A cDNA that codes for a nucleolar RNA helicase has recently been cloned in our laboratory using an autoimmune serum from a patient with watermelon stomach disease. Later studies in our laboratory show that this autoantibody is also present in some patients with connective tissue diseases. This 100-kDa autoantigen, referred to as RH-II/Gu protein, is a unique bifunctional enzyme which possesses a 5' to 3' ATP-dependent RNA unwinding activity and a GTP-stimulated RNA folding/annealing activity. The two activities are oppositely regulated and reside in separate domains of the same polypeptide. This is the first mammalian prototype nucleolar RNA helicase to be identified. Electron microscopy shows its localization to nucleolar structures associated with transcription, early processing and maturation of rRNA. It is hypothesized that RH-II/Gu is involved in rDNA transcription, binding and/or unwinding of small nucleolar RNAs which direct site-specific cleavages and modifications of the pre-rRNA, or in the proper folding of rRNA prior to binding of ribosomal proteins. The roles of RNA helicases and RNA annealing proteins are well-established in the processing of pre-mRNA, but not in the processing of mammalian pre-rRNA. The long term objective of this proposed project is to do structural and functional characterizations of RH-II/Gu.
The specific aims are (1) to do structural analyses of the RH-II/Gu protein including crystal structure of its RNA folding domain, (2) to determine the role(s) of RH-II/Gu protein in ribosomal RNA biogenesis, and (3) to examine structural features of the substrates recognized by RH-II/Gu. The results from this study will provide useful information on structural characterization and regulation of RH-II/Gu, and on the mechanism of mammalian rRNA production, a less characterized biochemical pathway compared to pre-mRNA splicing. RH-II/Gu may be an excellent chemotherapeutic target, and structural characterization of this enzyme may lead to drug development. This study may also provide an impetus for understanding the pathological roles of RH-II/Gu on the elicitation of autoimmune responses.
