteine residues form two disulfide bonds (Cys50-Cys58, Cys93-Cys102 in the precursor), conferring partial structure. Oxidized CP12 first binds GAPDH (forming a binary complex), then PRK is recruited to form a ternary PRK/GAPDH/CP12 complex. Both enzymes are inactive in this complex. PMID:12846565
The stoichiometry of the full complex: 2 PRK dimers + 2 GAPDH tetramers + CP12 PMID:12846565
CP12 is a conditionally disordered protein (CDP):
- Reduced CP12: fully intrinsically disordered, no stable secondary structure PMID:12846565
- Oxidized CP12: gains alpha-helical content and partial structure from disulfide bonds, but remains highly flexible PMID:12846565
- CP12 has properties similar to "intrinsically unstructured proteins" involved in regulating macromolecular complexes PMID:12846565
CP12 binds copper (Cu2+) and nickel (Ni2+) ions:
- Cu2+ binding: Kd = 26 ± 1 μM PMID:16259044
- Ni2+ binding: Kd = 11 ± 1 μM PMID:16259044
- Does NOT bind Fe2+ or Zn2+ PMID:16259044
- Cu2+ catalyzes re-formation of disulfide bonds in reduced CP12, promoting the oxidized (active linker) form PMID:16259044
- Similarity to copper chaperones from Arabidopsis thaliana noted PMID:16259044
- His74 mutation to Leu has no impact on metal-ion binding PMID:16259044
Important note on metal binding: While CP12 binds Cu2+ and Ni2+ in vitro, the biological significance of this metal binding is debated. The copper-catalyzed oxidation of CP12 thiols may be relevant in vivo, but the primary known function of CP12 is as a regulatory scaffold, not as a dedicated metal-binding protein. The metal binding annotations should be viewed as secondary/moonlighting characteristics.
CRISPR-Cas9 knockout of the single CP12 gene in C. reinhardtii PMID:35269851:
- Growth rates: nearly identical between WT and ΔCP12
- GAPDH: abundance and activity unchanged in ΔCP12
- PRK: abundance and specific activity significantly reduced in ΔCP12
- CP12 protects PRK from irreversible inactivation in vitro (redox-independent)
- Multiple proteins in redox homeostasis and stress response were more abundant in ΔCP12
- CP12 described as a "moonlighting protein" with functions beyond CBB regulation
id: A6Q0K5
gene_symbol: CP12
product_type: PROTEIN
status: DRAFT
taxon:
id: NCBITaxon:3055
label: Chlamydomonas reinhardtii
description: CP12 is a small (~8.5 kDa), nuclear-encoded, conditionally disordered
chloroplast protein that functions as a redox-sensitive linker/scaffold mediating
the assembly of a supramolecular PRK/GAPDH/CP12 complex. In the dark, oxidized CP12
(with two intramolecular disulfide bonds) sequentially binds GAPDH then PRK, forming
a ternary complex that inactivates both Calvin cycle enzymes. In the light, thioredoxin-mediated
reduction of CP12 disulfide bonds causes the complex to dissociate, releasing active
enzymes. CP12 also binds Cu2+ and Ni2+ ions in vitro; Cu2+ catalyzes re-oxidation
of CP12 thiols, potentially linking metal homeostasis to Calvin cycle regulation.
existing_annotations:
- term:
id: GO:0009507
label: chloroplast
evidence_type: IEA
original_reference_id: GO_REF:0000044
review:
summary: Chloroplast localization is well-supported by multiple lines of evidence.
CP12 has a 27-residue chloroplast transit peptide confirmed by direct protein
sequencing (PMID:12846565), and IDA evidence from CAFA also supports this localization.
This IEA annotation from UniProt subcellular location mapping is consistent
with all other evidence.
action: ACCEPT
reason: Although this is an IEA annotation, it is fully consistent with the IDA
evidence (PMID:12846565) showing chloroplast localization via transit peptide
identification and direct protein sequencing. The UniProt record confirms chloroplast
transit peptide residues 1-27 with experimental evidence.
supported_by:
- reference_id: PMID:12846565
supporting_text: CP12 is an 8.5-kDa nuclear-encoded chloroplast protein
- reference_id: PMID:16259044
supporting_text: The small chloroplast protein CP12
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:24863370
review:
summary: This annotation records CP12 interaction with PRK (P19824) based on fluorescence
correlation spectroscopy (FCS) experiments (PMID:24863370). While the interaction
is real and well-documented, GO:0005515 (protein binding) is too generic per
curation guidelines. CP12 binds the enzyme PRK as part of its core regulatory
function; GO:0019899 (enzyme binding) is more informative and already annotated.
action: MODIFY
reason: Per curation guidelines, GO:0005515 (protein binding) is uninformative.
