eIF4E-binding proteins are differentially modified after ammonia versus intracellular calcium activation of sea urchin unfertilized eggs.

Fertilization of sea urchin eggs triggers a rise of protein synthesis mainly dependent on the cap-binding protein eIF4E, which is released from its repressor 4E-BP and associates with eIF4G. Association of eIF4G with eIF4E is a crucial event for the onset of the first mitotic division following fertilization. Artificial activation of unfertilized eggs with the calcium ionophore A23187 results in the activation of protein synthesis comparable to the one triggered by fertilization, while increasing the intracellular pH by ammonia treatment results in partial activation of protein synthesis. Nevertheless, artificial activation does not induce the mitotic division. Here we investigate the effect of calcium ionophore and ammonia treatment of unfertilized eggs on eIF4E and its two antagonist partners, 4E-BP and eIF4G. We show that the addition of calcium ionophore to unfertilized eggs induces permanent dissociation between eIF4E and 4E-BP, whereas a reversible dissociation of the complex occurs after ammonia treatment. The regulation of the complex correlates with permanent or reversible 4E-BP disappearance depending on the treatment used to trigger artificial activation. Furthermore, while calcium ionophore treatment of unfertilized eggs induces eIF4G modifications comparable to those observed following fertilization, ammonia treatment does not. These results suggest that ionophore and ammonia treatments of unfertilized eggs induce differential protein synthesis activation by targeting eIF4E availability and specific regulation through its two partners 4E-BP and eIF4G.