Functional consequences of mutating conserved SF2 helicase motifs in the Type III restriction endonuclease EcoP15I translocase domain

For efficient DNA hydrolysis, Type III restriction endonuclease EcoP15I interacts with two inversely oriented recognition sites in an ATP-dependent process. EcoP15I consists of two methylation (Mod) subunits and a single restriction (Res) subunit yielding a multifunctional enzyme complex able to methylate or to hydrolyse DNA. Comprehensive sequence alignments, limited proteolysis and mass spectroscopy suggested that the Res subunit is a fusion of a motor or translocase (Tr) domain of superfamily II helicases and an endonuclease domain with a catalytic PD…EXK motif. In the Tr domain, seven predicted helicase motifs (I, Ia, II–VI), a recently discovered Q-tip motif and three additional regions (IIIa, IVa, Va) conserved among Type III restriction enzymes have been identified that are predicted to be involved in DNA binding and ATP hydrolysis. Because DNA unwinding activity for EcoP15I (as for bona fide helicases) has never been found and EcoP15I ATPase rates are only low, the functional importance of the helicase motifs and regions was questionable and has never been probed systematically. Therefore, we mutated all helicase motifs and conserved regions predicted in Type III restriction enzyme EcoP15I and examined the functional consequences on EcoP15I enzyme activity and the structural integrity of the variants by CD spectroscopy. The resulting eleven enzyme variants all, except variant IVa, are properly folded showing the same secondary structure distribution as the wild-type enzyme. Classical helicase motifs I–VI are important for ATP and DNA cleavage by EcoP15I and mutations therein led to complete loss of ATPase and cleavage activity. Among the catalytically inactive enzyme variants three preserved the ability to bind ATP. In contrast, newly assigned motifs Q-tip, Ia and Va are not essential for EcoP15I activity and the corresponding enzyme variants were still catalytically active. DNA binding was only marginally reduced (2–7 fold) in all enzyme variants tested.