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

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Abstract
Human DNA helicase VIII (HDH VIII) was isolated in the course of a systematic study of the DNA unwinding enzymes present in human cells. From a HeLa cell nuclear extract a protein with an Mr of 68 kDa in SDS-PAGE was isolated, characterised and microsequenced. The enzyme shows ATP- and Mg2+-dependent activity is not stimulated by RPA, prefers partially unwound 3′-tailed substrates and moves along the bound strand in the 5′ to 3′ direction. HDH VIII can also unwind partial RNA/DNA and RNA/RNA duplexes. Microsequencing of the polypeptide showed that this enzyme corresponds to G3BP, an element of the Ras pathway which binds specifically to the GTPase-activating protein. HDH VIII/G3BP is analogous to the heterogeneous nuclear ribonucleoproteins and contains a sequence rich in RGG boxes similar to the C-terminal domain of HDH IV/nucleolin, another DNA and RNA helicase.