Synthesis and turn-over of the replicative Cdc6 protein during the HeLa cell cycle.

The human replication protein Cdc6p is translocated from its chromatin sites to the cytoplasm during the replication phase (S phase) of the cell cycle. However, the amounts of Cdc6p on chromatin remain high during S phase implying either that displaced Cdc6p can rebind to chromatin, or that Cdc6p is synthesized de novo. We have performed metabolic labeling experiments and determined that [35S]methionine is incorporated into Cdc6p at similar rates during the G1 phase and the S phase of the cell cycle. Newly synthesized Cdc6p associates with chromatin. Pulse-chase experiments show that chromatin-bound newly synthesized Cdc6p has a half life of 2-4 h. The results indicate that, once bound to chromatin, pulse-labeled new Cdc6p behaves just as old Cdc6p: it dissociates and eventually disappears from the nucleus. The data suggest a surprisingly dynamic behaviour of Cdc6p in the HeLa cell cycle.