Gene: CAF120 / YNL278W · Saccharomyces cerevisiae (NCBITaxon:559292)
UniProt: P53836 (CA120_YEAST), 1060 aa
Focus type: function_assignment — does the gene product directly have this molecular function?
Hypothesis under review: CAF120 has protein kinase activity (GO:0004672)
Current annotation: GO:0004672 · evidence IBA · reference GO_REF:0000033 (GO_Central phylogenetic annotation)
The seed hypothesis that CAF120 possesses protein kinase activity (GO:0004672) is refuted. Three independent lines of evidence converge: (1) programmatic domain analysis shows P53836 carries only a Pleckstrin-homology (PH) domain and its fungal derivative — there is no protein-kinase domain of any kind; (2) a direct scan of the 1060-residue sequence finds none of the conserved eukaryotic protein-kinase (ePK) catalytic motifs required for phosphotransfer; and (3) the experimentally supported function of Caf120 is regulatory/adaptor in nature — a meiotic crossover regulator physically linked to the Exo1–MutLγ recombination machinery and, per UniProt curation, associated with the CCR4-NOT context — with no report of catalytic activity.
The GO:0004672 annotation is an IBA phylogenetic mis-propagation. It is co-annotated with GO:0007165 (signal transduction, also IBA) in the same record, and PH domains are shared with genuine kinases (e.g., AKT/PKB-type AGC kinases carry a PH module adjacent to their catalytic domain). The most parsimonious interpretation is that the GO_Central pipeline propagated "protein kinase activity + signal transduction" from a PH-domain-containing kinase clade to CAF120, which possesses the PH module but not the kinase domain.
The recommended curation action is to remove GO:0004672 from CAF120 — not to retain it, generalize it to a broader transferase/kinase term, or replace it with a more specific kinase term. This judgment is a structural-impossibility argument (the catalytic residues are physically absent) rather than merely "not yet tested," which makes the refutation strong. The residual uncertainty is limited to whether some wholly non-canonical catalytic mechanism could exist, for which there is no evidence.
Domain analysis via the InterPro API (queried this run) returns a single, internally consistent picture: P53836 is a Pleckstrin-homology (PH) domain protein. The matched signatures include Pfam PF00169 (PH domain) and PF25381 (fungal PH-like), PROSITE PS50003 (PH profile), SMART SM00233 (PH), Gene3D 2.30.29.30 (PH-domain-like β-sandwich fold), the InterPro PH-like superfamily IPR011993, and the dedicated fungal family IPR058155 ("Skg3/CAF120-like PH domain").
Critically, no kinase signatures are present at all. There is no Pfam PF00069 (Pkinase), no InterPro IPR000719 (protein kinase domain), and no PANTHER protein-kinase family assignment. The PH-like β-sandwich fold is structurally unrelated to the bilobal α/β protein-kinase fold. A PH domain is a lipid/protein-interaction module that recruits proteins to membranes or partners; it confers no catalytic activity by itself. The existence of a dedicated fungal family term (IPR058155) built around the PH module — not a catalytic domain — indicates the correct family placement for CAF120 is a PH-adaptor family, not a kinase family.
A direct motif scan of the full 1060-aa P53836 sequence tested each conserved catalytic element of the canonical ePK domain. All were absent:
| Catalytic element | Consensus | Function | Present in P53836? |
|---|---|---|---|
| Glycine-rich loop (P-loop) | G-x-G-x-x-G | Positions ATP phosphates | No |
| β3 lysine (VAIK) | V-A-I-K | Anchors α/β phosphates of ATP | No |
| Catalytic loop | H-R-D | Catalytic aspartate (proton acceptor) | No |
| Mg-binding loop | D-F-G | Coordinates Mg²⁺ | Only stray "DFG" tripeptides in N-terminal/disordered region, not in a catalytic domain |
| PROSITE PS00107 | ATP-binding kinase signature | ATP binding | No match |
| PROSITE PS00108 | Ser/Thr kinase active-site signature | Catalysis | No match |
The simultaneous absence of the P-loop, the VAIK lysine, and the HRD aspartate is decisive. These form the minimal catalytic core: the P-loop and lysine bind and orient ATP, while the HRD aspartate is the catalytic base that deprotonates the substrate hydroxyl. Even pseudokinases — catalytically dead but retaining the fold — usually preserve recognizable remnants of these motifs. CAF120 has none of the scaffold ordered within a ~250–300-aa domain, indicating it does not belong to the protein-kinase superfamily at all. The two "DFG" tripeptides that occur by chance fall in the N-terminus and a disordered region rather than within any structured catalytic module, so they are not evidence of a kinase.
