Pioneer-factor activity requires stable chromatin occupancy mediated by both sequence-specific binding and disordered protein domains
Pioneer transcription factors overcome the restrictive barrier imposed by chromatin to drive cell-fate specification, yet how their domains collectively support this activity remains unclear. Here, we use the deeply conserved pioneer factor Grainy head to define the protein-intrinsic features that govern pioneering activity. By integrating biochemistry, genomics and quantitative live-cell imaging, we determined that both the conserved DNA-binding domain and the extended, intrinsically disordered N-terminus are required for the stable chromatin occupancy that supports access to closed chromatin and the induction of chromatin accessibility. The disordered N-terminus supports pioneer activity through interactions that do not rely on strict amino acid sequence but instead overall composition. While our results show that pioneering activity depends on the combinatorial contributions of structured and disordered domains, mitotic retention depends solely on sequence-specific DNA binding. These results support stable chromatin occupancy mediated by multiple protein domains as necessary for pioneering function and that this is separable from the mechanisms required for mitotic retention.