Dual ionic controls for the activation of protein synthesis at fertilization

Abstract

The general metabolic activation of the sea urchin egg at fertilization is dependent on a release of intracellular stores of calcium and the subsequent transient elevation of intracellular Ca²⁺ (refs 1–3). However, this elevation does not by itself lead to increased macromolecular synthesis and development but initiates steps which result in a long-term elevation of intracellular pH (refs 4–6). Among the developmental processes dependent on the elevation of intracellular pH is the large acceleration in the rate of protein synthesis at fertilization⁶. Weak penetrating bases such as ammonia can be used to mimic the processes resulting in an increase in intracellular pH and so show the corresponding increases in protein synthesis rate^6–9. Conversely, it is possible to demonstrate a gradual but complete shut down of protein synthesis if the intracellular pH is reduced to the unfertilized level with penetrating weak acids⁶. However, the rate of protein synthesis in ammonia-activated eggs lags behind that of fertilized controls even though ammonia activation can result in an intracellular pH increase greater than occurs in the fertilized egg^6,8,10. This result has led to the suggestion that factors other than intracellular pH may be regulating protein synthesis following fertilization^6,10. To investigate the possibility that the Ca²⁺ transient may have such a role, we measured the rate of amino acid incorporation in eggs that were activated in various ionic conditions which enabled the effects of Ca²⁺ and pH changes to be studied separately. Our results, reported here, show that if intracellular pH is elevated, increases in intracellular Ca²⁺ play an additional part in the activation of protein synthesis at fertilization.