Cellular studies of MrDb (DDX18)

Helicases are present in viruses, prokaryotes, and eukaryotes, and several of them have been linked to human diseases. Here we study the role of one putative DEAD-box RNA helicase, MrDb (DDX18), in tumor cells. We show that MrDb is a nucleolar protein ubiquitously expressed in tumor cells and that it is more abundantly expressed in proliferating cells. Inhibition of MrDb with dominant negative mutant or a shRNA reduces tumor cell proliferation without inducing a cell cycle arrest or apoptosis. These findings suggest that MrDb is important for cell proliferation and that its inhibition could prevent tumor cell proliferation.

Biochemical study of recombinant PcrA from Staphylococcus aureus for the development of screening assays

Helicases are ubiquitous enzymes, which utilize the energy liberated during nucleotide triphosphate hydrolysis to separate double-stranded nucleic acids into single strands. These enzymes are very attractive targets for the development of new antibacterial compounds. The PcrA DNA helicase from Staphylococcus aureus is a good candidate for drug discovery. This enzyme is unique in the genome of S. aureus and essential for this bacterium. Furthermore, it has recently been published that it is possible to identify inhibitors of DNA helicases such as PcrA. In this report, we study the properties of recombinant PcrA from S. aureus purified from Escherichia coli to develop ATPase and helicase assays to screen for inhibitors.

Study of the ATP-binding site of helicase IV from Escherichia coli

Helicases contain conserved motifs involved in ATP/magnesium/nucleic acid binding and in the mechanisms coupling nucleotide hydrolysis to duplex unwinding. None of these motifs are located at the adenine-binding pocket of the protein. We show here that the superfamily I helicase, helicase IV from Escherichia coli, utilizes a conserved glutamine and conserved aromatic residue to interact with ATP. Other superfamily I helicases such as, UvrD/Rep/PcrA also possess these residues but in addition they interact with adenine via a conserved arginine, which is replaced by a serine in helicase IV. Mutation of this serine residue in helicase IV into histidine or methionine leads to proteins with unaffected ATPase and DNA-binding activities but with low helicase activity. This suggests that residues located at the adenine-binding pocket, in addition to be involved in ATP-binding, are important for efficient coupling between ATP hydrolysis and DNA unwinding.

Inhibition of DNA helicases with DNA-competitive inhibitors

Helicases form an attractive protein family for drug discovery because they are involved in various human diseases. In this report, we show that it is possible to inhibit both the ATPase and the helicase activities of a DNA helicase with dibenzothiepins that bind at its nucleic acid binding site. These results suggest a drug discovery strategy to inhibit DNA helicases.

TFIIH contains a PH domain involved in DNA nucleotide excision repair

The human general transcription factor TFIIH is involved in both transcription and DNA repair. We have identified a structural domain in the core subunit of TFIIH, p62, which is absolutely required for DNA repair activity through the nucleotide excision repair pathway. Using coimmunoprecipitation experiments, we showed that this activity involves the interaction between the N-terminal domain of p62 and the 3' endonuclease XPG, a major component of the nucleotide excision repair machinery. Furthermore, we reconstituted a functional TFIIH particle with a mutant of p62 lacking the N-terminal domain, showing that this domain is not required for assembly of the TFIIH complex and basal transcription. We solved its three-dimensional structure and found an unpredicted pleckstrin homology and phosphotyrosine binding (PH/PTB) domain, uncovering a new class of activity for this fold.

TFIIH Transcription Factor, a Target for the Rift Valley Hemorrhagic Fever Virus

The Rift Valley fever virus (RVFV) is the causative agent of fatal hemorrhagic fever in humans and acute hepatitis in ruminants. We found that infection by RVFV leads to a rapid and drastic suppression of host cellular RNA synthesis that parallels a decrease of the TFIIH transcription factor cellular concentration. Using yeast two hybrid system, recombinant technology, and confocal microscopy, we further demonstrated that the nonstructural viral NSs protein interacts with the p44 component of TFIIH to form nuclear filamentous structures that also contain XPB subunit of TFIIH. By competing with XPD, the natural partner of p44 within TFIIH, and sequestering p44 and XPB subunits, NSs prevents the assembly of TFIIH subunits, thus destabilizing the normal host cell life. These observations shed light on the mechanism utilized by RVFV to evade the host response.

Basal transcription defect discriminates between xeroderma pigmentosum and trichothiodystrophy in XPD patients

Mutations in the XPD gene result in xeroderma pigmentosum (XP) and trichothiodystrophy (TTD), the phenotypes of which are often intricate. To understand the genotype/phenotype relationship, we engineered recombinant TFIIHs in which XPD subunits carry amino acid changes found in XPD patients. We demonstrate that all the XPD mutations are detrimental for XPD helicase activity, thus explaining the NER defect. We also show that TFIIH from TTD patients, but not from XP patients, exhibits a significant in vitro basal transcription defect in addition to a reduced intracellular concentration. Moreover, when XPD mutations prevent interaction with the p44 subunit of TFIIH, transactivation directed by certain nuclear receptors is inhibited, regardless of TTD versus XP phenotype, thus explaining the overlapping symptoms. The implications of these mutations are discussed using a structural model of the XPD protein. Our study provides explanations for the nature and the severity of the various clinical features.

Transcriptional activators stimulate DNA repair

To counteract the deleterious effects of genotoxic injury, cells have set up a sophisticated network of DNA repair pathways. We show that Gal4-VP16 and RAR transcriptional activators stimulate nucleotide excision repair (NER). This DNA repair activation is not coupled to transcription since it occurs in Cockayne syndrome cells (which are transcription-coupled repair deficient) and is observed in vitro in the presence of alpha-amanitin and in the absence of the basal transcription factors. Using a reconstituted dual incision assay, we also show that binding of activators to their cognate sequences induces a local chromatin remodeling mediated by ATP-driven chromatin remodeling and acetyltransferase activities to facilitate DNA repair.

p52 Mediates XPB function within the transcription/repair factor TFIIH

To further our understanding of the transcription/DNA repair factor TFIIH, we investigated the role of its p52 subunit in TFIIH function. Using a completely reconstituted in vitro transcription or nucleotide excision repair (NER) system, we show that deletion of the C-terminal region of p52 results in a dramatic reduction of TFIIH NER and transcription activities. This mutation prevents promoter opening and has no effect on the other enzymatic activities of TFIIH. Moreover, we demonstrate that intact p52 is needed to anchor the XPB helicase within TFIIH, providing an explanation for the transcription and NER defects observed with the mutant p52. We show that these two subunits physically interact and map domains involved in the interface. Taken together, our results show that the p52/Tfb2 subunit of TFIIH regulates the function of XPB through pair-wise interactions as described previously for p44 and XPD.