Abstract
A p38(MAPK) homolog Mipk (meiosis-inhibited protein kinase) was cloned from seastar oocytes. This 40-kDa protein shares approximately 65% amino acid identity with mammalian p38-alpha isoforms. Mipk was one of the major tyrosine-phosphorylated proteins in immature oocytes arrested at the G(2)/M transition of meiosis I. The tyrosine phosphorylation of Mipk was increased in response to anisomycin, heat, and osmotic shock of oocytes. During 1-methyladenine-induced oocyte maturation, Mipk underwent tyrosine dephosphorylation and remained dephosphorylated in mature oocytes and during the early mitotic cell divisions until approximately 12 h after fertilization. At the time of differentiation and acquisition of G phases in the developing embryos, Mipk was rephosphorylated on tyrosine. In oocytes that were microinjected with Mipk antisense oligonucleotides and subsequently were allowed to mature and become fertilized, differentiation was blocked. Because MipK antisense oligonucleotides and a dominant-negative (K62R)Mipk when microinjected into immature oocytes failed to induce germinal vesicle breakdown, inhibition of Mipk function was not sufficient by itself to cause oocyte maturation. These findings point to a putative role for Mipk in cell cycle control as a G-phase-promoting factor.
Highlights
The meiotic maturation of an immature oocyte into a fertilizable egg is commonly the first developmental step in an organism
In seastar oocytes, whose maturation is induced with the hormone 1-methyladenine, several protein kinases appear to contribute to the global protein phosphorylation that precedes germinal vesicle breakdown (GVBD) [5]
In Xenopus laevis Erk2 appears to play a role in the formation of the active maturation-promoting factor (MPF) complex, whereas in the seastar, maximal maturation-activated protein kinase (Mapk) stimulation occurs after the initiation of GVBD and later than MPF activation [20]
Summary
The meiotic maturation of an immature oocyte into a fertilizable egg is commonly the first developmental step in an organism. In seastar oocytes, whose maturation is induced with the hormone 1-methyladenine, several protein kinases appear to contribute to the global protein phosphorylation that precedes GVBD [5]. Some of these kinases, such as the phosphatidylinositol 3-kinase [6], ribosomal S6 kinase [7,8,9], and casein kinase 2 [10] appear to mediate the re-entry of G0-arrested somatic cells into the G1 phase of the cell cycle, despite the profound differences in the end points. Erk, Mapk possesses a Thr-Glu-Tyr motif, which must be phosphorylated on the tyrosine residue by a Mek-like kinase for maximal activation.. The occurrence of other MAP kinases besides Mapk has not been reported in the seastar
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
