Uptake of valine and alanine by a neutral amino-acid carrier in sea urchin eggs: cyclic variations in the early cleavage stage

Abstract

The characteristics of valine and alanine uptake (respectively, preferential substrates of the L and A or ASC systems in mammalian cells) were studied in sea urchin eggs before and 40 min after fertilization. Substrate concentration dependence showed that in unfertilized eggs alanine is absorbed linearly up to an external concentration of 20mm, whereas valine uptake presented a saturable kinetic with aKm of 6 μm. Competition experiments showed that valine is absorbed by a carriermediated transport resembling the L system. Fertilization develops a new Na-dependent system, resembling the ASC system which is specific for neutral amino acids but does not discriminate between them. This system is superimposed on that of the unfertilized egg. In fertilized eggs, amino-acid transport displayed cyclic variations which have been previously associated with cell cleavage. We have found that eggs prevented from cleavage by treatment with antimitotic undergo a sequence of periodic amino-acid uptake timed with the mitotic cycle of untreated eggs. In addition, artificially activated eggs (A23187) which failed to divide showed a time course of amino-acid uptake similar to that observed in fertilized eggs. Furthermore, these variations are independent of protein synthesis. These results suggest to us that a biological clock exists in the cytoplasm or cortex of sea urchin eggs, which may be involved in timing the cell cycle.