Nucleolar RNA Helicase Research Articles

Adriamycin Inhibits Human RH II/Gu RNA Helicase Activity by Binding to Its Substrate

RNA helicases are enzymes important in RNA synthesis, processing, transport, and turnover. Human nucleolar RNA helicase II/Gu protein (RH II/Gu) was expressed in a baculovirus system. The purified recombinant RH II/Gu protein has RNA helicase activity on a 5′ tailed ds RNA substrate in vitro. We found that Adriamycin, a widely used anticancer drug, inhibited RH II/Gu helicase activity in a dose-dependent manner with an IC50 of 40 μM. Adriamycin bound to the RNA substrate, and the binding was disrupted by boiling or treatment with 1% SDS, suggesting that the binding of Adriamycin to RNA is reversible. Adriamycin was also found by gel electrophoresis to bind to yeast tRNA to form slow-migrating complexes. These results suggest that Adriamycin can inhibit RNA synthesis or processing by binding to RNA substrates.

Mapping and Characterization of the Functional Domains of the Nucleolar Protein RNA Helicase II/Gu

RNA helicase II/Gu (RH-II/Gu) is a nucleolar RNA helicase of the DEAD-box superfamily. In this study, the functional domains of RH-II/Gu molecule were mapped by fusing the protein or its deletion mutants with a green fluorescence protein and subsequently transfecting or microinjecting the recombinant constructs into HeLa cells. In addition to the identification of a nuclear localization signal (NLS) in the N-terminus and a nucleolar targeting signal in the central helicase domain, a hidden NLS and a nucleolar targeting signal were found in the C-terminal arginine/glycine-rich domain. RH-II/Gu colocalized with fibrillarin, a component of the dense fibrillar region of the nucleolus. Overexpression of the entire RH-II/Gu protein or specific domains of the protein in HeLa cells did not interfere with the normal distribution of fibrillarin. However, when the helicase domain was truncated, the distribution pattern of fibrillarin was distorted. Microinjection of the wild-type RH-II/Gu cDNA into the nucleus of HeLa cells did not disrupt normal cell growth. However, when cells were injected with mutant DNA, only a small percentage of HeLa cells progressed through the cell cycle. Analysis of centrosomes in transfected cells demonstrated that most of the mutant-expressing cells were arrested early in the cell cycle. The results suggest that each of the structural domains of RH-II/Gu is necessary for cell growth and cell cycle progression.

Green fluorescent protein tag for studies of drug-induced translocation of nucleolar protein RH-II/Gu.

We have constructed a human osteogenic sarcoma cell line, U-2 OS/GFP-Gu, that expresses nucleolar RNA helicase RH-II/Gu tagged with green fluorescent protein (GFP). The presence of a GFP tag does not inhibit RNA helicase, RNA folding and ATPase activities of RH-II/Gu protein. The derived cell line responds to cytotoxic agents like the parental cell line U-2 OS. In the presence of either actinomycin D or toyocamycin, the GFP-RH-II/Gu fusion protein translocates from the nucleolus to the nucleoplasm in the same way as the translocation of endogenous RH-II/Gu. The drug-induced translocation of GFP-RH-II/Gu is easily monitored by direct observation of live cells in vivo. This cell line can be used to screen cytotoxic drugs and to study the mechanisms of drug-induced translocation of RH-II/Gu. The cellular localization of RH-II/Gu during the cell cycle-dependent formation of the nucleolus is readily monitored. Real-time results are obtained more quickly without the disadvantages associated with cell fixation and immunofluorescence-based staining.

