The p34cdc2 kinase, which regulates the G2/M transition in eukaryotic cells, is composed of a complex between p34cdc2 and cyclin-B. The activity of this kinase, and intracellular levels of cyclin-B, achieve a maximum in the G2/M phases of both HeLa and Chinese hamster ovary (CHO) cells. The responses of p34cdc2 and cyclin-B proteins were compared between the two cell lines under conditions of G2 arrest induced by the DNA damaging agent, etoposide. p34cdc2 is a major tyrosine phosphorylated protein in the S- and G2-phases of both HeLa and CHO cells, although fundamental differences appear to exist in p34cdc2 regulation; approximately 80% of this protein is found in its hyperphosphorylated form in CHO cells compared to a maximum of 30% in HeLa cells. A brief exposure to etoposide causes both cell lines to arrest in G2, with a concomittant increase in cyclin-B levels and accumulation of hyperphosphorylated p34cdc2. Failure to activate the p34cdc2 kinase following etoposide treatment to levels comparable with synchronous G2/M-phase cells is not due to inhibition of p34cdc2/cyclin-B complex formation, but relies more on an inability to tyrosine dephosphorylate p34cdc2. Thus, regardless of apparent differences in p34cdc2/cyclin-B regulation between a human tumor and an immortalized rodent cell line, both appear to express the same G2 checkpoint for etoposide-induced DNA strand breaks, that of inhibition of p34cdc2 tyrosine dephosphorylation.

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