Regulation of replication origin licensing by ORC phosphorylation reveals a two-step mechanism for Mcm2-7 ring closing

How similar are the molecular mechanisms of yeast and metazoan genome replication initiation?

DNA replication start sites are licensed for replication when two hexameric ring-shaped motors of the replicative helicase are loaded as an inactive double hexamer around duplex DNA. Activation requires untwisting of the double helix and ejection of one DNA strand from the central channel of each helicase ring. The process of replication initiation is best understood in yeast, thanks to reconstitution with purified yeast proteins, which allowed systematic structural analysis of the replication initiation process. Orthologs of most yeast replication factors have been identified in higher eukaryotes; however, reconstitution of metazoan replication initiation is still in its infancy, with double hexamer loading but not activation having been achieved. Nonetheless, artificial intelligence-driven structure prediction and cryo-EM studies on native complexes, combined with cell-based and cell-free approaches, are starting to provide insights into metazoan replication initiation mechanisms. Here, we describe the emerging picture.

Unidirectional MCM translocation away from ORC drives origin licensing

The MCM motor of the eukaryotic replicative helicase is loaded as a double hexamer onto DNA by the Origin Recognition Complex (ORC), Cdc6, and Cdt1. ATP binding supports formation of the ORC-Cdc6-Cdt1-MCM (OCCM) helicase-recruitment complex where ORC-Cdc6 and one MCM hexamer form two juxtaposed rings around duplex DNA. ATP hydrolysis by MCM completes MCM loading but the mechanism is unknown. Here, we used cryo-EM to characterise helicase loading with ATPase-dead Arginine Finger variants of the six MCM subunits. We report the structure of two MCM complexes with different DNA grips, stalled as they mature to loaded MCM. The Mcm2 Arginine Finger-variant stabilises DNA binding by Mcm2 away from ORC/Cdc6. The Arginine Finger-variant of the neighbouring Mcm5 subunit stabilises DNA engagement by Mcm5 downstream of the Mcm2 binding site. Cdc6 and Orc1 progressively disengage from ORC as MCM translocates along DNA. We observe that duplex DNA translocation by MCM involves a set of leading-strand contacts by the pre-sensor 1 ATPase hairpins and lagging-strand contacts by the helix-2-insert hairpins. Mutating any of the MCM residues involved impairs high-salt resistant DNA binding in vitro and double-hexamer formation assessed by electron microscopy. Thus, ATPase-powered duplex DNA translocation away from ORC underlies MCM loading.

MCM2-7 ring closure involves the Mcm5 C-terminus and triggers Mcm4 ATP hydrolysis

The eukaryotic helicase MCM2-7, is loaded by ORC, Cdc6 and Cdt1 as a double-hexamer onto replication origins. The insertion of DNA into the helicase leads to partial MCM2-7 ring closure, while ATP hydrolysis is essential for consecutive steps in pre-replicative complex (pre-RC) assembly. Currently it is unknown how MCM2-7 ring closure and ATP-hydrolysis are controlled. A cryo-EM structure of an ORC-Cdc6-Cdt1-MCM2-7 intermediate shows a remodelled, fully-closed Mcm2/Mcm5 interface. The Mcm5 C-terminus (C5) contacts Orc3 and specifically recognises this closed ring. Interestingly, we found that normal helicase loading triggers Mcm4 ATP-hydrolysis, which in turn leads to reorganisation of the MCM2-7 complex and Cdt1 release. However, defective MCM2-7 ring closure, due to mutations at the Mcm2/Mcm5 interface, leads to MCM2-7 ring splitting and complex disassembly. As such we identify Mcm4 as the key ATPase in regulating pre-RC formation. Crucially, a stable Mcm2/Mcm5 interface is essential for productive ATP-hydrolysis-dependent remodelling of the helicase.

