PLK1 Activation in Late G2 Sets Up Commitment to Mitosis

TLDR

Commitment to mitosis must be tightly coordinated with DNA replication to preserve genome integrity, and while the timely activation of CyclinB1‑Cdk1 in late G2 triggers mitotic entry, the upstream regulatory mechanisms remain unclear. The study aims to identify the upstream regulators that activate CyclinB1‑Cdk1 and trigger mitotic entry. Plk1 is essential for mitotic entry, activating just before CyclinB1‑Cdk1, phosphorylating Cdc25C1 to promote entry, and its activation depends on CyclinA2‑Cdk, revealing Plk1’s critical role in CyclinB1‑Cdk1 activation and mitotic commitment.

Abstract

Commitment to mitosis must be tightly coordinated with DNA replication to preserve genome integrity. While we have previously established that the timely activation of CyclinB1-Cdk1 in late G2 triggers mitotic entry, the upstream regulatory mechanisms remain unclear. Here, we report that Polo-like kinase 1 (Plk1) is required for entry into mitosis during an unperturbed cell cycle and is rapidly activated shortly before CyclinB1-Cdk1. We determine that Plk1 associates with the Cdc25C1 phosphatase and induces its phosphorylation before mitotic entry. Plk1-dependent Cdc25C1 phosphosites are sufficient to promote mitotic entry, even when Plk1 activity is inhibited. Furthermore, we find that activation of Plk1 during G2 relies on CyclinA2-Cdk activity levels. Our findings thus elucidate a critical role for Plk1 in CyclinB1-Cdk1 activation and mitotic entry and outline how CyclinA2-Cdk, an S-promoting factor, poises cells for commitment to mitosis.