Fig. 5: Mechanisms of hDH loading.

Model of proposed ORC6-independent and ORC6-dependent MCM loading mechanisms. a, MCM can be loaded via an ORC6-independent pathway, via two inverted hOCCM complexes that load two hSHs in a process that requires ATP hydrolysis. Free diffusion along duplex DNA would then lead to hDH formation. b, In a variation of the same mechanism, two hOCCMs assembled around duplex DNA are free to diffuse and form a double OCCM. ATP hydrolysis then promotes hDH formation. c, MCM loading with full-length ORC1 can occur in an ORC6-dependent manner and might go through the hMO* intermediate. hMO* might recruit a hSH through an ORC6 interaction with N-terminal MCM. Following hSH release, the same ORC could recruit a second hSH via C-terminal MCM interaction (hOCCM intermediate), resulting in two N-terminally facing hSHs that can assemble a hDH. d, Alternatively, a first hSH could be loaded via OCCM. ORC6 would then interact with the N-terminal domain of hSH forming hMO*. A structural change would then occur, which causes hMO* to transition to a yMO-like state, in which the human ORC is competent for the recruitment of a second hSH, eventually leading to hDH formation. However, we did not observe formation of a yMO-like complex with the human proteins.