CP12's interaction with PRK is its core molecular function -- it acts as a scaffold/linker
that binds PRK to form the regulatory PRK/GAPDH/CP12 complex. GO:0019899 (enzyme
binding) is already annotated for the GAPDH interaction and is equally appropriate
for the PRK interaction, as PRK is an enzyme (phosphoribulokinase, EC 2.7.1.19).
proposed_replacement_terms:
- id: GO:0019899
label: enzyme binding
supported_by:
- reference_id: PMID:24863370
supporting_text: we characterize the diffusion dynamics and hydrodynamic radii
of CP12 from Chlamydomonas reinhardtii upon binding to GAPDH and PRK using
fluorescence correlation spectroscopy experiments
- reference_id: PMID:12846565
supporting_text: oxidized, but not reduced, CP12 acts as a linker in the assembly
of the complex
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:24863370
review:
summary: This annotation records CP12 interaction with GAPDH (P50362) based on
FCS experiments (PMID:24863370). Same issue as the PRK protein binding annotation
-- GO:0005515 is too generic. The GAPDH interaction is already covered by the
enzyme binding annotation from PMID:12846565.
action: MODIFY
reason: GO:0005515 is uninformative per curation guidelines. CP12's interaction
with GAPDH is its core function as a regulatory linker. GO:0019899 (enzyme binding)
is more appropriate, as GAPDH is an enzyme (glyceraldehyde-3-phosphate dehydrogenase,
EC 1.2.1.13). This interaction is already annotated as enzyme binding from PMID:12846565.
proposed_replacement_terms:
- id: GO:0019899
label: enzyme binding
supported_by:
- reference_id: PMID:24863370
supporting_text: we characterize the diffusion dynamics and hydrodynamic radii
of CP12 from Chlamydomonas reinhardtii upon binding to GAPDH and PRK using
fluorescence correlation spectroscopy experiments
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:12846565
review:
summary: This annotation records CP12 interaction with both PRK (P19824) and GAPDH
(P50362) based on reconstitution assays and SPR binding studies (PMID:12846565).
As with the other protein binding annotations, GO:0005515 is too generic. The
interactions are real but better captured by GO:0019899 (enzyme binding).
action: MODIFY
reason: GO:0005515 is uninformative per curation guidelines. CP12 acts as a linker
for two Calvin cycle enzymes; enzyme binding (GO:0019899) is the appropriate
term. The original paper clearly demonstrates CP12 binding to both PRK and GAPDH
through reconstitution assays and surface plasmon resonance.
proposed_replacement_terms:
- id: GO:0019899
label: enzyme binding
supported_by:
- reference_id: PMID:12846565
supporting_text: oxidized, but not reduced, CP12 acts as a linker in the assembly
of the complex, and we propose a model in which CP12 associates with GAPDH,
causing its conformation to change. This GAPDH/CP12 complex binds PRK to form
a half-complex (one unit).
- term:
id: GO:0009507
label: chloroplast
evidence_type: IDA
original_reference_id: PMID:12846565
review:
summary: Direct experimental evidence for chloroplast localization. The PMID:12846565
study determined the N-terminal sequence of the mature protein (residues 28-32),
confirming cleavage of the chloroplast transit peptide and thereby demonstrating
chloroplast import. UniProt lists this as transit peptide residues 1-27 with
evidence from PMID:12846565.
action: ACCEPT
reason: Strong IDA evidence from direct protein sequencing confirming the transit
peptide cleavage site. This is the primary experimental evidence for chloroplast
localization and directly demonstrates that CP12 is imported into the chloroplast.
supported_by:
- reference_id: PMID:12846565
supporting_text: CP12 is an 8.5-kDa nuclear-encoded chloroplast protein
- term:
id: GO:0019899
label: enzyme binding
evidence_type: IPI
original_reference_id: PMID:12846565
review:
summary: 'Enzyme binding is CP12''s core molecular function. CP12 acts as a regulatory
linker that binds two Calvin cycle enzymes: GAPDH (P50362, glyceraldehyde-3-phosphate
dehydrogenase) and PRK (P19824, phosphoribulokinase). The WITH column specifies
GAPDH (P50362). This was demonstrated through reconstitution assays and SPR
binding studies (PMID:12846565), and independently confirmed by FCS (PMID:24863370).'
action: ACCEPT
reason: This is the most informative molecular function term for CP12's core activity.
