The Gly-Arg-rich C-terminal domain of pea nucleolin is a DNA helicase that catalytically translocates in the 5′- to 3′-direction

Nucleolin is a major nucleolar phosphoprotein of exponentially growing eukaryotic cells. Here we report the cloning, purification, and characterization of the C-terminal glycine/arginine-rich (GAR) domain of pea nucleolin. The purified recombinant protein (17 kDa) shows ATP-/Mg2+-dependent DNA helicase and ssDNA-/Mg2+-dependent ATPase activities. The enzyme unwinds DNA in the 5′- to 3′-direction, which is the first report in plant for this directional activity. It unwinds forked/non-forked DNA with equal efficiency. The anti-nucleolin antibodies immunodepleted the activities of the enzyme. The DNA interacting ligands nogalamycin, daunorubicin, actinomycin C1, and ethidium bromide were inhibitory to DNA unwinding (with Ki values of 0.40, 2.21, 8.0, and 9.0 μM, respectively) and ATPase (with Ki values of 0.43, 1.65, 4.6, and 7.0 μM, respectively) activities of the enzyme. This study confirms that the unwinding and ATPase activities of pea nucleolin resided in the GAR domain. This study should make important contribution to our better understanding of DNA transaction in plants, mechanism of DNA unwinding, and the mechanism by which these ligands can disturb genome integrity.