Activation and role of mitogen-activated protein (MAP) kinase (MAPK) during mitosis are still matters of controversy in early embryos. We report here that an ERK-like protein is present and highly phosphorylated in unfertilized sea urchin eggs. This MAPK becomes dephosphorylated after fertilization and a small pool of it is transiently reactivated during mitosis. The phosphorylated ERK-like protein is localized to the nuclear region and then to the mitotic poles and the mitotic spindle. Treatment of eggs after fertilization with two different MEK inhibitors, PD 98059 and U0126, at low concentrations capable to selectively induce dephosphorylation of this ERK-like protein, or expression of a dominant-negative MEK1/2, perturbed mitotic progression. Our results suggest that an ERK-like cascade is part of a control mechanism that regulates mitotic spindle formation and the attachment of chromosomes to the spindle during the first mitosis of the sea urchin embryo.
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