β-pairing RFdiffusion was built for this kind of problem, generating binder backbones that form direct hydrogen-bonded β-sheet contacts with the target strand. Lengths from 80–200 aa were sampled and hotspot anchors were placed on both the disordered loop and the ordered RING domain. In practice, the RING domain didn't contribute much, while the loop was captured consistently and with clean geometry. ProteinMPNN was used for sequence design, designs were validated with ColabFold and Boltz-2, and the best were redesigned with ProteinMPNN and folded again.