Midnolin-directed protein degradation beyond ubiquitin.

Targeted protein degradation has emerged as a powerful way to eliminate disease-driving proteins rather than merely inhibit them. However, most current approaches depend on ubiquitin ligases, whereas midnolin bypasses E3 recruitment and substrate ubiquitination while converging on the same 26S proteasome used for canonical ubiquitin-dependent degradation. Recent structural, biochemical, and disease studies have defined how midnolin recognizes β-strand-forming degrons, positions substrates at the 26S proteasome, and contributes to interferon regulatory factor 4 regulation, while engineered midnolin-targeting chimeras and midnolin-based targeting chimeras have redirected this pathway toward β-catenin and c-Myc. By linking endogenous pathway biology with engineered degrader pharmacology, this review compares midnolin-directed degradation with proteolysis-targeting chimeras and molecular glue degraders and identifies the design, delivery, and safety principles that will determine whether it becomes a useful ubiquitin-independent complement to existing degraders.