Domain structure of human nuclear DNA helicase II (RNA helicase A)

Full-length human nuclear DNA helicase II (NDH II) was cloned and overexpressed in a baculovirus-derived expression system. Recombinant NDH II unwound both DNA and RNA. Limited tryptic digestion produced active helicases with molecular masses of 130 and 100 kDa. The 130-kDa helicase missed a glycine-rich domain (RGG-box) at the carboxyl terminus, while the 100-kDa form missed both its double-stranded RNA binding domains (dsRBDs) at the amino terminus and its RGG-box. Hence, the dsRBDs and the RGG-box were dispensable for unwinding. On the other hand, the isolated DEXH core alone could neither hydrolyze ATP nor unwind nucleic acids. These enzymatic activities were not regained by fusing a complete COOH or NH2 terminus to the helicase core. Hence, an active helicase required part of the NH2 terminus, the DEXH core, and a C-terminal extension of the core. Both dsRBDs and the RGG-box were bacterially expressed as glutathione S-transferase fusion proteins. The two dsRBDs had a strong affinity to double-stranded RNA and cooperated upon RNA binding, while the RGG-box bound preferentially to single-stranded DNA. A model is suggested in which the flanking domains influence and regulate the unwinding properties of NDH II.