Crystal structure of an ATPase-active form of rad51 homolog from Methanococcus voltae -: Insights into potassium dependence

Homologous gene recombination is crucial for the repair of DNA. A superfamily of recombinases facilitate a central strand exchange reaction in the repair process. This reaction is initiated by coating single-stranded DNA (ssDNA) with recombinases in the presence of ATP and Mg2+ co-factors to form helical nucleoprotein filaments with elevated ATPase and strand invasion activities ( 1). At the amino acid sequence level, archaeal RadA and Rad51 and eukaryal Rad51 and meiosis-specific DMC1 form a closely related group of recombinases distinct from bacterial RecA ( 2). Unlike the extensively studied Escherichia coli RecA (EcRecA), increasing evidences on yeast and human recombinases imply that their optimal activities are dependent on the presence of a monovalent cation, particularly potassium (3-5). Here we present the finding that archaeal RadA from Methanococcusvoltae (MvRadA) is a stringent potassium-dependent ATPase, and the crystal structure of this protein in complex with the non-hydrolyzable ATP analog adenosine 5'-(beta,gamma-iminotriphosphate), Mg2+, and K+ at 2.4 Angstrom resolution. Potassium triggered an in situ conformational change in the ssDNA-binding L2 region concerted with incorporation of two potassium ions at the ATPase site in the RadA crystals preformed in K+-free medium. Both potassium ions were observed in contact with the gamma-phosphate of the ATP analog, implying a direct role by the monovalent cations in stimulating the ATPase activity. Cross-talk between the ATPase site and the ssDNA-binding L2 region visualized in the MvRadA structure provides an explanation to the co-factor-induced allosteric effect on RecA-like recombinases.