The origin recognition complex (ORC) of Saccharomyces cerevisiae binds origin 
DNA and cooperates with Cdc6 and Cdt1 to load the replicative helicase MCM2-7 
onto DNA. Helicase loading involves two MCM2-7 hexamers that assemble into a 
double hexamer around double-stranded DNA. This reaction requires ORC and Cdc6 
ATPase activity, but it is unknown how these proteins control MCM2-7 double 
hexamer formation. We demonstrate that mutations in Cdc6 sensor-2 and Walker A 
motifs, which are predicted to affect ATP binding, influence the ORC-Cdc6 
interaction and MCM2-7 recruitment. In contrast, a Cdc6 sensor-1 mutant affects 
MCM2-7 loading and Cdt1 release, similar as a Cdc6 Walker B ATPase mutant. 
Moreover, we show that Orc1 ATP hydrolysis is not involved in helicase loading 
or in releasing ORC from loaded MCM2-7. To determine whether Cdc6 regulates 
MCM2-7 double hexamer formation, we analysed complex assembly. We discovered 
that inhibition of Cdc6 ATPase restricts MCM2-7 association with origin DNA to a 
single hexamer, while active Cdc6 ATPase promotes recruitment of two MCM2-7 
hexamer to origin DNA. Our findings illustrate how conserved Cdc6 AAA+ motifs 
modulate MCM2-7 recruitment, show that ATPase activity is required for MCM2-7 
hexamer dimerization and demonstrate that MCM2-7 hexamers are recruited to 
origins in a consecutive process.