Interplay of the Tfb1 Pleckstrin homolog domain with Rad2 and Rad4 in transcription coupled and global genomic nucleotide excision repair

Transcription-coupled repair (TCR) and global genomic repair (GGR) are two subpathways of nucleotide excision repair (NER). The TFIIH subunit Tfb1 contains a Pleckstrin homology domain (PHD), which was shown to interact with one PHD-binding segment (PB) of Rad4 and two PBs (PB1 and PB2) of Rad2 in vitro. Whether and how the different Rad2 and Rad4 PBs interact with the same Tfb1 PHD, and whether and how they affect TCR and GGR within the cell remain mysterious. We found that Rad4 PB constitutively interacts with Tfb1 PHD, and the two proteins may function together within one stable module for damage recognition in TCR and GGR. Rad2 PB1 interacts with Tfb1 PHD at a basal level when NER is inactive, and their interaction augments in a late step of NER. The augmented interaction contributes to both TCR and GGR. Rather than interacting with Tfb1 PHD, Rad2 PB2 constrains the basal interaction between Rad2 PB1 and Tfb1 PHD, thereby permitting their augmented interaction after NER is activated in the cell. Remarkably, the constraint also contributes to TCR and GGR. Our results elucidate the intricate interplay among Tfb1, Rad4, and Rad2 during NER, and unveil previously unexpected regulation mechanisms for NER.