HUMAN DNA HELICASE-IV IS NUCLEOLIN, AN RNA HELICASE MODULATED BY PHOSPHORYLATION

The cDNA encoding human DNA helicase IV (HDH IV), a 100-kDa protein wh ich unwinds DNA in the 5' to 3' direction with respect to the bound st rand, was cloned and sequenced. It was found to be identical to the hu man cDNA encoding nucleolin, a ubiquitous eukaryotic protein essential for pre-ribosome assembly. HDH IV/nucleolin can unwind RNA-RNA duplex es, as well as DNA-DNA and DNA-RNA duplexes. Phosphorylation of HDH IV /nucleolin by cdc2 kinase and casein kinase II enhanced its unwinding activity in an additive way. The Gly-rich C-terminal domain possesses a limited ATP-dependent duplex-unwinding activity which contributes to the helicase activity of HDH IV/nucleolin.