Two primary studies characterize Caf120, and neither reports or implies catalytic kinase activity:
Meiotic crossover regulation. In a proteomic dissection of homologous-recombination enzyme networks, Caf120 was identified among the meiosis-specific interactors of the MutLγ–Exo1 crossover machinery and classified as a regulator of crossing-over (PMID: 31351878). This places Caf120 in a physical/regulatory relationship with the recombination apparatus — an adaptor role, not an enzymatic one.
Stress-dependent relocalization. In a proteome-wide survey of protein location and function under stress, Caf120 was one of several proteins that change subcellular location under DNA-damaging conditions (PMID: 25028499). Regulated relocalization is consistent with a PH-domain adaptor redeployed in response to signals; no kinase activity is described.
UniProt further curates CAF120 within the CCR4-NOT context (transcription regulation / mRNA deadenylation) and notes it "Belongs to the CAF120 family." A "Phosphoprotein" keyword marks CAF120 as a kinase substrate/target, not a kinase — a common source of confusion, but the opposite of the hypothesis. None of these curated or experimental descriptions invokes phosphotransfer chemistry.
The evidence assembles into a clear model of how a spurious kinase annotation arose and why it should be removed.
WHAT CAF120 ACTUALLY IS WHAT THE IBA CLAIMS
---------------------------- --------------------------
1060-aa protein "protein kinase activity"
| |
[ PH domain (PF00169) ] requires bilobal ePK fold
[ fungal Skg3/CAF120 PH ] |
[ IPR058155 family ] requires: GxGxxG P-loop
| VAIK Lys
lipid / protein binding HRD Asp
adaptor & recruitment DFG motif
| |
regulatory / substrate roles: NONE present in a catalytic
- meiotic crossover regulation domain in P53836
(Exo1/MutLgamma, PMID 31351878) |
- stress relocalization => structurally impossible
(PMID 25028499) to catalyze phosphotransfer
- CCR4-NOT context (UniProt)
- phosphoprotein = KINASE TARGET,
not kinase
How the mis-annotation propagated. GO:0004672 is annotated IBA (Inferred from Biological Ancestor) via GO_REF:0000033, the GO_Central phylogenetic pipeline, in which an experimentally supported annotation on one family member is propagated to un-characterized members in the same inferred ancestral clade. PH domains occur in many signaling proteins, and a subset of bona fide kinases — notably the AGC-family AKT/PKB — carry a PH module adjacent to their kinase domain. If the reference tree grouped CAF120's PH module with the PH modules of such kinases, "protein kinase activity" and "signal transduction" leaf annotations could propagate to CAF120 even though it has only the PH module. The co-occurrence of GO:0007165 (signal transduction) IBA in the same record is the fingerprint of exactly this PH-domain-driven mis-grouping.
The correct molecular picture. CAF120 is a non-catalytic PH-domain adaptor. Its PH β-sandwich most plausibly mediates membrane or protein interactions that position CAF120 within regulatory assemblies — the meiotic recombination network and the CCR4-NOT context. Its function is to localize and connect, not to phosphorylate.
| Citation | Evidence type | Supports/Refutes | Claim tested | Key finding | Context | Confidence / limitations |
|---|---|---|---|---|---|---|
| InterPro API (this run; IPR011993, IPR058155; Pfam PF00169, PF25381; PROSITE PS50003; SMART SM00233; Gene3D 2.30.29.30) | Structural/evolutionary (domain) | Refutes | Does CAF120 contain a protein-kinase domain? | Only PH domain + PH-like superfamily + fungal "Skg3/CAF120-like PH"; no kinase domain (no PF00069, no IPR000719, no PANTHER kinase family) | S. cerevisiae P53836 sequence | High. Absence of a kinase Pfam/InterPro hit is definitive for the ePK family |
| Sequence motif scan (this run) | Computational | Refutes | Are ePK catalytic motifs present? | None found: no GxGxxG P-loop, no VAIK Lys, no HRD, no PS00107/PS00108. Stray "DFG" tripeptides fall in N-terminal/disordered region, not a catalytic domain | 1060-aa sequence | High. Genuine ePKs require these motifs ordered within a ~250–300-aa domain |
| UniProt P53836 (Swiss-Prot, reviewed) | Review/database | Competing (different function) | What is CAF120's function? | CCR4-NOT context (transcription regulation / mRNA deadenylation); "Belongs to the CAF120 family"; keyword Phosphoprotein (substrate, not enzyme) | S. cerevisiae | High (manually curated). Function is adaptor/regulatory, not catalytic |
| PMID: 31351878 | Mutant/interaction (primary) | Competing | Native role of Caf120? | Identified as a regulator of meiotic crossing-over, a meiosis-specific interactor of the MutLγ–Exo1 recombination machinery | Yeast meiosis | Medium-high. No kinase activity implied |
| PMID: 25028499 | Localization (primary) | Qualifies | Condition-dependent behavior? | Caf120 changes subcellular location under DNA damage | Yeast stress proteomics | Medium. Regulatory relocalization, not enzymatic |
| GO:0004672 IBA + GO:0007165 IBA (GO_Central, GO_REF:0000033) | Computational (phylogenetic) | Source of hypothesis | Origin of kinase annotation | Both terms IBA from the same reference; kinase+signal-transduction pairing on a PH-only protein is a classic mis-propagation signature | GO_Central | The IBA is the artefact being evaluated, not independent support |
Lead (requires curator verification): Remove GO:0004672 (protein kinase activity, MF) from CAF120. The IBA lacks any catalytic-domain basis and conflicts with the curated molecular identity of the protein. Do not replace it with any other kinase/transferase MF term.
