Abstract
A 95-kDa protein in Xenopus oocytes, Xp95, was shown to be phosphorylated from the first through the second meiotic divisions during progesterone-induced oocyte maturation. Xp95 was purified and cloned. The Xp95 protein sequence exhibited homology to mouse Rhophilin, budding yeast Bro1, and Aspergillus PalA, all of which are implicated in signal transduction. It also contained three conserved features including seven conserved tyrosines, a phosphorylation consensus sequence for the Src family of tyrosine kinases, and a proline-rich domain near the C terminus that contains multiple SH3 domain-binding motifs. We showed the following: 1) that both Xp95 isolated from Xenopus oocytes and a synthetic peptide containing the Src phosphorylation consensus sequence of Xp95 were phosphorylated in vitro by Src kinase and to a lesser extent by Fyn kinase; 2) Xp95 from Xenopus oocytes or eggs was recognized by an anti-phosphotyrosine antibody, and the relative abundance of tyrosine-phosphorylated Xp95 increased during oocyte maturation; and 3) microinjection of deregulated Src mRNA into Xenopus oocytes increased the abundance of tyrosine-phosphorylated Xp95. These results suggest that Xp95 is an element in a tyrosine kinase signaling pathway that may be involved in progesterone-induced Xenopus oocyte maturation.
Highlights
Xenopus oocyte maturation is the process through which fully grown Xenopus oocytes mature into unfertilized eggs [1,2,3]
The G2/M block of stage VI oocytes is released by the steroid hormone progesterone, which initiates a series of events that lead to induction of the first and second meiotic divisions
Many of the components in the maturation induction pathway are still unknown. It is unclear how Mos and Raf protein kinases are activated following progesterone stimulation; how activation of MAP kinase by Raf and Mos protein kinases leads to the activation of Cdc25 phosphatase; and how the phosphorylation orchestrated by multiple mitotic kinases induces meiotic divisions
Summary
Xenopus oocyte maturation is the process through which fully grown Xenopus oocytes (stage VI) mature into unfertilized eggs [1,2,3]. These results suggest that Xp95 is an element in a tyrosine kinase signaling pathway that may be involved in progesterone-induced Xenopus oocyte maturation.
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