Cyclin A is required at two points in the human cell cycle

Cyclins play a fundamental role in regulating cell cycle events in all eukaryotic cells. The human cyclin A gene was identified as the site of integration of hepatitis B virus in a hepatocarcinoma cell line; in addition, cyclin A is associated with the E2F transcription factor in a complex which is dissociated by the E1A oncogene product. Such findings suggest that cyclin A is a target for oncogenic signals. We have now found that DNA synthesis and entry into mitosis are inhibited in human cells micro-injected with anti-cyclin A antibodies at distinct times. Cyclin A binds both cdk2 and cdc2, giving two distinct cyclin A kinase activities, one appearing in S phase, the other in G₂. These results suggest that cyclin A defines novel control points of the human cell cycle.