The hexameric ring structure of the Escherichia coli RuvB branch migration protein

The RuvB protein is part of the homologous recombination machinery in prokaryotic cells. Many studies have shown that RuvB is organized into hexameric rings functioning as DNA pumps at Holliday junctions, using ATP hydrolysis to drive branch migration. Structures now exist for two RuvB proteins, as well as for several structurally homologous proteins, including the replication factor-C small subunit (RFCS). Two models for the possible hexameric organization of RuvB subunits have been proposed, based upon the hexameric structures of NSF and HslU, two AAA-ATPases involved in vesicle fusion and proteolysis, respectively. We have used electron microscopy to generate an improved three-dimensional reconstruction of the double hexamers formed by Escherichia coli RuvB on double-stranded DNA. We find that an atomic model of the hexameric RFCS provides a significantly better fit to the RuvB hexamer than do the models for RuvB generated from NSF and HslU. This suggests that there may be a highly conserved structure for many proteins involved in different aspects of DNA replication, recombination, transcription and repair.