The Cdc6 protein has been suggested as a loader for the eukaryotic MCM helicase. Archaeal replication machinery represents a core version of that in eukaryotes. In the Current work, three eukaryotic Orc1/Cdc6 homologs (SsoCdc6-1, -2, and -3) from crenarchaeon Sulfolobus solfataricus were shown to have totally different effects on the interactions with SsoMCM helicase. SsoCdc6-2 stimulates the binding of the SsoMCM onto the origin DNA, but SsoCdc6-1 and SsoCdc6-3 significantly inhibit the loading activities, and these inhibitive effects can not be reversed by the stimulation of SsoCdc6-2. Using pull-down assays, we showed that three SsoCdc6 proteins interacted physically with the SsoMCM. Furthermore, the C-terminal domains of SsoCdc6 proteins were shown to physically and functionally affect the interactions with SsoMCM. This is the first report on the divergent functions of multiple eukaryote-like Orc1/Cdc6 proteins on regulating the loading of the MCM helicase onto the origins in the archaeon. (c) 2007 Elsevier Inc. All rights reserved.