BGPT: Paper Review: C-terminal amides mark proteins for degradation via SCF

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Quick Explanation Copied

Core claim (paper): C-terminally amidated proteins (“CTAPs”) are rapidly cleared via SCF–FBXO31, where FBXO31 acts as a reader that binds the terminal amide with nanomolar affinity while strongly excluding unmodified carboxylates.

Most important critique: The mechanism is impressively reductionist (semi-synthetic reporters + orthogonal binding/ubiquitylation/clearance assays), but large parts of “in vivo” evidence rely on fragment-generated CTAP discovery from MS workflows and contextual oxidative-stress models. Key uncertainties remain: (i) how quantitatively CTAP levels scale across tissues and physiological stress regimes; (ii) whether alternative fragmentation/amidation routes or non-CTAP damages also contribute; (iii) generality beyond the tested substrates and models.

Long Explanation

Paper

C-terminal amides mark proteins for degradation via SCF–FBXO31

Nature • published 29 Jan 2025 • DOI: 10.1038/s41586-024-08475-w

What’s new?

CTAPs (C-terminally amidated “modified amino acid degrons”) are presented as a chemical, sequence-agnostic proteasome trigger read by FBXO31 within SCF.

Key mechanism

FBXO31 binds the terminal amide (not carboxylate) and SCF–FBXO31 ubiquitylates amidated clients for proteasomal clearance.

Physiological story

Oxidative fragmentation is proposed to generate amidated neo-C termini in vivo that FBXO31 can then recognize under stress.

Figure A — Binding selectivity: amide vs carboxylate

Reported dissociation constants (Kd) for FBXO31-SKP1 binding to the screened amidated C-terminal peptide versus its carboxylate form, plus a small set of example terminal-amino-acid dependences for X–CONH2.

Figure B — Breadth: FBXO31 binds many amidated termini

The paper’s pooled in vitro peptide interaction screen reports thousands of tested C-terminal variants and highlights that amidated termini are enriched among FBXO31 binders relative to unmodified carboxy termini.

Figure C — Oxidative stress linkage: number of CTAPs and putative tissue proteins

Using reanalysis criteria for CTAP-associated amidation signatures, the paper reports counts of CTAP cleavage sites and distinct proteins per tissue sample, as well as a mapping onto oxidative fragmentation logic.

Figure D — Stress-activated CRLs: FBXO31 ranks among activated cullin-RING complexes

In active CRL profiling after acute H2O2 challenge in K562 cells, the paper reports FBXO31 as one of the strongest stress-activated CRLs and reports that oxidative stress increases CRL targeting of many proteins, with haemoglobin subunits highlighted among the rewired targets.

Figure E — D334N mutation redirects FBXO31: predicted neosubstrate engagement scale

The paper reports that when the dominant FBXO31 D334N variant is expressed (in an F-box–mediated SCF context), it co-purifies with many proteins not detected for WT FBXO31, and it identifies SUGT1 and other essential factors as neosubstrates.

Model map — CTAP → FBXO31 → ubiquitylation → proteasome clearance

This diagram is a compact restatement of the paper’s mechanism and stress-generation hypothesis.

Stepwise evaluation (what is strongly supported vs what is more tentative)

1) Chemical causality in cells (amide is sufficient)

The strongest evidence comes from semi-synthetic reporters: in-cell degradation kinetics show that a terminal amide enables rapid clearance, while the corresponding unmodified C-terminus is stable; proteasome/ubiquitylation inhibition prevents CTAP clearance and lysosome inhibition does not.

Skeptical note: “Sufficient” was tested against a limited set of reporter architectures; however, the authors explicitly report multiple contexts and peptide/protein reporters, which reduces—but cannot eliminate—the chance that this is a reporter-specific artifact.

2) Identification of the reader machinery (CRISPR → FBXO31; requirement for SCF assembly)

A genome-wide CRISPR screen in inducible Cas9 K562 cells identifies FBXO31 as the top CTAP-clearance factor, with additional hits consistent with SCF/CRL biology (e.g., CUL1 and COP9 signalosome subunits). FBXO31 CRISPRi/KO stabilizes CTAP reporters; rescue by wild-type FBXO31 restores degradation, while loss of the F-box domain prevents rescue.

