DNA helicases associated with DNA polymerases from human cells

A crude DNA polymerase fraction partially purified from a low salt extract of HeLa cells was fractionated on a hydroxylapatite column by an elution with a linear gradient of potassium phosphate. By this procedure, DNA polymerase α, δ and ε were separated from each other. DNA helicase activities were detected in the DNA polymerase α and δ fractions but not in the ε fraction. Characterization of DNA helicases after further purification on heparin column revealed that the DNA helicase in the DNA polymerase α fraction required ATP (or dATP) while that in the DNA polymerase δ could utilize CTP (or dCTP) in addition to ATP (or dATP). Both DNA helicases translocated on single-stranded DNA in the same direction of 3' to 5'. By a repeated gel-filtration on Superose 6 (SMART system), activities of DNA polymerase a and were eluted at positions of approx. 600 kDa and 400 kDa, respectively, and the activities of DNA helicases were well associated with those of corresponding DNA polymerases. These results strongly suggest that the DNA helicases described here are physically associated with DNA polymerase α and δ to make large complexes.