Folding scFv–Antigen Complexes at Scale
1. The paper introduces SCALE (scFv–Ag CompLex Ensembles): a large benchmarking resource to stress-test modern cofolding models specifically on scFv–antigen docking, where correct epitope engagement and binding geometry are often the true failure mode.
2. SCALE is built from a curated subset of 3,800 experimentally solved Ab–Ag complexes from SAbDab, standardized by converting each heavy/light antibody into an scFv sequence (VH (GGGGS)3 linker VL) paired with a single antigen chain, then deduplicated by scFv–antigen sequence identity.
3. Using multiple state-of-the-art folding backbones (AlphaFold 2.3 Multimer, AlphaFold 3, Boltz-2, Chai-1, Pairmixer) and diverse inference-time settings (random seeds, recycling depth, optional antibody MSA, optional templates), the authors generate ~197,900 predicted scFv–Ag complexes—an ensemble-centric view rather than single-shot evaluation.
4. Interface accuracy is evaluated with DockQ against experimental references, separately for VH–Ag and VL–Ag interfaces (and often summarized as their mean). Across ~200k predictions, most DockQ scores are very low; near-native interfaces (DockQ > 0.8) exist but are rare for every model, indicating that correct docking is not consistently recovered even when tertiary folds look confident.
5. Best-of-ensemble behavior is notably better than the typical sample: with enough trials, many targets yield at least one “acceptable” interface (DockQ > 0.23). Still, 879/3,800 complexes never exceed DockQ 0.23 under any tested setting, while only 256 exceed 0.23 across all conditions—highlighting a large “hard target” regime.
6. The study finds strong coupling between VH–Ag and VL–Ag interface quality (Pearson r = 0.958), with VL–Ag slightly harder on average. This suggests that when docking fails, it often fails globally (wrong pose/epitope) rather than in only one variable domain.
7. A central result: confidence metrics commonly used in binder pipelines (ipTM, ipSAE, pDockQ, pDockQ2, AbEpiScore) correlate well with DockQ when pooling all predictions globally, but perform poorly at selecting the best structure within each target’s ensemble (low per-complex correlations and low Top-1 accuracy). In other words, they separate “easy vs hard targets” better than “best vs second-best pose for the same target.”
8. The paper documents a key practical pitfall: high single-chain confidence does not imply correct complex formation. Many predictions have high pLDDT yet extremely low DockQ, underscoring a decoupling between confident tertiary structure and correct quaternary docking.
9. Inference-time choices matter but mainly for best-case outcomes: more recycling can occasionally refine already-good predictions toward near-native interfaces without shifting the median much; additional seed sampling often shows diminishing returns, but a subset of targets benefits substantially. Including an antibody MSA improves the frequency of higher-quality interfaces even though antibody sequences may have limited evolutionary signal, while templates provide only modest gains in this setting.
10. Physics-based interface descriptors computed with PyRosetta (e.g., shape complementarity, estimated binding energy, clash/repulsion terms) correlate with DockQ roughly as well as learned confidence scores for ranking, suggesting that reranking/scoring—possibly combining model confidences with physical interaction features—is a major bottleneck for scalable scFv–Ag docking.
💻Code:
huggingface.co/datasets/ravi…
📜Paper:
biorxiv.org/content/10.64898…
#ComputationalBiology #ProteinFolding #Antibodies #StructuralBiology #Benchmark #AlphaFold #Docking #MachineLearning #Bioinformatics