Multiple mechanisms for licensing human replication origins

Loading of replicative helicases is obligatory for the assembly of DNA replication machineries. The eukaryotic MCM2-7 replicative helicase motor is deposited onto DNA by the origin recognition complex (ORC) and co-loader proteins as a head-to-head double hexamer to license replication origins. Although extensively studied in budding yeast1-4, the mechanisms of origin licensing in multicellular eukaryotes remain poorly defined. Here we use biochemical reconstitution and electron microscopy to reconstruct the human MCM loading pathway. We find that unlike in yeast, the ORC6 subunit of the ORC is not essential for-but enhances-human MCM loading. Electron microscopy analyses identify several intermediates en route to MCM double hexamer formation in the presence and absence of ORC6, including a DNA-loaded, closed-ring MCM single hexamer intermediate that can mature into a head-to-head double hexamer through multiple mechanisms. ORC6 and ORC3 facilitate the recruitment of the ORC to the dimerization interface of the first hexamer into MCM-ORC (MO) complexes that are distinct from the yeast MO complex5,6 and may orient the ORC for second MCM hexamer loading. Additionally, MCM double hexamer formation can proceed through dimerization of independently loaded MCM single hexamers, promoted by a propensity of human MCM2-7 hexamers to self-dimerize. This flexibility in human MCM loading may provide resilience against cellular replication stress, and the reconstitution system will enable studies addressing outstanding questions regarding DNA replication initiation and replication-coupled events in the future.

Replication licensing regulated by a short linear motif within an intrinsically disordered region of origin recognition complex

In eukaryotes, the origin recognition complex (ORC) faciliates the assembly of pre-replicative complex (pre-RC) at origin DNA for replication licensing. Here we show that the N-terminal intrinsically disordered region (IDR) of the yeast Orc2 subunit is crucial for this process. Removing a segment (residues 176-200) from Orc2-IDR or mutating a key isoleucine (194) significantly inhibits replication initiation across the genome. These Orc2-IDR mutants are capable of assembling the ORC-Cdc6-Cdt1-Mcm2-7 intermediate, which exhibits impaired ATP hydrolysis and fails to be convered into the subsequent Mcm2-7-ORC complex and pre-RC. These defects can be partially rescued by the Orc2-IDR peptide. Moreover, the phosphorylation of this Orc2-IDR region by S cyclin-dependent kinase blocks its binding to Mcm2-7 complex, causing a defective pre-RC assembly. Our findings provide important insights into the multifaceted roles of ORC in supporting origin licensing during the G1 phase and its regulation to restrict origin firing within the S phase.

Multiple pathways for licensing human replication origins

The loading of replicative helicases constitutes an obligatory step in the assembly of DNA replication machineries. In eukaryotes, the MCM2-7 replicative helicase motor is deposited onto DNA by the origin recognition complex (ORC) and co-loader proteins as a head-to-head MCM double hexamer to license replication origins. Although extensively studied in the budding yeast model system, the mechanisms of origin licensing in higher eukaryotes remain poorly defined. Here, we use biochemical reconstitution and electron microscopy (EM) to reconstruct the human MCM loading pathway. Unexpectedly, we find that, unlike in yeast, ORC's Orc6 subunit is not essential for human MCM loading but can enhance loading efficiency. EM analyses identify several intermediates en route to MCM double hexamer formation in the presence and absence of Orc6, including an abundant DNA-loaded, closed-ring single MCM hexamer intermediate that can mature into a head-to-head double hexamer through different pathways. In an Orc6-facilitated pathway, ORC and a second MCM2-7 hexamer are recruited to the dimerization interface of the first hexamer through an MCM-ORC intermediate that is architecturally distinct from an analogous intermediate in yeast. In an alternative, Orc6-independent pathway, MCM double hexamer formation proceeds through dimerization of two independently loaded single MCM2-7 hexamers, promoted by a propensity of human MCM2-7 hexamers to dimerize without the help of other loading factors. This redundancy in human MCM loading pathways likely provides resilience against replication stress under cellular conditions by ensuring that enough origins are licensed for efficient DNA replication. Additionally, the biochemical reconstitution of human origin licensing paves the way to address many outstanding questions regarding DNA replication initiation and replication-coupled events in higher eukaryotes in the future.

The Origin Recognition Complex: From Origin Selection to Replication Licensing in Yeast and Humans

Understanding human DNA replication through the study of yeast has been an extremely fruitful journey. The minichromosome maintenance (MCM) 2-7 genes that encode the catalytic core of the eukaryotic replisome were initially identified through forward yeast genetics. The origin recognition complexes (ORC) that load the MCM hexamers at replication origins were purified from yeast extracts. We have reached an age where high-resolution cryoEM structures of yeast and human replication complexes can be compared side-by-side. Their similarities and differences are converging as alternative strategies that may deviate in detail but are shared by both species.