Human DNA helicase VIII: a DNA and RNA helicase corresponding to the G3BP protein, an element of the Ras transduction pathway

Human DNA helicase VIII (HDH VIII) was isolated in the course of a systemat ic study of the DNA unwinding enzymes present in human cells. From a HeLa c ell nuclear extract a protein with an M-r of 68 kDa in SDS-PAGE was isolate d, characterised and microsequenced. The enzyme shows ATP- and Mg2+-depende nt activity is not stimulated by RPA, prefers partially unwound 3'-tailed s ubstrates and moves along the bound strand in the 5' to 3' direction. HDH V III Can also unwind partial RNA/DNA and RNA/RNA duplexes. Microsequencing o f the polypeptide showed that this enzyme corresponds to G3BP, an element o f the Ras pathway which binds specifically to the GTPase-activating protein . HDH VIII/G3BP is analogous to the heterogeneous nuclear ribonucleoprotein s and contains a sequence rich in RGG boxes similar to the C-terminal domai n of HDH IV/nucleolin, another DNA and RNA helicase.