Structure of the Rad50 x Mre11 DNA repair complex from Saccharomyces cerevisiae by electron microscopy

The RAD50 gene of Saccharomyces cerevisiae is one of several genes required for recombinational repair of double-strand DNA breaks during vegetative growth and for initiation of meiotic recombination. Rad50 forms a complex with two other proteins, Mre11 and Xrs2, and this complex is involved in double-strand break formation and processing. Rad50 has limited sequence homology to the structural maintenance of chromosomes (SMC) family of proteins and shares the same domain structure as SMCs: N- and C-terminal globular domains separated by two long coiled-coils. However, a notable difference is the much smaller non-coil hinge region between the two coiled-coils. We report here a structural analysis of full-length S. cerevisiae Rad50, alone and in a complex with yeast Mre11 by electron microscopy. Our results confirm that yeast Rad50 does have the same antiparallel coiled-coil structure as SMC proteins, but with no detectable globular hinge domain. However, the molecule is still able to bend sharply in the middle to bring the two catalytic domains together, indicating that the small hinge domain is flexible. We also demonstrate that Mre11 binds as a dimer between the catalytic domains of Rad50, bringing the nuclease activities of Mre11 in close proximity to the ATPase and DNA binding activities of Rad50.