Biochemical and structural studies of the midnolin Catch domain bound with both wild-type and mutant IRF4 peptides reveal the molecular basis for its broad substrate specificity

The newly discovered midnolin-proteasome pathway is a unique ubiquitin-independent mechanism for degrading nuclear proteins, which is crucial for maintaining cellular protein homeostasis. The Catch domain of midnolin is essential for substrate recognition and binding, yet the underlying mechanism for its broad substrate specificity remains elusive. Transcription factor IRF4, essential for the functions of B and T cells, is a substrate of midnolin. This study presents comprehensive biochemical and structural analyses of the human midnolin Catch domain in complexes with both wild-type and mutant IRF4 peptides. The crystal structure of the Catch-IRF4 complex reveals that the Catch domain creates a substrate-binding groove at the interface of the Catch1 and Catch2 subdomains, recognizing and binding to the 215-QVTGTFYAC-223 sequence motif of IRF4. The binding motif of IRF4 forms a β-strand that is embedded into the substrate-binding groove, resulting in an antiparallel five-stranded β-sheet. The interactions between the IRF4 peptide and the Catch domain are predominantly hydrophobic and exhibit high spatial complementarity. Additionally, the biochemical, modeling and structural data indicate that the V2 and A8 positions of the IRF4 sequence motif can be substituted with other hydrophobic or small polar residues (G/A/V/L/I/M/P/F/Y/C/S/T), but not with large polar and charged residues (D/N/E/Q/H/K/R). The G4 position can be replaced by Ser, while the F6 position can be substituted with Tyr. These results suggest that the Catch domain can recognize and bind to a variety of substrates containing the sequence motif x[G/A/V/L/I/M/P/F/Y/C/S/T]x[G/S]x[F/Y]x[G/A/V/L/I/M/P/F/Y/C/S/T]x or briefly the G/SxF/Y motif (where x represents polar residues) located in an unstructured or loop region on the protein surface, and the hydrophobic interactions and spatial complementarity between the binding motifs of substrates and the Catch domain govern the substrate specificity. Collectively, these findings elucidate the molecular basis for midnolin's broad substrate specificity.