Characterization of the enzymatic activity of hChIR1, a novel human DNA helicase

Recently, we cloned two highly related human genes, hChlR1 (DDX11) and hChl R2 (DDX12), which appear to be homologs of the Saccharomyces cerevisiae CHL 1 gene. Nucleotide sequence analysis suggests that these genes encode new m embers of the DEAH family of:DNA helicases, While the enzymatic activity of CHL1 has not been characterized, the protein is required for the maintenan ce of high fidelity chromosome segregation in yeast. Here we report that th e hChlR1 protein is a novel human DNA helicase, We have expressed and purif ied hChlR1 using a baculo-virus system and analyzed its enzymatic activity. The recombinant hChlR1 protein possesses both ATPase and DNA helicase acti vities that are strictly dependent on DNA, divalent cations and ATP, These activities are abolished by a single amino acid substitution in the ATP-bin ding domain. The hChlR1 protein can unwind both DNA/DNA and RNA/DNA substra tes. It has a preference for movement in the 5' --> 3' direction on short s ingle-stranded DNA templates, However, unlike other DNA helicases, the hChl R1 DNA helicase can translocate along single-stranded DNA in both direction s when substrates have a very long single-stranded DNA region. The enzymati c activities of hChlR1 suggest that DNA helicases are required for maintain ing the fidelity of chromosome segregation.