The Rad50 zinc-hook is a structure joining Mre11 complexes in DNA recombination and repair

The Mre11 complex (Mre11 Rad50 Nbs1) is central to chromosomal maintenance and functions in homologous recombination, telomere maintenance and sister chromatid association. These functions all imply that the linked binding of two DNA substrates occurs, although the molecular basis for this process remains unknown. Here we present a 2.2 A crystal structure of the Rad50 coiled-coil region that reveals an unexpected dimer interface at the apex of the coiled coils in which pairs of conserved Cys-X-X-Cys motifs form interlocking hooks that bind one Zn(2+) ion. Biochemical, X-ray and electron microscopy data indicate that these hooks can join oppositely protruding Rad50 coiled-coil domains to form a flexible bridge of up to 1,200 A. This suggests a function for the long insertion in the Rad50 ABC-ATPase domain. The Rad50 hook is functional, because mutations in this motif confer radiation sensitivity in yeast and disrupt binding at the distant Mre11 nuclease interface. These data support an architectural role for the Rad50 coiled coils in forming metal-mediated bridging complexes between two DNA-binding heads. The resulting assemblies have appropriate lengths and conformational properties to link sister chromatids in homologous recombination and DNA ends in non-homologous end-joining.     

Structural basis of highly conserved ribosome recycling in eukaryotes and archaea

Ribosome-driven protein biosynthesis is comprised of four phases: initiation, elongation, termination and recycling. In bacteria, ribosome recycling requires ribosome recycling factor and elongation factor G, and several structures of bacterial recycling complexes have been determined. In the eukaryotic and archaeal kingdoms, however, recycling involves the ABC-type ATPase ABCE1 and little is known about its structural basis. Here we present cryo-electron microscopy reconstructions of eukaryotic and archaeal ribosome recycling complexes containing ABCE1 and the termination factor paralogue Pelota. These structures reveal the overall binding mode of ABCE1 to be similar to canonical translation factors. Moreover, the iron-sulphur cluster domain of ABCE1 interacts with and stabilizes Pelota in a conformation that reaches towards the peptidyl transferase centre, thus explaining how ABCE1 may stimulate peptide-release activity of canonical termination factors. Using the mechanochemical properties of ABCE1, a conserved mechanism in archaea and eukaryotes is suggested that couples translation termination to recycling, and eventually to re-initiation.