CP12 is defined by its ability to bind and regulate GAPDH and PRK through redox-dependent
complex assembly. Enzyme binding accurately captures this scaffold/linker function.
The evidence is strong, based on multiple in vitro reconstitution approaches.
supported_by:
- reference_id: PMID:12846565
supporting_text: oxidized, but not reduced, CP12 acts as a linker in the assembly
of the complex, and we propose a model in which CP12 associates with GAPDH,
causing its conformation to change
- reference_id: PMID:24863370
supporting_text: We quantify a hydrodynamic radius of 3.4 ± 0.2 nm for the CP12
protein with an increase up to 5.2 ± 0.3 nm upon complex formation with GAPDH
and PRK
- term:
id: GO:0009507
label: chloroplast
evidence_type: ISS
original_reference_id: GO_REF:0000024
review:
summary: ISS annotation based on sequence similarity to Arabidopsis thaliana CP12
(O22914). Consistent with the IDA evidence from PMID:12846565 and the IEA annotation.
This is redundant with the IDA evidence but not incorrect.
action: ACCEPT
reason: Fully consistent with the IDA evidence (PMID:12846565). The ISS transfer
from A. thaliana CP12 is appropriate given that chloroplast localization is
a conserved feature of all CP12 family members, which universally possess chloroplast
transit peptides.
supported_by:
- reference_id: PMID:12846565
supporting_text: CP12 is an 8.5-kDa nuclear-encoded chloroplast protein
- term:
id: GO:0032991
label: protein-containing complex
evidence_type: ISS
original_reference_id: GO_REF:0000024
review:
summary: CP12 is part of the PRK/GAPDH/CP12 supramolecular complex. The annotation
to the generic GO:0032991 (protein-containing complex) is correct but very broad.
The full complex stoichiometry is 2 PRK dimers + 2 GAPDH tetramers + CP12 (PMID:12846565).
A more specific GO complex term would be ideal, but no dedicated term for the
PRK/GAPDH/CP12 complex currently exists in GO.
action: ACCEPT
reason: While GO:0032991 is generic, there is no more specific GO term available
for the PRK/GAPDH/CP12 complex. The annotation is accurate -- CP12 is indeed
a component of a well-characterized protein-containing complex. The ISS transfer
from A. thaliana CP12 is appropriate since the PRK/GAPDH/CP12 complex is conserved
across photosynthetic organisms. Proposing a new GO term for this complex could
be considered.
supported_by:
- reference_id: PMID:12846565
supporting_text: It forms part of a core complex of two dimers of phosphoribulokinase
(PRK), two tetramers of glyceraldehyde 3-phosphate dehydrogenase (GAPDH),
and CP12
- term:
id: GO:0080153
label: negative regulation of reductive pentose-phosphate cycle
evidence_type: ISS
original_reference_id: GO_REF:0000024
review:
summary: 'This is the core biological process annotation for CP12. CP12 negatively
regulates the Calvin-Benson-Bassham (reductive pentose-phosphate) cycle by sequestering
PRK and GAPDH into an inactive complex in the dark. The ISS transfer from A.
thaliana CP12 (O22914) is well-justified, as this regulatory function is conserved
across all characterized CP12 proteins. CRISPR-Cas9 knockout of CP12 in C. reinhardtii
(PMID:35269851) provides direct IMP-grade evidence in the target organism — delta-CP12
cells show abolished DTT-dependent regulation of GAPDH activity and 3-fold reduced
PRK specific activity, confirming that CP12 is required in vivo for dark regulation
of the Calvin cycle. An upgraded experimental evidence annotation (e.g. IMP
from PMID:35269851) should be considered by the curation source.'
action: ACCEPT
reason: GO:0080153 precisely captures CP12's core biological process function.
