Protein synthesis in sea urchin eggs: a "late" response to fertilization.

Fertilization of echinoderm eggs results in a complex series of metabolic activations, resulting in greater than 30 changes within the first ten minutes after insemination. The most prominent of these are modifications in structure, ' increases in respiration rate2 and coenzyme content,3 increases in substrate uptake,4-6 transient proteolytic activity,7 and a decrease in external pH.8 These changes are possibly related to enzymic activations leading to the synthesis of lipids, proteins, and nucleic acids required for cell division and differentiation. Studies on the temporal sequence of these diverse physiological reactions could provide insights into the mechanisms and interrelationships of these changes. Thus far, such temporal data are available on the light-scattering and external pH changes9 (resulting from structural changes in the cell cortex), activation of NAD kinase3 (resulting in NADP and NADPH synthesis), and activation of respiration.2 The present report concerns the temporal relationships of the above events to the postfertilization increase in protein synthesis, and also attempts to resolve contradictory results regarding amino acid incorporation in unfertilized eggs. Previous studies have shown that the rate of protein synthesis is low or negligible in unfertilized eggs, and increases markedly after fertilization.6 10-12 This increased rate is apparently dependent on mRNA already present in unfertilized egg cytoplasm, since it is unaffected by either actinomycin D13 or enucleation.14 Current hypotheses regarding activation mechanisms of this increased protein synthesis, which need not be mutually exclusive, implicate (1) structural changes in ribosomes resulting from protease activation at fertilization,'5 (2) synthesis of a factor(s) controlling mRNA translation rate,16 or (3) energy-dependent processes involved in mRNA attachment.'7 In the present study, effects of changes in cellular amino acid permeability following fertilization" 6 were minimized by "preloading" unfertilized eggs with radioactive amino acid, and incorporation kinetics measured by sampling at close intervals following fertilization. The results indicate that the activation of protein synthesis is actually a "late" response to fertilization, since increased synthesis does not begin until six to ten minutes after insemination. The results also show that unfertilized eggs transport and concentrate leucine and valine, and incorporate these amino acids into protein; that a sizeable amount of added leucine is converted to compounds not involved with protein synthesis; and that the rate of this conversion is accelerated by fertilization.