Identification of an RNA-stimulated NTPase in the predicted helicase sequence of the Rubella virus nonstructural polyprotein.

The replicative proteins of Rubella virus are generated from a polyprotein that is translated from the 5'-terminal segment of the viral genome. The determination of the genome sequence and the description of amino acid sequence motifs which are proposed to be characteristic for helicase proteins have indicated that the polyprotein region located between amino acid residues 1300 and 1600 represents the Rubella virus helicase. We have expressed a segment comprising the sequences between the amino acid residues A (1225) and R (1664) as part of a glutathione S-transferase fusion protein in Escherichia coli. We show that this protein contains a nucleoside triphosphatase activity which hydrolyses all eight ribonucleoside- and deoxyribonucleoside triphosphates. The activity of the protein, determined by ATP hydrolysis, was influenced by the presence of single-stranded RNA; it was stimulated about 1.7 fold in the presence of poly(U), poly(C), or poly(dT) and inhibited to half its activity in the presence of poly(G). These functions represent characteristic helicase partial functions and provide experimental support for the predicted localization of the helicase in the nonstructural polyprotein.