[A 45 kDa phosphorylated protein, resistant to alkaline treatment, appears at the time of rupture of the nuclear envelope during the 1st meiotic division of the Xenopus oocyte].

Xenopus oocytes were prelabeled with 32PO4 and induced to mature by progesterone treatment (1 microM). At the time of the breakdown of the germinal vesicle (nucleus), an alkali stable 45 kDa phosphoprotein appears in the 165 000 X g oocyte supernatant. Phospho-amino acid analysis shows that the 45 kDa protein is phosphorylated at threonine residues.