Figure 3. Characterisation of the ZRANB3 HNH active site.

(a) Overlay of the T4 endonuclease VII (yellow) and the ZRANB3 HNH (white) structures. Electrostatic surface potential of the HNH domain shows an overlap of the electronegative patch (red) with the Mg²⁺ ion (purple) in the T4 endonuclease VII active site. (b) Overlay of the T4 endonuclease VII (yellow, labels in italics) and the ZRANB3 HNH (white) structures. Mg²⁺ ion in T4 endonuclease is shown as a purple sphere. His1021, Asp1020 and His1045 in the HNH structure overlap with the active site residues of T4 endonuclease VII. (c) Nuclease assay with the wild-type and mutant ZRANB3 proteins in the presence and absence of ATP. Reactions were analysed by native poly-acrylamide gel electrophoresis. Indicated are mobilities of the fluorescently labelled DNA substrate and product. (d) Nuclease assay with non-hydrolysable ATP homologues. Shown are controls with the wild-type ZRANB3 and the ATPase dead K65R mutant in the presence of ATP.