Extended Data Fig. 5: A role for Orc6 in modulating MCM loading.
From: Mechanism of head-to-head MCM double-hexamer formation revealed by cryo-EM
a, Two elements connect ORC and the N-terminal face of MCM. One is Orc6 and the second is DNA, which is solvent-exposed between the ORC and MCM complexes owing to the bend induced by complex formation. b, Orc6 contains a domain architecture preserved in the related TFiiB transcription factor29. Although the precise mode of DNA engagement for the N- and C-terminal domains of TFiiB and Orc6 differ, notable conservation can be detected. c, Sequence alignment between the N-terminal domain of TFiiB and the N-terminal domain of Orc6. The N-terminal domain of Orc6 contacts DNA through a conserved lysine that is also found in TFiiB. Mutation of the equivalent lysine in Drosophila Orc6 affects DNA binding in vitro, as well as replication in extracts and cells30. d, A conserved helix18,31 of the Orc6 C-terminal domain (Orc6C) touches the N-terminal helical bundle of Mcm5. The Orc6 N-terminal domain (Orc6N) touches the N-terminal helical bundle of Mcm2. Together, the N-terminal and C-terminal domains of Orc6 latch across the Mcm2–Mcm5 gate. e, No steric clash can be detected between Orc6 and Cdt1 when MO and MCM–Cdt1 are superposed via the N-terminal domain of Mcm2. However, the C-terminal domain of Orc6 severely clashes with the N-terminal domain of Mcm5 in this configuration. Only Orc6 from MO is shown in the MCM–Cdt1 superposed structure.
