RNA‐unwinding and RNA‐folding Activities of RNA Helicase II/Gu — Two Activities in Separate Domains of the Same Protein

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Abstract
The human RNA helicase II/Gu protein (RH‐TI/Gu) is a member of the D‐E‐A‐D box protein family. It is a unique enzyme, which possesses an ATP‐dependent RNA‐unwinding activity and has an RNA‐folding activity that introduces an intramolecular secondary structure in single‐stranded RNA. This report shows that these two enzymatic activities are distinct. ATP[S], GTP and low concentrations of ATP enhance the RNA‐folding activity of RH‐II/Gu but not the RNA‐helicase activity. High concentrations of ATP are required for the helicase activity but are inhibitory to the RNA‐folding activity. Mg2 is required for the helicase activity but not for the RNA‐folding reaction. Affinity‐purified anti‐(RH‐II/Gu) polyclonal Ig inhibit the RNA‐unwinding activity but not the folding activity. Mutations of the DEVD sequence, which corresponds to the DEAD box, and the SAT motif enhanced RNA‐folding activity of RH‐II/Gu but completely inhibited the RNA‐helicase activity. A mutant that lacks the COOH‐terminal 76 amino acid residues, including the four FRGQR repeats, had unwinding activity but did not catalyze the folding of a single‐stranded RNA. The two enzymatic activities of RH‐II/Gu reside in distinct domains. Amino acids 1–650 are active in the RNA‐unwinding reaction but lack RNA‐folding activity. Amino acids 646–801 fold single‐stranded RNA but lack helicase activity. This report shows distinct RNA‐unwinding and RNA‐folding activities residing in separate domains within the same protein.