S6 phosphorylation is regulated at multiple levels in Xenopus laevis oocytes

It is known that the 40s ribosomal protein S6 undergoes a dramatic increase in its level of phosphorylation during Xenopus oocyte meiotic maturation in response to progesterone stimulation. During prophase arrest, the majority of S6 has 0 moles phosphate per mole protein; this increases to 4-5 moles phosphate per mole protein by the time of germinal vesicle breakdown (GVBD). Our in vitro and in vivo studies indicate that the accumulation of phosphate on S6 is the net result of a 4-5-fold increase in S6 kinase activity and a 30-50% decrease in the rate of dephosphorylation and/or turnover of phosphate groups on S6 in maturing oocytes. In addition, the level of phosphorylation of S6 on 80s monosomes injected into non-hormone-stimulated oocytes was unexpectedly high. This indicates that the S6 kinase/phosphatase ratio in prophase arrested oocytes is higher than anticipated from previous studies. This observation implies that the majority of the oocyte ribosomes may be sequestered from any S6 kinase during meiotic prophase. Furthermore, these observations suggest that a portion of the increased accumulation of phosphate on S6 may be the result of increased accessibility of the ribosomes to S6 kinase during oocyte meiotic maturation.