CP12 coordinates the dark inactivation of GAPDH and PRK, two key Calvin cycle
enzymes, by assembling them into an inactive supramolecular complex. This is
the most specific and accurate BP term for CP12's role. The ISS annotation is
consistent with strong direct experimental evidence in C. reinhardtii from both
reconstitution experiments (PMID:12846565) and CRISPR knockout studies (PMID:35269851),
so an IMP upgrade in the source GOA is warranted.
supported_by:
- reference_id: PMID:12846565
supporting_text: oxidized, but not reduced, CP12 acts as a linker in the assembly
of the complex
- reference_id: PMID:16259044
supporting_text: a PRK/GAPDH/CP12 complex that is involved in CO2 assimilation
in photosynthetic organisms. The redox state of CP12 regulates its role as
a protein linker.
- reference_id: PMID:35269851
supporting_text: The chloroplast protein CP12 is involved in the dark/light
regulation of the Calvin-Benson-Bassham cycle, in particular, in the dark
inhibition of two enzymes
- reference_id: PMID:35269851
supporting_text: in the ∆CP12-Cr cell extracts, the NADPH-dependent activity
of GAPDH displayed a linear curve regardless of DTT
- term:
id: GO:0005507
label: copper ion binding
evidence_type: IDA
original_reference_id: PMID:16259044
review:
summary: IDA evidence from ESI-MS experiments demonstrating specific Cu2+ binding
by CP12 with Kd = 26 +/- 1 uM (PMID:16259044). Importantly, Cu2+ also catalyzes
re-formation of CP12 disulfide bonds, potentially linking copper to Calvin cycle
regulation. The authors note sequence similarity between CP12 and copper chaperones
from A. thaliana. However, the primary biological function of CP12 is as a regulatory
scaffold, not as a dedicated copper-binding protein; the in vivo significance
of copper binding remains uncertain.
action: KEEP_AS_NON_CORE
reason: The copper binding is real (IDA evidence, specific binding with measured
Kd), but it represents a secondary/potential moonlighting function rather than
CP12's core evolved function. CP12's primary role is as a redox-dependent linker
for the PRK/GAPDH complex. The copper-catalyzed thiol oxidation may be physiologically
relevant as it could promote CP12's active (oxidized) conformation, but this
remains speculative. Retaining as non-core appropriately reflects the evidence.
supported_by:
- reference_id: PMID:16259044
supporting_text: The oxidized protein bound specifically Cu2+ and Ni2+ (Kd of
26+/-1 microM and 11+/-1 microM, respectively); other cations such as Fe2+
and Zn2+ did not bind
- reference_id: PMID:16259044
supporting_text: Cu2+ catalyzes the re-formation of the disulfide bonds of the
reduced CP12, leading to recovery of the fully oxidized CP12 that is then
able to bind a Cu2+ ion
- term:
id: GO:0016151
label: nickel cation binding
evidence_type: IDA
original_reference_id: PMID:16259044
review:
summary: IDA evidence from ESI-MS experiments showing specific Ni2+ binding by
CP12 with Kd = 11 +/- 1 uM (PMID:16259044). The binding is more specific than
Cu2+ (lower Kd) but the biological relevance is even less clear. Nickel is not
known to play a significant role in chloroplast metabolism in green algae. The
His74 mutation had no impact on metal binding, suggesting the binding site may
involve other residues.
action: KEEP_AS_NON_CORE
reason: The nickel binding is experimentally demonstrated (IDA) with good specificity
(Kd = 11 uM, no binding of Fe2+ or Zn2+), but there is no evidence for an in
vivo role. Unlike copper, nickel does not catalyze CP12 disulfide bond formation,
so a regulatory connection to CP12's primary function is not established. This
is a secondary characteristic best kept as non-core.
supported_by:
- reference_id: PMID:16259044
supporting_text: The oxidized protein bound specifically Cu2+ and Ni2+ (Kd of
26+/-1 microM and 11+/-1 microM, respectively); other cations such as Fe2+
and Zn2+ did not bind
- reference_id: PMID:16259044
supporting_text: the high similarity between CP12 and copper chaperones from
Arabidopsis thaliana, as judged by hydrophobic cluster analysis, provides
additional evidence for the relevance of metal binding for the in vivo situation
- term:
id: GO:0050821
label: protein stabilization
evidence_type: IMP
original_reference_id: PMID:35269851
review:
summary: Proposed new annotation reflecting a redox-independent moonlighting function
of CP12. PMID:35269851 demonstrates that CP12 stabilizes PRK against irreversible
inactivation both in vitro (recombinant CP12 prevents irreversible loss of PRK
activity in a redox-independent manner; DTT can restore activity only when CP12
is present) and in vivo (ΔCP12 cells show 6.5-fold reduction in PRK protein
abundance with no change in mRNA, consistent with loss of post-translational
stabilization). Site-directed mutagenesis identifies specific residues (D36,
E39, E40, W35, H47) required for this protective function.
