Structural basis of TFIIH activation for nucleotide excision repair
Genomes are constantly threatened by DNA damage, but cells can remove a large variety of DNA lesions by nucleotide excision repair (NER). Mutations in NER factors compromise cellular fitness and cause human diseases such as Xeroderma pigmentosum (XP), Cockayne syndrome and trichothiodystrophy. The NER machinery is built around the multisubunit transcription factor IIH (TFIIH), which opens the DNA repair bubble, scans for the lesion, and coordinates excision of the damaged DNA single strand fragment. TFIIH consists of a kinase module and a core module that contains the ATPases XPB and XPD. Here we prepare recombinant human TFIIH and show that XPB and XPD are stimulated by the additional NER factors XPA and XPG, respectively. We then determine the cryo-electron microscopy structure of the human core TFIIH-XPA-DNA complex at 3.6 Å resolution. The structure represents the lesion-scanning intermediate on the NER pathway and rationalizes the distinct phenotypes of disease mutations. It reveals that XPB and XPD bind double- and single-stranded DNA, respectively, consistent with their translocase and helicase activities. XPA forms a bridge between XPB and XPD, and retains the DNA at the 5′-edge of the repair bubble. Biochemical data and comparisons with prior structures explain how XPA and XPG can switch TFIIH from a transcription factor to a DNA repair factor. During transcription, the kinase module inhibits the repair helicase XPD. For DNA repair, XPA dramatically rearranges the core TFIIH structure, which reorients the ATPases, releases the kinase module and displaces a 'plug' element from the DNA-binding pore in XPD. This enables XPD to move by ~80 Å, engage with DNA, and scan for the lesion in a XPG-stimulated manner. Our results provide the basis for a detailed mechanistic analysis of the NER mechanism.
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