Effects of Cytotoxic Drugs on Translocation of Nucleolar RNA Helicase RH-II/Gu

Some cytotoxic drugs cause translocation of nucleophosmin/B23 and other nucleolar proteins to the nucleoplasm. The present study shows that these drugs caused a similar translocation of RH-II/Gu, a nucleolar RNA helicase. Other nucleolar proteins including p120, UBF, RNA polymerase I large subunit, fibrillarin, p40, and Ren-1 did not translocate. A 2-h treatment of MCF-7 breast cancer cells with 0.008 or 0.16 μMactinomycin D resulted in translocation of RH-II/Gu to the nucleoplasm; these effects were not reversed by 100 μMguanosine. The effects of 0.008 μMactinomycin D, but not 0.16 μMactinomycin D, on the translocation of RH-II/Gu were reversed when the drug was removed. However, the effects of 0.008 or 0.16 μMactinomycin D on the translocation of nucleophosmin/B23 were not reversible. The translocation effects of 50 μMtoyocamycin on RH-II/Gu were reversed when the drug was replaced with fresh medium. RH-II/Gu mostly relocalized to the nucleoli within 15 min after toyocamycin was withdrawn; only partial relocalization of nucleophosmin/B23 occurred 40 h after removal of the drug. The effects of toyocamycin were not blocked by 100 μMguanosine. Mycophenolic acid (50 μM,2-h treatment) caused partial translocation of RH-II/Gu; this effect was slowly reversed upon drug removal and was inhibited by 100 μMguanosine, in a manner similar to the effects of mycophenolic acid on the localization of nucleophosmin/B23. This study shows similarities and differences in the drug-induced translocation and relocalization of RH-II/Gu and nucleophosmin/B23. Analysis of translocation of specific nucleolar proteins may offer a quantitative approach to assessment of potency and duration of effects of cytotoxic agents.

Autoantibodies to a nucleolar RNA helicase protein in patients with connective tissue diseases.

An autoantibody to a nucleolar RNA helicase protein (Gu) was recently discovered in a patient with gastric antral vascular ectasia or watermelon stomach, a disorder that is increasingly being described in systemic sclerosis (SSc). The present study was undertaken to determine whether anti-Gu antibodies occur in connective tissue diseases (CTD) and, if so, to determine their frequencies and any clinical or immunogenetic associations. Anti-Gu antibodies were determined by Western blotting of glutathione-purified glutathione S transferase-Gu fusion proteins against consecutive antinucleolar antibody-positive sera (HEp-2 cell substrate) collected over a 5-year period in a rheumatology antinuclear antibody (ANA) testing laboratory. Anti-Gu antibodies were found in 11 (10%) of 108 antinucleolar antibody-positive sera. The subjects with anti-Gu antibodies included 3 of 46 patients with SSc (7%), 3 of 17 patients with systemic lupus erythematosus (18%), 4 of 9 patients with undifferentiated CTD (44%), and 1 healthy relative of an SSc patient. None of the anti-Gu-positive patients had any symptoms suggestive of watermelon stomach. Increased frequencies of both HLA-DQA1*0501 and DQB1*0301 were found, but only DQB1*0301 maintained statistical significance after correction. Anti-Gu (nucleolar RNA helicase) antibodies occur in low frequencies in patients with CTDs who have antinucleolar antibodies by ANA testing, but they are not specific for SSc or the watermelon stomach lesion.

Cloning and Characterization of Gu/RH-II Binding Protein

Gu/RNA helicase II (Gu/RH-II) is the first reported mammalian nucleolar RNA helicase that is a member of the D-E-A-D (Asp-Glu-Ala-Asp) box family of proteins. It has an ATP-dependent RNA unwinding (helicase) activity and a separate RNA folding activity (introduction of intramolecular secondary structure into single-stranded RNA). To determine which proteins may bind to Gu/RH-II, a yeast two-hybrid system was used. A cDNA which encoded a protein, called Gu/RH-II binding protein or GBP, was isolated and sequenced. The GBP protein is localized to the nucleus in speckled or diffuse nucleoplasmic patterns. The GBP mRNA level is highest in testis, 9- to 49-fold greater than other tissues. When GBP interacts with Gu/RH-II, proteolytic cleavage of Gu/RH-II occurs; the amino-terminal portion of Gu/RH-II is critical for this proteolysis.