Crystal structure of archaeal recombinase RadA: A snapshot of its extended conformation

Homologous recombination of DNA plays crucial roles in repairing severe DNA damage and in generating genetic diversity. The process is facilitated by a superfamily of recombinases: bacterial RecA, archaeal RadA and Rad51, and eukaryal Rad51 and DMC1. These recombinases share a common ATP-dependent filamentous quaternary structure for binding DNA and facilitating strand exchange. We have determined the crystal structure of Methanococcus voltae RadA in complex with the ATP analog AMP-PNP at 2.0 Angstrom resolution. The RadA filament is a 106.7 Angstrom pitch helix with six subunits per turn. The DNA binding loops L1 and L2 are located in close proximity to the filament axis. The ATP analog is buried between two RadA subunits, a feature similar to that of the active filament of Escherichia coli RecA revealed by electron microscopy. The disposition of the N-terminal domain suggests a role of the Helix-hairpin-Helix motif in binding double-stranded DNA.