action: NEW
reason: PMID:35269851 provides both in vivo (knockout) and in vitro (recombinant
protein + mutagenesis) evidence that CP12 specifically protects PRK from irreversible
inactivation in a redox-independent manner. This is mechanistically distinct
from CP12's redox-dependent regulatory role in the PRK/GAPDH/CP12 complex and
warrants a separate annotation. GO:0050821 (protein stabilization) accurately
captures this function as the term covers maintenance of protein structure/integrity
and prevention of aggregation/degradation.
supported_by:
- reference_id: PMID:35269851
supporting_text: Isolated PRK lost irreversibly its activity over-time in vitro,
which was prevented in the presence of recombinant CP12 in a redox-independent
manner
- reference_id: PMID:35269851
supporting_text: in the presence of CP12, the addition of DTT restored the initial
PRK activity, indicating that CP12 prevented this irreversible inactivation
- reference_id: PMID:35269851
supporting_text: a significant difference was observed for PRK amount, with
a 6.5-fold decrease of PRK amount in ΔCP12-Cr compared to the WT-Cr strain
core_functions:
- description: Redox-dependent scaffold/linker that binds GAPDH and PRK to assemble
an inactive PRK/GAPDH/CP12 supramolecular complex, negatively regulating the Calvin-Benson-Bassham
cycle in the dark
molecular_function:
id: GO:0019899
label: enzyme binding
directly_involved_in:
- id: GO:0080153
label: negative regulation of reductive pentose-phosphate cycle
locations:
- id: GO:0009507
label: chloroplast
in_complex:
id: GO:0032991
label: protein-containing complex
supported_by:
- reference_id: PMID:12846565
supporting_text: oxidized, but not reduced, CP12 acts as a linker in the assembly
of the complex, and we propose a model in which CP12 associates with GAPDH,
causing its conformation to change. This GAPDH/CP12 complex binds PRK to form
a half-complex (one unit).
- reference_id: PMID:24863370
supporting_text: We quantify a hydrodynamic radius of 3.4 +/- 0.2 nm for the CP12
protein with an increase up to 5.2 +/- 0.3 nm upon complex formation with GAPDH
and PRK
- reference_id: PMID:35269851
supporting_text: the abundance of PRK and its specific activity were significantly
reduced in deltaCP12, as revealed by relative quantitative proteomics
references:
- id: GO_REF:0000024
title: Manual transfer of experimentally-verified manual GO annotation data to orthologs
by curator judgment of sequence similarity
findings: []
- id: GO_REF:0000044
title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
vocabulary mapping, accompanied by conservative changes to GO terms applied by
UniProt
findings: []
- id: PMID:12846565
title: 'The small protein CP12: a protein linker for supramolecular complex assembly.'
findings:
- statement: Oxidized CP12 acts as a redox-sensitive linker for assembly of the
PRK/GAPDH/CP12 supramolecular complex; reduced CP12 cannot reconstitute the
complex.
supporting_text: oxidized, but not reduced, CP12 acts as a linker in the assembly
of the complex
- statement: The complex has a stoichiometry of 2 PRK dimers, 2 GAPDH tetramers,
and CP12. CP12 first binds GAPDH, then PRK is recruited.
supporting_text: It forms part of a core complex of two dimers of phosphoribulokinase
(PRK), two tetramers of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and
CP12
- statement: Oxidized CP12 is mainly alpha-helical and flexible; reduced CP12 is
mainly unstructured, consistent with intrinsically disordered protein properties.
supporting_text: Oxidized CP12 is mainly composed of alpha helix and coil segments,
and is extremely flexible, while reduced CP12 is mainly unstructured
- statement: CP12 is a nuclear-encoded chloroplast protein with a transit peptide
confirmed by N-terminal sequencing of the mature protein (residues 28-32).
supporting_text: CP12 is an 8.5-kDa nuclear-encoded chloroplast protein
- id: PMID:16259044
title: 'Mass spectrometric analysis of the interactions between CP12, a chloroplast
protein, and metal ions: a possible regulatory role within a PRK/GAPDH/CP12 complex.'
findings:
- statement: CP12 specifically binds Cu2+ (Kd = 26 uM) and Ni2+ (Kd = 11 uM) but
not Fe2+ or Zn2+, as shown by ESI-MS.
