The budding-yeast initiator·co-loader complex ORC·DNA·Cdc6 can adopt an autoinhibited state in which the N-terminal cyclin box domain of Orc6 (Orc6-CBN^{\mathrm{N}}) docks onto the ORC·Cdc6 ring, occupies the Mcm7-binding site used during replicative-helicase recruitment, and thereby stalls Mcm2-7 loading in response to cyclin-dependent kinase (CDK) phosphorylation (Schmidt et al., 2022). Whether functionally analogous inhibitory states exist in metazoans or other eukaryotes to temporally coordinate Mcm2-7 double-hexamer formation and prevent DNA re-replication is unresolved. Later work has substantially settled one sub-question — phosphorylation of Orc6 alone suffices to inhibit helicase loading, while Orc2 phosphorylation acts through a distinct complementary mechanism (Amasino et al., 2023; Wu et al., 2024) — and the first human ORC-CDC6-DNA and human OCCM (ORC-CDC6-Cdt1-MCM2-7 intermediate) structures and reconstituted human licensing reactions have been reported without revealing any CDK-controlled autoinhibited state; but no study has directly tested phosphorylated metazoan ORC·CDC6·DNA for such a state, captured the cis/trans docking mode of yeast Orc6-CBN^{\mathrm{N}}, or defined the conformational alterations that trigger ATP hydrolysis at the ORC ATPase sites. The open problem is therefore to establish whether initiator autoinhibition is a conserved principle of licensing control — identifying or definitively excluding analogous inhibitory states in a metazoan system — and to complete the molecular mechanism of the yeast state: the phosphorylation-to-docking coupling and the ATPase-activation trigger for ORC release from DNA.

Background

Eukaryotic cells duplicate their genome exactly once per cell division cycle by temporally separating two events. During G1 phase, replication origins are licensed by loading the replicative helicase — the ring-shaped Mcm2-7 ATPase hexamer — onto double-stranded DNA as an inactive head-to-head double hexamer. During S phase, licensed helicases are activated to unwind DNA at replication forks. Licensing begins when the six-subunit origin recognition complex (ORC) binds DNA and recruits the loading factors Cdc6 and Cdt1, which together deposit a first Mcm2-7 hexamer; a second hexamer is then loaded in the opposite orientation so that the two dock N-terminal face to N-terminal face. Re-licensing within the same cycle would cause DNA re-replication and genome instability, so cyclin-dependent kinases (CDKs) block helicase loading outside G1 by multiple mechanisms; in budding yeast (Saccharomyces cerevisiae), one such mechanism is direct CDK phosphorylation of the ORC subunits Orc2 and Orc6.

Schmidt et al. (2022) determined cryo-electron microscopy structures of S. cerevisiae ORC·DNA·Cdc6 at 2.5–2.7 Å resolution. ORC and the loading machinery are built from AAA+-family ATPase modules (in Orc1, Orc4, Orc5 and Cdc6, and in all six Mcm2-7 subunits) whose ATP-binding pockets form at subunit interfaces — including the Orc1/Orc4 site whose hydrolysis is required for licensing — and each subunit also carries a winged-helix (WH) domain used for DNA and protein contacts. The structures revealed that the N-terminal cyclin box domain of Orc6 (Orc6-CBN^{\mathrm{N}}; Orc6 is a two-domain protein whose cyclin box folds resemble the fold of cyclins), normally flexibly tethered by a linker to the C-terminal domain, can dock laterally onto the ORC·Cdc6 ring precisely where the WH domain of the Mcm7 subunit must bind during helicase recruitment. This autoinhibited ORC·DNA·Cdc6 state is thus incompatible with productive Mcm2-7 loading: docking of Orc6-CBN^{\mathrm{N}} and of Mcm2-7 are mutually exclusive, and deleting the Orc6 N-terminal region relieves CDK-mediated inhibition of Mcm7 recruitment. The authors propose that under high CDK activity, phosphorylation drives formation of this state — with Orc6-CBN^{\mathrm{N}} docking either in cis (from the same ORC) or in trans (from a second ORC molecule) — coupling origin licensing to the cell cycle and averting re-replication.

The question is immediately cross-species. Metazoan Orc6 orthologs (for example human and Drosophila) associate only weakly with the ORC1-5 subcomplex, and their CBN^{\mathrm{N}}–CBC^{\mathrm{C}} linker is substantially shorter — likely too short to reach across the ORC·Cdc6 ring, although trans-docking onto a second ORC remains conceivable. Structural studies of Drosophila ORC·DNA·Cdc6 (Schmidt & Bleichert, 2020) had not resolved the Orc6 cyclin box domains at all. Metazoans also prevent re-replication by other means — for instance the inhibitor geminin, which blocks the loading factor Cdt1, and cell-cycle-regulated degradation or export of ORC1 and CDC6 — so whether a yeast-like autoinhibited initiator state exists outside fungi, or whether metazoans rely entirely on alternative strategies, is untested. Finally, even on the yeast side the mechanism is incomplete: the available ORC·DNA·Cdc6 structures show pre-hydrolysis ATPase configurations, and the conformational alterations in Mcm2-7-containing loading intermediates that activate ATP hydrolysis by ORC itself and trigger ORC release from DNA have not been defined.

Problem Statement

From Schmidt et al. (2022), updated for literature progress through 2026, the open problem is to establish whether initiator autoinhibition is a general principle of eukaryotic origin licensing control and to complete its molecular mechanism. Three linked components jointly serve this single objective:

(1) Cross-species existence. Determine whether metazoan or other non-Saccharomyces eukaryotic initiator·co-loader assemblies (for example human or Drosophila ORC·CDC6·DNA, with or without Orc6) can adopt a CDK-controlled or otherwise functionally analogous autoinhibited state — via cis-docking of an Orc6 cyclin box domain, trans-docking across two ORC molecules, or a different inhibitory conformation — that blocks Mcm2-7 loading and thereby temporally coordinates the sequential steps of Mcm2-7 double-hexamer formation and prevents re-replication. A rigorous demonstration that no such state exists in a representative metazoan system is an equally valid answer to this component.

(2) Assembly mechanism in yeast. Establish at the structural level how phosphorylation couples to autoinhibition: whether phosphorylation of Orc6 alone is sufficient to drive Orc6-CBN^{\mathrm{N}} docking onto the ORC·Cdc6 ring, and whether docking occurs in cis or in trans. (The functional counterpart of this question — whether Orc6 phosphorylation alone, versus both Orc2 and Orc6 phosphorylation, is required to inhibit loading — has been substantially answered (Amasino et al., 2023; Wu et al., 2024).)

(3) Disassembly mechanism. Define, in Mcm2-7-containing loading intermediates, the precise conformational alterations that activate ATP hydrolysis at the ORC ATPase sites (notably the Orc1/Orc4 site) and trigger ORC release from DNA — the transition that the existing 2.5–2.7 Å ORC·DNA·Cdc6 structures, which show pre-hydrolysis configurations, could not capture.

The three components are linked, and the objective is a mechanism that connects any positive metazoan finding mechanistically to the yeast autoinhibited state.

References

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