Potential blind spot: While the CRISPR screen supports necessity, it does not alone establish that FBXO31 is sufficient to drive clearance for all endogenous CTAPs—sufficiency for endogenous substrates depends on the stress-generated CTAP mapping and client validation steps.

3) Orthogonality: binding and productive ubiquitylation are amide-dependent

The authors provide two strong orthogonal biochemical links: (i) peptide binding by FBXO31-SKP1 via fluorescence polarization, and (ii) productive SCF-FBXO31 ubiquitylation only of amide-bearing clients (not carboxylate) in vitro and with strict discrimination when mixed substrates are co-present.

4) Broad client sampling: pocket recognition yields “sequence-agnostic” chemical surveillance

The peptide library screen indicates FBXO31 binds a large number of amidated termini (841 amidated binders vs 73 unmodified binders) with only modest preferences for residue identity at the terminal position, and follow-up reporter assays show that top motifs become destabilized when amidated and the effect is rescued by FBXO31 knockdown.

Key logic check: “Agnostic to sequence motifs” is consistent with large binder diversity, but the terminal residue identity still modulates binding affinities (e.g., strong reduction for X=Asp-CONH2 vs X=Phe-CONH2). So the surveillance is chemically specific (amide) but not perfectly uniform across all amide-bearing termini.

5) Oxidative stress and CTAP formation: MS discovery + functional client validation

The physiological claim has two major links: (i) CTAPs can form via oxidative fragmentation and amidation chemistry consistent with a neo-C terminus signature in proteomics; (ii) FBXO31 clients increase under oxidative stress, and amidated peptide fragments derived from specific clients drive FBXO31-dependent reporter degradation.

Primary uncertainty (important): MS-based detection of amidated neo-termini is sensitive to search parameters, confidence thresholds, and fragmentation artifacts. The paper’s stringent approach helps, but the remaining question is quantification and causality in intact physiology: which CTAPs form, where, when, and how often they drive measurable flux through FBXO31-dependent clearance pathways.

6) Disease mutation: mechanism is re-routed binding selectivity, not merely loss of function

A dominant D334N mutation is used as a mechanistic probe: it abolishes CTAP recognition, yet SCF–FBXO31(D334N) binds and ubiquitylates many neosubstrates. The authors report specific motif dependence for D334N neosubstrates (basic residue at -3 and hydrophobic at -1 relative to the C terminus), and that the toxic activity requires SCF assembly (F-box).

Counterpoints / limitations / blind spots (critical but fair)

Cell-model dependence: Many mechanistic links are demonstrated in human cell lines and with delivered reporters; generality across primary tissues and physiological oxidative stress rhythms is not fully established by this paper alone.
Proteomics discovery biases: Identification of amidated neo-termini depends on search settings and confidence thresholds; missing data could bias the inferred distribution of CTAPs.
Mechanistic completion: The paper establishes the reader/ligation logic for CTAPs; however, it does not prove that all oxidative protein damage that leads to degradation proceeds via CTAP formation exclusively. Alternative MAADs or other fragmentation chemistries may also contribute.

Related follow-up (useful to readers): FBXO31 recruiting chemical degrons

A later study (not part of this paper) reports translating FBXO31 recognition of terminal amide degron chemistry into small-molecule degraders by engineering terminal amide-functionalized ligands that recruit FBXO31 to proteins for proteasome degradation. This provides external support that the amide-reader concept can be harnessed for engineered degradation systems, but it is still a distinct experimental line (chemical recruiters rather than CTAPs produced by oxidative fragmentation).

Fast “how to falsify” checklist (actionable experiments conceptually)

Below are mechanistic falsification routes aligned to the paper’s central claims (amide reader selectivity and CTAP generation → FBXO31-dependent degradation):

Demonstrate in a physiologically relevant system that CTAPs (amidated neo-C termini) are not required for stress-induced clearance of the candidate FBXO31 clients—i.e., show that blocking amidation chemistry does not preserve client abundance when oxidative fragmentation occurs.
Show that FBXO31 pocket mutations abolish CTAP recognition in cells but do not prevent clearance of endogenous CTAP-linked clients under oxidative stress.
Quantify the flux contribution: determine what fraction of oxidative proteostasis decline is actually attributable to CTAP→FBXO31, rather than other damage-to-degradation routes.

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Updated: April 30, 2026