Ultraviolet light-induced G2 phase cell cycle checkpoint blocks cdc25-dependent progression into mitosis

Area of Science:

• Cell cycle regulation
• DNA damage response
• Molecular biology

Background:

• Cells possess checkpoints to prevent replication or segregation of damaged DNA.
• The G2 checkpoint delays cell division in response to DNA damage, such as that caused by ultraviolet (U.V.) radiation.

Purpose of the Study:

• To investigate the molecular mechanisms underlying the U.V.-induced G2 cell cycle delay.
• To determine how U.V. radiation affects the activation of key cell cycle regulators like cyclin B1/cdc2 and cyclin A/cdk.

Main Methods:

• Analysis of cyclin and cyclin-dependent kinase (cdk) complex expression and localization.
• Investigation of cdc25 phosphatase activity and localization in U.V.-irradiated cells.
• Cellular fractionation to separate cytoplasmic and nuclear components.

Main Results:

• U.V. radiation induces a G2 delay characterized by the accumulation of inactive cyclin B1/cdc2 and cyclin A/cdk complexes.
• Activation of these complexes is blocked at the cdc25-dependent step.
• While cdc25B and cdc25C phosphatases accumulate, their mitotic phosphorylation and activation are absent in U.V.-irradiated cells.
• Cyclin B1/cdc2 accumulates in the cytoplasm, while cdc25B and cdc25C accumulate in the nucleus, leading to physical separation and impaired activation.

Conclusions:

• The U.V.-induced G2 checkpoint operates by preventing the activation of cyclin B1/cdc2 and cyclin A/cdk complexes.
• This block is partly due to the cytoplasmic localization of cyclin B1/cdc2 and the nuclear localization of activated cdc25 phosphatases.
• The U.V. checkpoint is likely positioned at the S/G2 transition or early G2 phase, preceding cyclin A/cdk2 activation.