Pathway Map Details
Cell cycle_Start of DNA replication in early S phase
Object list (links open in MetaCore):
E2F1, Histone H1, ORC5L, RPA2, DNA polymerase alpha/primase, PP2A catalytic, RPA3, DP1, CDC45L, E2F1/DP1 complex, CDC7, MCM4, HP1 alpha, CDK2, Geminin, ORC2L, ORC4L, Cyclin E, ORC3L, MCM3, DNA, MCM10, DRF1, ASK (Dbf4), CDC18L (CDC6), MCM5, ORC6L, Cdt1, RPA1, ORC1L, MCM2, MCM4/6/7 complex
DNA replication and S phase progression
DNA replication is a key event of cell proliferation.
Replication begins with the ordered assembly of a multiprotein complex called the prereplicative complex (pre-RC). Pre-RC formation occurs during late M and early G1 phases of the cell cycle and licenses the DNA for replication during S phase. Transcription of m any eukaryotic DNA replication machinery genes is activated by E2F transcriptional factors (e.g. E2F1/DP1 dimer).
Pre-RC consists of the origin recognition complex ( ORC complex ), the cell division cycle 6 homolog ( CDC18L ), Cdt1 and the minichromosome maintenance protein complex ( MCM complex ). Pre-RC components are recruited to the origins in a stepwise manner beginning with the ORC. Subunit 1 of ORC complex ( ORC1L ) may be recruited to origins by chromobox protein homolog 5 ( HP1 ) . Subunits ORC2L, ORC3L, ORC4L and ORC5L form core complex, its stability is regulated by ORC1L , .
ORC complex recruits CDC18L and Cdt1, which are both required for the subsequent loading of the MCM complex . CDC18L is capable of inducing the binding between Cdt1 and MCM complex . The MCM complex is a heterohexamer which plays a role of the replicative helicase in eukaryotic cells. Each of the six subunits is required for replication . It is shown that MCM2 and MCM3/5 are regulatory subunits. MCM2 and MCM3/5 inhibit the helicase activity of the MCM4/6/7  by converting its double trimer structure into a heterotetramer or heteropentamer, respectively .
Then cell division cycle 7-related protein kinase 2 ( CDK2) and/or cell division cycle 7-related protein kinase ( CDC7 ) and minichromosome maintenance deficient 10 protein ( Mcm10 ) stimulate recruiting of CDC45-related protein ( CDC45L ) to the Pre-RC . Recruiting CDC45L and some other proteins promote maturation Pre-RC in pre-initiation complex (pre-IC). CDC45L recruits CDK2 to the replication foci, resulting in histon H1 phosphorylation, chromatin decondensation, and facilitation of fork progression .
In addition, replication proteins ( RPA ) are bound with the origin. There are three forms of RPA ( RPA1, RPA2 and RPA3 ). Trimeric complex of these proteins is required for an association with DNA .
Serine/threonine protein phosphatase 2A ( PP2A ) activates by dephosphorylation DNA polymerase alpha/primase in late G1 phase. DNA polymerase alpha/primase is the only enzyme capable of initiating DNA synthesis de novo by first synthesizing an RNA primer and then extending the primer by polymerization to produce a short 30-nucleotide DNA extension. DNA polymerase alpha/primase forms complex with MCM2 in early S-phase. Then formation of new MCM2/ DNA polymerase alpha/primase complexes might be prevented by CDK2/ CyclinA phosphorylation of DNA polymerase alpha/primase in late-S-phase .
CDK2/ Cyclin s also inhibit by phoshorylation some eukaryotic DNA replication machinery elements in late S-phase. There are MCM4/6/7 helicase , CDC18L  and Cdt1. ORC1L, CDC18L, Cyclin A are inhibited by ubiqitination with help of APC/hCdh complex. Cdt1 is regulated by Geminin and ubiqitination with help SCF complex.
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