Trapped in translocation: Stalling of XPD on a crosslinked DNA substrate

The super family 2 (SF2) helicase XPD is a central component of the general transcription factor II H (TFIIH) which is essential for transcription and nucleotide excision DNA repair (NER)1. Within these two processes XPDs helicase function is vital for NER but not for transcription initiation, where XPD only acts as a scaffold for other factors 2. We deciphered one of the most enigmatic steps in XPD helicase action: the active separation of dsDNA and its stalling upon approaching an interstrand crosslink, one of the most severe DNA damages in the cell, using cryo EM. Furthermore, the structure clearly shows how dsDNA is separated and reveals a highly unusual involvement of the Arch domain in active dsDNA separation. Combined with mutagenesis and biochemical analyses, we identify distinct functional residues important for helicase activity. Surprisingly, those areas also affect core TFIIH translocase activity, revealing a yet unencountered function of XPD within the TFIIH scaffold. Importantly, our structure provides a basis for XPD damage recognition and further suggests how the NER bubble could be formed, leading to a model for the location of the XPG nuclease relative to the excised damage.