supporting_text: The oxidized protein bound specifically Cu2+ and Ni2+ (Kd of
26+/-1 microM and 11+/-1 microM, respectively); other cations such as Fe2+ and
Zn2+ did not bind
- statement: Cu2+ catalyzes re-oxidation of reduced CP12, promoting formation of
the disulfide bonds required for linker activity.
supporting_text: Cu2+ catalyzes the re-formation of the disulfide bonds of the
reduced CP12, leading to recovery of the fully oxidized CP12
- statement: CP12 shows sequence similarity to copper chaperones from Arabidopsis
thaliana by hydrophobic cluster analysis, suggesting in vivo relevance of metal
binding.
supporting_text: the high similarity between CP12 and copper chaperones from Arabidopsis
thaliana, as judged by hydrophobic cluster analysis, provides additional evidence
for the relevance of metal binding for the in vivo situation
- id: PMID:24863370
title: Conformational modulation and hydrodynamic radii of CP12 protein and its
complexes probed by fluorescence correlation spectroscopy.
findings:
- statement: FCS confirms CP12 binding to GAPDH and PRK, with hydrodynamic radius
increasing from 3.4 nm (free CP12) to 5.2 nm upon complex formation.
supporting_text: We quantify a hydrodynamic radius of 3.4 ± 0.2 nm for the CP12
protein with an increase up to 5.2 ± 0.3 nm upon complex formation with GAPDH
and PRK
- statement: N-terminal and C-terminal cysteine mutants show different structural
behavior during unfolding, suggesting distinct roles for the two disulfide bonds
in mediating GAPDH vs PRK binding.
supporting_text: The different behavior of the CP12 mutant proteins during hydrophobic
collapse transition is a direct clue to different structural orientations of
the CP12 mutant proteins
- id: PMID:35269851
title: Reduction in Phosphoribulokinase Amount and Re-Routing Metabolism in Chlamydomonas
reinhardtii CP12 Mutants.
findings:
- statement: CRISPR-Cas9 knockout of CP12 in C. reinhardtii results in reduced PRK
abundance and activity but normal GAPDH levels and near-normal growth.
supporting_text: the abundance of PRK and its specific activity were significantly
reduced in ΔCP12, as revealed by relative quantitative proteomics
- statement: CP12 protects PRK from irreversible inactivation in a redox-independent
manner, suggesting functions beyond Calvin cycle regulation.
supporting_text: in the presence of CP12, the addition of DTT restored the initial
PRK activity, indicating that CP12 prevented this irreversible inactivation
- id: PMID:24523724
title: 'The CP12 protein family: a thioredoxin-mediated metabolic switch?'
findings:
- statement: Review establishing CP12 as a conserved redox-sensitive protein found
across all oxygenic phototrophs whose only clearly defined function is thioredoxin-mediated
regulation of the Calvin cycle via GAPDH/PRK/CP12 complex assembly/disassembly.
supporting_text: The only clearly defined function for CP12 in any organism is
in the thioredoxin-mediated regulation of the Calvin-Benson cycle
- statement: CP12 may have broader regulatory roles beyond Calvin cycle regulation,
with evidence suggesting involvement in stress responses and additional metabolic
functions.
supporting_text: CP12 may have a broader role in the regulation of metabolism,
over and above the well established role of CP12 in the regulation of the Calvin-Benson
cycle
- id: PMID:30923119
title: Arabidopsis and Chlamydomonas phosphoribulokinase crystal structures complete
the redox structural proteome of the Calvin-Benson cycle.
findings:
- statement: Crystal structures of PRK from both Arabidopsis and Chlamydomonas reveal
that the regulatory cysteines are connected by a flexible clamp loop unique to
eukaryotic PRKs, completing the structural understanding of all redox-regulated
Calvin cycle enzymes.
supporting_text: the regulatory cysteines are 13 Å apart and connected by a flexible
region exclusive to photosynthetic eukaryotes-the clamp loop-which is believed
to be essential for oxidation-induced structural rearrangements
- id: PMID:39036361
title: The primary carbon metabolism in cyanobacteria and its regulation.
findings:
- statement: Review positioning CP12 as a principal redox regulator of the Calvin
cycle especially in cyanobacteria, noting new metabolomics evidence that CP12
variants affect acclimation to external glucose under diurnal conditions and
CO2 fluctuations.
