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Several important cellular processes, including transcription, nucleotide excision repair and cell cycle control are mediated by the multifaceted interplay of subunits within the general transcription factor II H (TFIIH).
A better understanding of the molecular structure of TFIIH is the key to unravel the mechanism of action of this versatile protein complex within these pathways. This becomes especially important in the context of severe diseases like xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy, that arise from single point mutations in some of the TFIIH subunits.
In an attempt to structurally characterize the TFIIH complex, we harnessed the qualities of the eukaryotic thermophile Chaetomium thermophilum, a remarkable fungus, which has only recently been recognized as a novel model organism. Homologues of TFIIH from C. thermophilum were expressed in E. coli, purified to homogeneity and subsequently utilized for crystallization trials and biochemical studies.
The results of the present work include the first crystal structure of the p34 subunit of TFIIH, comprising the N-terminal domain of the protein. The structure revealed a von Willebrand Factor A (vWA) like fold, which is generally known to be involved in a multitude of protein-protein interactions. Structural comparison allowed to delineate similarities as well as differences to already known vWA domains, providing insight into the role of p34 within TFIIH. These results indicate that p34 assumes the role of a structural scaffold for other TFIIH subunits via its vWA domain, while likely serving additional functions, which are mediated through its
C-terminal zinc binding domain and are so far unknown.
Within TFIIH p34 interacts strongly with the p44 subunit, a positive regulator of the XPD helicase, which is required for regulation of RNA Polymerase II mediated transcription and essential for eukaryotic nucleotide excision repair. Based on the p34 vWA structure putative protein-protein interfaces were analyzed and binding sites for the p34 p44 interaction suggested. Continuous crystallization efforts then led to the first structure of a p34 p44 minimal complex, comprising the N-terminal vWA domain of p34 and the C-terminal C4C4 RING domain of p44. The structure of the p34 p44 minimal complex verified the previous hypothesis regarding the involved binding sites. In addition, careful analysis of the complex interface allowed to identify critical residues, which were subsequently mutated and analyzed with respect to their significance in mediating the p34 p44 interaction, by analytical size exclusion chromatography, electrophoretic mobility shift assays and isothermal titration calorimetry. The structure of the p34 p44 complex also revealed a binding mode of the p44 C4C4 RING domain, which differed from that of other known RING domains in several aspects, supporting the hypothesis that p44 contains a novel variation of this domain.
XPD is a 5‘-3‘ helicase of the superfamily 2. As part of the transcription factor IIH it functions in transcription initiation and nucleotide excision repair. This work focus on the role of XPD in nucleotide excision repair. NER is a DNA repair pathway unique for its broad substrate range. In placental mammals NER is the only repair mechanism able to remove lesions induced by UV-light. NER can be divided into four different steps that are conserved between pro- and eukaryotes. Step 1 consists of the initial damage recognition, during step 2 the putative damage is verified, in step 3 the verified damage is excised and in the 4th and final step the resulting gap in the DNA is refilled. XPD was shown to be involved in the damage verification step. It was possible to solve the first apo XPD structure by a MAD approach using only the endogenous iron from the iron sulfur cluster. Based on the apo XPD structure several questions arise: where is DNA bound? Where is DNA separated? How is damage verification achieved? What is the role of the FeS cluster? These questions were addressed in this work. Hypothesis driven structure based functional mutagenesis was employed and combined with detailed biochemical characterization of the variants. The variants were analyzed by thermal unfolding studies to exclude the possibility that the overall stability could be affected by the point mutation. DNA binding assays, ATPase assays and helicase assays were performed to delineate amino acid residues important for DNA binding, helicase activity and damage recognition. A structure of XPD containing a four base pair DNA fragment was solved by molecular replacement. This structure displays the polarity of the translocated strand with respect to the helicase framework. Moreover the properties of the FeS cluster were studied by electron paramagnetic resonance to get insights into the role of the FeS cluster. Furthermore XPD from Ferroplasma acidarmanus was investigated since it was shown that it is stalled at CPD containing lesions. The data provide the first detailed insight into the translocation mechanism of a SF2B helicase and reveal how polarity is achieved. This provides a basis for further anlayses understanding the combined action of the helicase and the 4Fe4S cluster to accomplish damage verification within the NER cascade.