Cyclin A2 localises in the cytoplasm at the S/G2 transition to activate Plk1

Cyclin A2 is a key regulator of the cell cycle, implicated both in DNA replication and mitotic entry. Cyclin A2 participates in feedback loops that activate mitotic kinases in G2-phase, but why active Cyclin A2-CDK2 during S phase does not trigger mitotic kinase activation remains unclear. Here we describe a change in localisation of Cyclin A2 from being only nuclear to both nuclear and cytoplasmic at the S/G2 border. We find that Cyclin A2-CDK2 can activate the mitotic kinase PLK1 through phosphorylation of Bora, and that only cytoplasmic Cyclin A2 interacts with Bora and PLK1. Expression of predominately cytoplasmic Cyclin A2 or phospho-mimicking PLK1 T210D can partially rescue a G2 arrest caused by Cyclin A2 depletion. Cytoplasmic presence of Cyclin A2 is restricted by p21, in particular after DNA damage. Cyclin A2 chromatin association during DNA replication and additional mechanisms contribute to Cyclin A2 localisation change in G2 phase. We find no evidence that such mechanisms involve G2 feedback loops and suggest that cytoplasmic appearance of Cyclin A2 at the S/G2 transition functions as a trigger for mitotic kinase activation. SynopsisMain mitotic kinases as PLK1 are activated at the S/G2 transition. A change in Cyclin A2 localisation at the S/G2 transition enables activation of PLK1. Main points-Cyclin A2 appears in the cytoplasm at the S/G2 transition -Association with replicating chromatin and p21 restricts Cyclin A2 to the nucleus -DNA damage ensures nuclear Cyclin A2 through p21 -Cytoplasmic Cyclin A2 initiates PLK1 activation Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=182 SRC="FIGDIR/small/191437v2_ufig1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@7e7e82org.highwire.dtl.DTLVardef@a3c9aforg.highwire.dtl.DTLVardef@10e11edorg.highwire.dtl.DTLVardef@baf6bd_HPS_FORMAT_FIGEXP M_FIG C_FIG