XGef Mediates Early CPEB Phosphorylation duringXenopusOocyte Meiotic Maturation

Abstract

Polyadenylation-induced translation is an important regulatory mechanism during metazoan development. During Xenopus oocyte meiotic progression, polyadenylation-induced translation is regulated by CPEB, which is activated by phosphorylation. XGef, a guanine exchange factor, is a CPEB-interacting protein involved in the early steps of progesterone-stimulated oocyte maturation. We find that XGef influences early oocyte maturation by directly influencing CPEB function. XGef and CPEB interact during oogenesis and oocyte maturation and are present in a c-mos messenger ribonucleoprotein (mRNP). Both proteins also interact directly in vitro. XGef overexpression increases the level of CPEB phosphorylated early during oocyte maturation, and this directly correlates with increased Mos protein accumulation and acceleration of meiotic resumption. To exert this effect, XGef must retain guanine exchange activity and the interaction with CPEB. Overexpression of a guanine exchange deficient version of XGef, which interacts with CPEB, does not enhance early CPEB phosphorylation. Overexpression of a version of XGef that has significantly reduced interaction with CPEB, but retains guanine exchange activity, decreases early CPEB phosphorylation and delays oocyte maturation. Injection of XGef antibodies into oocytes blocks progesterone-induced oocyte maturation and early CPEB phosphorylation. These findings indicate that XGef is involved in early CPEB activation and implicate GTPase signaling in this process.

MAPK interacts with XGef and is required for CPEB activation during meiosis inXenopusoocytes

“…Overexpression in oocytes of a guaninenucleotide-exchange-deficient version of XGef that is unable to activate small GTPases does not enhance early CPEB phosphorylation, as observed upon overexpression of wildtype XGef (Martinez et al, 2005). We therefore suspected that XGef influences early CPEB phosphorylation through activation of a Rho-family GTPase.…”

“…When XGef is overexpressed in Xenopus oocytes, we observe an increase in the level of phosphorylated CPEB during early meiosis, earlier elongation of poly (A) tails on Mos mRNA, earlier accumulation of Mos protein and an acceleration of meiotic progression by ~25% (Martinez et al, 2005;Reverte et al, 2003). A version of XGef that has very reduced interaction with CPEB functions in a dominantnegative manner and substantially impairs these events.…”

“…XGef interacts with CPEB throughout meiosis and is present with CPEB in a Mos messenger ribonucleoprotein (mRNP), suggestive of an ongoing role of XGef in either regulation of CPEB activity or CPEB-mediated processes. The stimulatory effect of XGef on meiosis correlates directly with the early, activating phosphorylation of CPEB (Martinez et al, 2005), suggesting that XGef has a role in the signaling mechanism leading to activation of CPEB.…”

Phospho-Signaling at Oocyte Maturation and Fertilization: Set Up for Embryogenesis and Beyond Part II. Kinase Regulators and Substrates

“…GEF activity of XGef is required for Mos synthesis. In addition, interaction between XGef is responsible for an increase in CPEB phosphorylation during oocyte maturation, which is important for CPEB activation (Martinez et al 2005). Further studies have shown that MAPK interacts with XGef and acts as a kinase of CPEB on Thr-22, Thr-164, Ser-184, and Ser-248 (Keady et al 2007).…”

Starfish andXenopusoocyte maturation

“…Progesterone causes a series of reactions culminating in the translation of the RNA for the protein Pp39mos, laying till that point inactive in the cytoplasm. This happens because a protein called CEBP gets phosphorylated by the effect of the protein Xgef (Martinez et al 2005) and binds to a sequence of this RNA (Howard et al 1999;Dickson et al 1999;Reverte et al 2003), made by 6 U and 1 A, causing the RNA polyadenylation (Hake & Richter 1994;Sheets et al 1994;Barkoff et al 1998) in 3'. At the same time the cap at its 5' end becomes methylated (Kuge et al 1998).…”