A Nucleolar RNA Helicase Recognized by Autoimmune Antibodies from a Patient With Watermelon Stomach Disease

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Abstract
Watermelon stomach is characterized by prominent stripes of ectatic vascular tissue in the stomach similar to stripes on a watermelon; in patients with this disorder chronic gastrointestinal bleeding occurs and approximately half of these patients have associated autoimmune disorders. In the serum of one patient, an antinucleolar antibody titer of 1:25 600 was found; the antibodies specifically recognized an approximately 100 kDa nucleolar protein, which we referred to as the 'Gu' protein. Its cDNA was cloned and sequenced. The Gu protein is a member of a new subgroup of RNA helicases, the DEXD box family. Gu protein fused with glutathione S-transferase contains ATP-dependent RNA helicase activity which preferably translocates in the 5'-->3' direction. Its RNA folding activity, RNA-dependent ATPase and dATPase activities, and its translocation direction are similar to those of RNA helicase II [Flores-Rozas, H. and Hurwitz, J. (1993) J. Biol. Chem. 268, 21372-21383]. Sequencing of 209 amino acids of RNA helicase II peptides showed 96.7% identity with the cDNA-derived amino acid sequence of the Gu protein. The precise biological roles of this RNA helicase in the biogenesis of ribosomal RNA and the pathogenesis of watermelon disease and autoimmune disorder require further study.