Abstract
In eukaryotic cells, DNA replication licensing is precisely regulated to ensure that the initiation of genomic DNA replication in S phase occurs once and only once for each mitotic cell division. A key regulatory mechanism by which DNA re-replication is suppressed is the S phase-dependent proteolysis of Cdt1, an essential replication protein for licensing DNA replication origins by loading the Mcm2-7 replication helicase for DNA duplication in S phase. Cdt1 degradation is mediated by CRL4Cdt2 ubiquitin E3 ligase, which further requires Cdt1 binding to proliferating cell nuclear antigen (PCNA) through a PIP box domain in Cdt1 during DNA synthesis. Recent studies found that Cdt2, the specific subunit of CRL4Cdt2 ubiquitin E3 ligase that targets Cdt1 for degradation, also contains an evolutionarily conserved PIP box-like domain that mediates the interaction with PCNA. These findings suggest that the initiation and elongation of DNA replication or DNA damage-induced repair synthesis provide a novel mechanism by which Cdt1 and CRL4Cdt2 are both recruited onto the trimeric PCNA clamp encircling the replicating DNA strands to promote the interaction between Cdt1 and CRL4Cdt2. The proximity of PCNA-bound Cdt1 to CRL4Cdt2 facilitates the destruction of Cdt1 in response to DNA damage or after DNA replication initiation to prevent DNA re-replication in the cell cycle. CRL4Cdt2 ubiquitin E3 ligase may also regulate the degradation of other PIP box-containing proteins, such as CDK inhibitor p21 and histone methylase Set8, to regulate DNA replication licensing, cell cycle progression, DNA repair, and genome stability by directly interacting with PCNA during DNA replication and repair synthesis.
Highlights
DNA replication licensing is tightly controlled to prevent re-replication of the genome during cell cycle progression in eukaryotic cells [1,2,3]
Progress revealed that Cdt1 and DNA replication licensing are regulated at multiple levels during cell cycle progression [7,8,9,10,11], providing new insights into the mechanism by which the fidelity of DNA replication and genome stability are controlled in the cell cycle
Recent studies revealed that there is a vast excess of Mcm2-7 helicase proteins relative to the Origin Recognition Complex (ORC) proteins assembled onto chromatin in G1 during the formation of pre-RC, the loading of the Mcm2-7 helicase to the replication origins are dependent on the prior binding of the ORC-Cdc6 complex [54,55,56,57]
Summary
DNA replication licensing is tightly controlled to prevent re-replication of the genome during cell cycle progression in eukaryotic cells [1,2,3]. The Orc or Orc deletion mutant cells have reduced Mcm helicase loading and their proliferation is critically dependent on Cdc, an ATPase that is essential to form the ORC ring structure that encircles the duplex DNA. Recent studies revealed that there is a vast excess of Mcm helicase proteins relative to the ORC proteins assembled onto chromatin in G1 during the formation of pre-RC, the loading of the Mcm helicase to the replication origins are dependent on the prior binding of the ORC-Cdc complex [54,55,56,57]. Cdt may coordinate several important processes, including chromatin structural dynamics, to promote the loading of the double Mcm replication helicase hexamer complex onto chromatin DNA template for replication licensing in the cell cycle
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