supporting_text: New results of metabolomic and redox level analyses on strains
with Cp12 variants extend the known role of Cp12 regulation towards the acclimation
to external glucose supply under diurnal conditions as well as to fluctuations
in CO2 levels in the light
- id: PMID:37298260
title: Conformational Disorder Analysis of the Conditionally Disordered Protein CP12
from Arabidopsis thaliana in Its Different Redox States.
findings:
- statement: SAXS analysis of Arabidopsis CP12 confirms it is a conditionally disordered
protein; reduced CP12 is highly disordered while oxidized CP12 gains partial
structural order through disulfide bond formation but remains flexible.
supporting_text: a small angle X-ray scattering (SAXS) analysis of recombinant
Arabidopsis CP12 (AtCP12) in a reduced and oxidized form confirmed the highly
disordered nature of this regulatory protein
- id: PMID:37719215
title: Phosphoribulokinase abundance is not limiting the Calvin-Benson-Bassham cycle
in Chlamydomonas reinhardtii.
findings:
- statement: PRK is present in moderate excess in C. reinhardtii; approximately 86%
of wild-type PRK content is sufficient for full photoautotrophic growth, and
overexpression does not increase growth.
supporting_text: Immunoblot and growth assays revealed that a PRK content of ≈86%
is sufficient to fully restore photoautotrophic growth
- id: PMID:31570616
title: Structural basis of light-induced redox regulation in the Calvin-Benson cycle
in cyanobacteria.
findings:
- statement: Cryo-EM structure of the cyanobacterial GAPDH-CP12-PRK ternary complex
reveals that CP12's N-terminal disulfide-stabilized helical hairpin plugs the
PRK active-site cleft, while the C-terminal region binds GAPDH and influences
substrate accessibility of all GAPDH active sites.
supporting_text: CP12 binding to GAPDH influences substrate accessibility of all
GAPDH active sites in the binary and ternary inhibited complexes
- statement: CP12 integrates responses from both the redox state (via disulfide bonds)
and metabolite availability (NAD(H)/NADP(H) cofactor preference) to coordinate
inhibition of GAPDH and PRK in the dark.
supporting_text: Our structural and biochemical data explain how CP12 integrates
responses from both redox state and nicotinamide dinucleotide availability to
regulate carbon fixation
proposed_new_terms:
- proposed_name: PRK-GAPDH-CP12 complex
proposed_definition: 'A protein complex consisting of phosphoribulokinase (PRK),
glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and the linker protein CP12.
The complex forms in the dark under oxidizing conditions and sequesters PRK and
GAPDH in an inactive state, thereby negatively regulating the Calvin-Benson-Bassham
cycle. Stoichiometry: 2 PRK dimers + 2 GAPDH tetramers + CP12.'
justification: No GO complex term currently exists for this well-characterized supramolecular
complex, which is conserved across photosynthetic organisms from cyanobacteria
to higher plants. The current annotation uses the generic GO:0032991 (protein-containing
complex).
proposed_parent:
id: GO:0032991
label: protein-containing complex
supported_by:
- reference_id: PMID:12846565
supporting_text: It forms part of a core complex of two dimers of phosphoribulokinase
(PRK), two tetramers of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and
CP12
suggested_questions:
- question: Is the copper ion binding by CP12 physiologically relevant in vivo, or
is it an in vitro artifact? Does copper-catalyzed re-oxidation of CP12 thiols
contribute to dark regulation of the Calvin cycle under normal conditions?
experts:
- Gontero B
- Launay H
- question: Should CP12's role in protecting PRK from irreversible inactivation (redox-independent)
be annotated separately from its role in dark/light regulation of the Calvin cycle
(redox-dependent)?
experts:
- Gontero B
suggested_experiments:
- hypothesis: CP12 copper binding is relevant in vivo for Calvin cycle regulation
description: Measure Calvin cycle enzyme activity and PRK/GAPDH/CP12 complex formation
in C. reinhardtii cells grown under copper-depleted vs. copper-replete conditions,
comparing wild-type and ΔCP12 strains. Use immunoprecipitation to assess complex
formation kinetics during light-to-dark transitions.
- hypothesis: CP12 has additional interaction partners beyond GAPDH and PRK
description: Perform in vivo crosslinking mass spectrometry (XL-MS) or proximity
labeling (BioID/TurboID) with tagged CP12 in C. reinhardtii to identify the full
interactome, including reported interactions with aldolase and malate dehydrogenase.