purified human G3BP, which they called HDH-VIII, in a systematic study of DNA-unwinding enzymes present in HeLa cells. They found that G3BP could unwind DNA and RNA partial duplexes in an ATP- and Mg(2+)-dependent manner, even though it lacks the canonical helicase domain. G3BP preferentially unwound partial duplex substrates having a 17-bp annealed portion and either a hanging 3-prime tail or hanging tails at both the 3-prime and 5-prime ends. It unwound DNA/DNA, RNA/DNA, and RNA/RNA substrates with comparable efficiency. G3BP failed to unwind a substrate with a 40-bp annealed section. It appeared to unwind unidirectionally by moving in the 5-prime-to-3-prime direction along the bound single-stranded DNA