Reasoning by GO aspect:
GO decision table:
| Term | Aspect | Current | Recommended action | Basis |
|---|---|---|---|---|
| GO:0004672 protein kinase activity | MF | IBA (GO_REF:0000033) | Remove | No kinase domain; no catalytic residues |
| GO:0007165 signal transduction | BP | IBA (co-annotation) | Flag / likely remove | Same PH-domain mis-propagation signature |
| Meiotic recombination regulation | BP | — | Candidate add (curator verify) | PMID 31351878 |
The molecular function under test is catalytic transfer of a phosphate from ATP to a protein substrate (phosphotransfer). This requires a spatially organized active site — a P-loop and β3 lysine to bind/orient ATP, an HRD aspartate as catalytic base, and a DFG aspartate to coordinate Mg²⁺. CAF120 lacks every one of these, so the direct gene-product activity hypothesized is physically precluded.
What CAF120 does have is a PH domain, whose direct molecular activity is binding (phosphoinositide lipids and/or protein partners) — a recruitment/scaffolding activity. Everything else attributed to CAF120 — regulation of meiotic crossovers, stress relocalization, CCR4-NOT association, and its status as a phosphoprotein — is either a downstream, context-dependent regulatory role mediated through interactions, or evidence that CAF120 is a kinase target. The kinase hypothesis conflates "participates in signaling/regulatory processes" (plausible for a PH adaptor) with "is itself a catalytic kinase" (unsupported and structurally impossible).
| Gap | What was checked | Why it matters | What would resolve it |
|---|---|---|---|
| Exact PANTHER/PAINT node that seeded the IBA | InterPro PANTHER endpoint returned no record this run | Confirms the propagation source and lets curators fix the tree annotation | Query GO_Central/PANTHER PAINT tree for the P53836 leaf and its annotated ancestral node |
| No direct in-vitro kinase assay exists for CAF120 | PubMed searches found only CCR4-NOT / meiosis / stress roles | A direct negative assay would be definitive, though absence of catalytic residues already predicts no activity | In-vitro autophosphorylation/substrate assay (predicted negative) |
| AlphaFold fold confirmation of non-PH regions | AF model exists (P53836); much of the sequence is predicted disordered | Rules out a cryptic kinase fold | Inspect AlphaFold pLDDT/domain segmentation; expect PH domain + long disordered tail, no kinase lobe |
| PH-domain ligand specificity | Domain identity clear, but specific ligand not determined here | A positive MF term depends on it | Structural docking / lipid-overlay (PIP-strip) assays |
Additional limitation: the refutation is primarily bioinformatic (domain + motif) plus the absence of any positive experimental kinase report. There is no wet-lab negative kinase assay in the literature; however, the physical absence of the catalytic residues provides a strong mechanism-based refutation, not merely an evidential one.
Computational checks executed during this investigation:
- UniProt REST fetch of P53836 (function, features, GO, cross-references, sequence).
- InterPro API domain enumeration for P53836 (PH-only domain architecture; PF00169, PF25381, PS50003, SM00233, Gene3D 2.30.29.30, IPR011993, IPR058155; no kinase family).
- Regex/PROSITE scan of the 1060-aa sequence for ePK catalytic motifs (GxGxxG, VAIK, HRD, DFG, PS00107, PS00108) — all negative within a catalytic domain.
CAF120 (P53836) is a Pleckstrin-homology-domain adaptor protein of the fungal Skg3/CAF120-like family, with experimentally supported regulatory roles in meiotic crossover control and stress-responsive relocalization, and a database-curated association with the CCR4-NOT context. It has no protein-kinase domain and lacks every conserved catalytic residue required for phosphotransfer. The GO:0004672 (protein kinase activity) IBA annotation is a PH-domain-driven phylogenetic over-annotation and should be removed rather than retained, generalized, or replaced with another catalytic term.