RBX1 (ROC1) is the catalytic RING subunit of Cullin-RING ligases (CRLs), which govern ubiquitin transfer to many cancer-relevant substrates. This makes RBX1 a high-leverage target, but also a difficult one: the interface is compact, metal-stabilized, and functionally constrained by E2 recruitment geometry. Prior work established that the ROC1/RBX1 RING region is the catalytic hub for E2 activation in CRL assemblies [1]. More specifically, structural/NMR and biochemical studies on RBX1 residues 40-108 identified the E2-facing alpha2 surface (including W87/R91/V93) as a key determinant of CDC34~ubiquitin recognition and ubiquitin-transfer competence [2]. These data directly support focusing design efforts on this hotspot-centered surface rather than diffuse surface binding.

There is also direct precedent that this RBX1 face is inhibitable at the protein-interface level: GLMN was shown structurally to bind RBX1 in a RBX1-CUL1 complex and mask the E2-binding surface, suppressing CRL chain formation [3]. In parallel, chemical biology studies demonstrated that CRL activity can be reduced by disrupting the same E2-recruitment pathway (e.g., Cdc34 recruitment blockade at the CUL1-RBX1 catalytic axis) [4]. Together, prior work argues that interfering with E2 engagement is a biologically validated strategy, and that RBX1-centered interface blockade is mechanistically meaningful.

Miniproteins are well suited to this target class because they can present dense, preorganized side-chain geometry to engage shallow protein-protein interfaces with high shape complementarity, while remaining experimentally practical (rapid synthesis, scalable screening, and sequence diversification). For RBX1 specifically, compact binders are attractive for selectively occupying the E2-proximal alpha2 region and perturbing catalytic complex formation, while allowing iterative optimization for specificity and developability. In short, our design hypothesis is that a small, high-affinity protein interface ligand can functionally compete with native E2 recruitment at a structurally validated RBX1 hotspot surface, providing a focused route toward CRL-pathway modulation.

References:

1. Furukawa et al., J Biol Chem (2002), ROC1 RING and E2 activation: https://pubmed.ncbi.nlm.nih.gov/11861641/
2. Spratt et al., J Biol Chem (2012), RBX1 40-108 and alpha2 hotspot (W87/R91/V93): https://pubmed.ncbi.nlm.nih.gov/22433864/
3. Duda et al., Mol Cell (2012), GLMN masks RBX1 E2-binding surface: https://pubmed.ncbi.nlm.nih.gov/22748924/
4. Wu et al., PNAS (2016), CRL inhibition via E2-recruitment disruption (suramin): https://pubmed.ncbi.nlm.nih.gov/27001857/