Development of polyHis-targeting chimeras (HisTACs) for targeted protein degradation

Induced-proximity degraders hold great promise for oncology, yet their utility remains constrained by the need for high-affinity ligands for each protein of interest. Tag-based degrader systems provide ligand-independent surrogates but are limited by large tag size, risk of tag-driven ubiquitination, and difficulty of engineering substantial tags into endogenous genes. To address these challenges, we developed a polyhistidine-directed degrader platform (HisTAC) that uses Ni²⁺-nitrilotriacetic acid (Ni²⁺-NTA) to engage His-tagged proteins and recruit CRBN or VHL E3 ligases.Cellular uptake was first confirmed with a fluorescent His₆-CPP/Ni-NTA probe, which exhibited rapid entry and widespread intracellular distribution. We then generated a HeLa cell line with endogenously His₆-HiBiT-tagged BRD4 and screened a panel of Ni²⁺-NTA-based degraders. Constructs containing two Ni²⁺-NTA groups displayed the highest potency, yielding >50% BRD4 loss at micromolar levels. MG-132 co-treatment fully blocked BRD4 depletion, confirming proteasome dependence. Immunoblotting further revealed preferential degradation of BRD4 long isoform, with moderate reduction of the short isoform. VHL-recruiting HisTACs also induced potent, isoform-selective BRD4 degradation with sub-micromolar efficacy toward the long isoform.To test the generalizability of this platform, we engineered a polyHis-HiBiT-tagged PSPC1 cell line-an RNA-binding protein lacking tractable ligands. Both CRBN- and VHL-based HisTAC degraders induced rapid, dose-dependent PSPC1 degradation, demonstrating the applicability of Ni²⁺-NTA-guided proximity to traditionally “undruggable” targets. Together, these results establish HisTAC degraders as a versatile, ligand-independent strategy for probing challenging cancer targets and accelerating degrader discovery and mechanistic studies in cancer research.