c-Ets-1 protein is hyperphosphorylated during mitosis

The ets-1 and ets-2 proto-oncogene products can serve as transcription factors and become phosphorylated in response to Ca(2+)-mediated signals. We have examined expression of Ets proteins during the cell cycle in cells synchronized by centrifugal elutriation or nocodazole-induced mitotic block. Both methods revealed the presence of a hyperphosphorylated isoform of Ets-1 during the mitotic phase. This isoform showed a characteristic mobility shift and was observed during mitosis in each of four cell lines (three human T-cell lines and a human astrocytoma) that express ets-1. In elutriated cells, only a small portion of the Ets-1 in cells from the G2/M fractions was hyperphosphorylated, while in nocodazole-arrested cells more of the Ets-1 was shifted. When cells were released from nocodazole arrest, this isoform disappeared within 1-2 h as cells completed mitosis and entered G1. This suggests that hyperphosphorylated Ets-1 is present transiently during early mitosis, before or around the time of the metaphase-anaphase transition. Exposure of unsynchronized cells to okadaic acid resulted in a dramatic hyperphosphorylation of virtually all Ets-1, suggesting that changes in cellular phosphatase activity are important for cell cycle regulation of Ets-1. Hyperphosphorylated Ets-1 appears to arise from multiple phosphorylations on serine in the exon 7-encoded domain of the protein and did not appear to alter sequence-specific DNA-binding activity. Although enhanced phosphorylation of Ets-2 was detected in nocodazole-arrested cells, no Ets-2 hyperphosphorylation was seen.