Intracellular pH and ribosomal protein S6 phosphorylation: role in translational control in Xenopus oocytes

The induction of amphibian oocyte maturation with progesterone as well as the activation of sea urchin eggs at the time of fertilization result in increased protein synthesis. The increase in both cases involves the recruitment of maternal mRNA onto polysomes. Further, it has been reported that sea urchin eggs, like full-grown Xenopus oocytes, contain no spare translational capacity based on the observation that injected heterologous mRNA is translated only at the expense of endogenous messages. The nature of the limiting component defined by such experiments is not known, but two factors which have been proposed to play a role in regulating protein synthesis are ribosomal protein S6 phosphorylation and intracellular pH. In the current paper, we review the literature and present new evidence on the roles intracellular pH and S6 phosphorylation have in regulating protein synthesis in Xenopus oocytes. We report that pHi does not increase between stage 3 and stage 6, yet the protein synthetic rate increases at least eight fold during the same period. Hence, we conclude that increasing pHi is not a prerequisite for increasing protein synthesis. Moreover, we present three arguments against increased ribosomal protein S6 phosphorylation being sufficient or necessary for increased protein synthesis in Xenopus oocytes. First, the level of S6 phosphorylation does not increase between stages 4 and 6, a period exhibiting a two to three fold increase in protein synthesis. Second, the injection of globin mRNA into stage-4 oocytes increases total protein synthesis two to three fold, but has no effect on S6 phosphorylation. Third, when the injection of globin mRNA into stage-4 oocytes is followed by an injection of MPF, a dramatic increase in S6 phosphorylation is seen, but total protein synthesis is not further stimulated.