Structure and function of Cdc6/Cdc18: Implications for origin recognition and checkpoint control

Cdc6/Cdc18 is a conserved and essential component of prereplication complex es. The 2.0 Angstrom crystal structure of an archaeal Cdc6 ortholog, in con junction with a mutational analysis of the homologous Cdc18 protein from Sc hizosaccharomyces pombe, reveals novel aspects of Cdc6/Cdc18 function. Two domains of Cdc6 form an AAA(+)-type nucleotide binding fold that is observe d bound to Mg ADP. A third domain adopts a winged-helix fold similar to kno wn DNA binding modules. Sequence comparisons show that the winged-helix dom ain is conserved in Orc1, and mutagenesis data demonstrate that this region of Cdc6/Cdc18 is required for function in vivo. Additional mutational anal yses suggest that nucleotide binding and/or hydrolysis by Cdc6/Cdc18 is req uired not only for progression through S phase, but also for maintenance of checkpoint control during S phase.