HORMONE-INDUCED MEIOTIC MATURATION IN XENOPUS OOCYTES OCCURS INDEPENDENTLY OF P70(S6K) ACTIVATION AND IS ASSOCIATED WITH ENHANCED INITIATION-FACTOR (EIF)-4F PHOSPHORYLATION AND COMPLEX-FORMATION

Hormone-induced meiotic maturation of the Xenopus oocyte is regulated by complex changes in protein phosphorylation. It is accompanied by a stimulation in the rate of translation, manifest at the level of polyp eptide chain initiation, At later times in the maturation process, thi s reflects an increased ability for mRNA to interact with the 40 S rib osomal subunit. In mammalian cells there is growing evidence for the r egulation of translation by phosphorylation of ribosomal protein S6 an d of initiation factors responsible for the binding of mRNA to ribosom es, In this report, we show that although the 70 kDa form of S6 kinase is activated within 1,5 hours in response to progesterone or insulin, a time critical for protein synthesis, its activation is not required for hormone-induced stimulation of translation rates or maturation, I n response to progesterone, activation of translation occurs in parall el with enhanced phosphate labelling of eIF-4 alpha and eIF-4 gamma an d eIF-4F complex formation, events which are thought to facilitate the interaction of eIF-4F with the mRNA cap structure, However, with insu lin, activation of translation occurs prior to detectable de novo phos phorylation of eIF-4F, although a small enhancement of turnover of pho sphate on elF-4 alpha may occur at this early time, With either hormon e, enhanced phosphate labelling of eIF-4 alpha is shown to reflect act ivation of eIF-4 alpha kinase(s), which co-incides temporally with act ivation of p42 MAP and p90(rsk) kinases, The possible role of initiati on factor modification on increased translation rates during meiotic m aturation is discussed.