Distinct roles for cyclins E and A during DNA replication complex assembly and activation.
Initiation of DNA replication is regulated by cyclin-dependent protein kinase 2 (Cdk2) in association with two different regulatory subunits, cyclin A and cyclin E (reviewed in ref. 1). But why two different cyclins are required and why their order of activation is tightly regulated are unknown. Using a cell-free system for initiation of DNA replication that is based on G1 nuclei, G1 cytosol and recombinant proteins, we find that cyclins E and A have specialized roles during the transition from G0 to S phase. Cyclin E stimulates replication complex assembly by cooperating with Cdc6, to make G1 nuclei competent to replicate in vitro. Cyclin A has two separable functions: it activates DNA synthesis by replication complexes that are already assembled, and it inhibits the assembly of new complexes. Thus, cyclin E opens a 'window of opportunity' for replication complex assembly that is closed by cyclin A. The dual functions of cyclin A ensure that the assembly phase (G1) ends before DNA synthesis (S) begins, thereby preventing re-initiation until the next cell cycle.
Coverley
Dawn
D
Hutchison/MRC Research Centre, MRC Cancer Cell Unit, Hills Road, Cambridge, CB2 2XZ, UK. dc17@york.ac.uk
Laman
Heike
H
Laskey
Ronald A
RA
D018797:Cell Cycle Proteins / Q000378:metabolism; D002474:Cell-Free System; D019925:Cyclin A / Q000502:physiology*; D019927:Cyclin E / Q000502:physiology*; D004261:DNA Replication; D016193:G1 Phase; D006801:Humans; D009687:Nuclear Proteins / Q000378:metabolism; D010766:Phosphorylation
C103706:CDC6 protein, human; D018797:Cell Cycle Proteins; D019925:Cyclin A; D019927:Cyclin E; D009687:Nuclear Proteins