Importin-9 recognizes the winged-helix fold of ETS transcription factors to mediate nuclear import.

Abstract

Protein trafficking between the cytoplasm and the nucleus is a fundamental process in eukaryotic cell biology. While linear nuclear localization signals (NLSs) are well characterized, many nuclear proteins lack a predictable NLS. Here, we identify the ETS domain, a DNA-binding winged-helix fold, from ETS family transcription factors as a structure-encoded NLS. We show that ETS domains mediate nuclear import through direct nanomolar affinity recognition by IPO9. Cryo-electron microscopy analysis of the EHF:IPO9 complex reveals that the IPO9 wraps around the ETS domain and engages structural features throughout the winged-helix fold. Biochemical studies demonstrate that the ETS domain DNA-binding helix is critical for importin recognition and for NLS activity in mammalian cells. Comparison of IPO9 bound to EHF and the histone H2A:H2B dimer reveals distinct interaction hotspots, illustrating how IPO9 employs unique combinatorial binding surfaces to accommodate structurally diverse cargos. These findings define a unique class of globular NLSs and highlight the adaptability of importins in recognizing distinct protein folds.