Structural domains involved in the RNA folding activity of RNA helicase II/Gu protein

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Abstract
RNA helicase II/Gu (RH II/Gu) is a nucleolar protein that unwinds dsRNA in a 5′ to 3′ direction, and introduces a secondary structure into a ssRNA. The helicase domain is at the N‐terminal three‐quarters of the molecule and the foldase domain is at the C‐terminal quarter. The RNA folding activity of RH II/Gu is not a mere artifact of its binding to RNA. This study narrows down the RNA foldase domain to amino acids 749–801 at the C‐terminus of the protein. Dissection of this region by deletion and site‐directed mutagenesis shows that the four FRGQR repeats, as well as the C‐terminal end bind RNA independently. These juxtaposed subdomains are both important for the RNA foldase activity of RH II/Gu. Mutation of either repeat 2 or repeat 4, or simultaneous mutation of Lys792, Arg793 and Lys797 at the C‐terminal end of RH II/Gu to alanines inhibits RNA foldase activity. The last 17 amino acids of RH II/Gu can be replaced by an RNA binding motif from nucleolar protein p120 without a deleterious effect on its foldase activity. A model is proposed to explain how RH II/Gu binds and folds an RNA substrate.