Domain conformations of the motor subunit of EcoR124I involved in ATPase activity and dsDNA translocation

Abstract

Bacterial type I restriction-modification systems are composed of three different subunits: one HsdS subunit is required for identification of target sequence and anchoring the enzyme complex on DNA; two HsdM subunits in the methyl-transferase complex serve for host genome modification accomplishing a protective function against self-degradation; two HsdR (or motor) subunits house ATP-dependent translocation and consequent cleavage of double stranded DNA activities. The crystal structure of the 120 kDa HsdR subunit of the Type I restriction-modification system EcoR124I in complex with ATP was recently reported. HsdR is organized into four approximately globular structural domains in a nearly square-planar arrangement: the N-terminal endonuclease domain, the RecA-like helicase domains 1 and 2 and the C-terminal helical domain. The near-planar arrangement of globular domains creates prominent grooves between each domain pair. The two helicase-like domains form a canonical helicase cleft in which double-stranded B-form DNA can be accommodated without steric clash. The helical domain, probably involved in complex assembly, exhibits only a few specific interactions with helicase 2 domain. Molecular mechanism of dsDNA translocation, cleavage and ATP hydrolysis has not been yet structurally investigated. Here we propose a translocation cycle of the restriction-modification system EcoR124I based on analysis of available crystal structures of superfamily 2 helicases, strutural modeling and complementary biochemical characterization of mutations introduced in sites potentially inportant for translocation in the HsdR motor subunit. Also a role of the extended region of the helicase motif III in ATPase activity of EcoR124I was probed.