Molecular Biology of the Cell Vol. 21, 3137–3148, September 15, 2010 KHDC1B Is a Novel CPEB Binding Partner Speciﬁcally Expressed in Mouse Oocytes and Early Embryos Congli Cai,* Keiko Tamai,† and Kathleen Molyneaux* *Department of Genetics, School of Medicine, Case Western Reserve University, Cleveland, OH 44106; and †Daiich Sankyo Co. Ltd., Tokyo 103-8426, Japan Submitted March 30, 2010; Revised July 9, 2010; Accepted July 16, 2010 Monitoring Editor: Kunxin Luo mRNAs required for meiotic maturation and early embryonic development are stored in growing oocytes. These transcripts are translationally repressed until hormonal cues trigger ovulation. Errors in translation underlie some cases of human infertility and are associated with ovarian germ cell tumors. However, it remains unclear how maternal transcripts are kept quiescent in mammals. This study describes a potential translational regulator, KHDC1B. KHDC1B is a member of a small family of KH-domain containing proteins speciﬁc to eutherian mammals. Two family members, KHDC1A and 1B, are highly expressed in oocytes. KHDC1A and 1B bind polyU agarose and form oligomers like other KH-domain proteins. The functions of these proteins were tested by expression in Xenopus embryos. KHDC1A caused cell death, whereas KHDC1B caused cleavage arrest. This arrest phenotype was rescued by coexpression of the mouse translational regulator cytoplasmic polyadenylation binding protein 1 (mCPEB1). Coimmunoprecipitation and coimmu- nostaining experiments conﬁrmed the functional interaction between KHDC1B and mCPEB1. Finally, KHDC1B levels and binding partners were shown to ﬂuctuate with the cell cycle. KHDC1B, via its interaction with mCEPB1, may regulate translation of mRNA targets required for oocyte maturation. INTRODUCTION Oocyte maturation and early embryonic development take place in the absence of transcription and rely on maternally stored mRNAs and proteins. The temporal and in some cases the spatial expression of these stored mRNAs control cell cycle progression and embryonic patterning until zy- gotic gene expression is initiated. Consequently, the regula- tion of mRNA stability, localization, and translation serve as the driving forces behind oogenesis and early embryogene- sis (Colegrove-Otero et al., 2005). Multiple translational regulators including members of the DAZ (deleted in azoospermia), PUF (Pumilio and FBF), STAR/QKI (Signal transduction and activator of mRNA/ Quaking), and Bicaudal C families have been shown to be vital for oocyte development with evidence accumulating from both invertebrate and vertebrate systems (Colegrove- Otero et al., 2005; Kimble and Crittenden, 2007). Perhaps the best studied of these factors, at least at the biochemical level, is cytoplasmic polyadenylation element binding protein (CPEB). CPEB is an RNA recognition motif (RRM) protein originally identiﬁed in Xenopus oocytes (Hake and Richter, 1994). CPEB controls the translation of speciﬁc mRNA tar- gets during meiotic maturation (Stebbins-Boaz et al., 1996) This article was published online ahead of print in MBoC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E10 – 03– 0255) on July 28, 2010. Address correspondence to: Congli Cai (cxc242@case.edu). © 2010 C. Cai et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by- nc-sa/3.0). and early embryonic development (Groisman et al., 2000). In growing oocytes, CPEB binds mRNA targets containing the cytoplasmic polyadenylation element (CPE), a U-rich motif (consensus UUUUUAU) found in the 3 -untranslated region (UTR) typically within 100 nucleotides of the polyadenyla- tion site. Initially, CPEB acts as a translational repressor by recruiting the eIF4E (eukaryotic initiation factor 4E) binding protein Maskin to the RNP complex (Stebbins-Boaz et al., 1999). Maskin binds the cap-associated eIF4E, blocking its interaction with eIF4G and preventing assembly of the ribo- some on the transcript. The hormonal signals that control oocyte maturation lead to the activation of the Aurora A/Eg2 kinase that phosphorylates CPEB (Mendez et al., 2000). Phospho-CPEB leads to activation of the cytoplasmic polyadenylase, xGld2 (Barnard et al., 2004; Papin et al., 2008), elongation of the polyA tail, displacement of Maskin, and assembly of an active translational complex. The core role for CPEB in translational control during oocyte maturation appears to be conserved in both inverte- brate and vertebrate model systems. However, many of the details of this process remain vague in mammals. An oocyte speciﬁc loss of mouse CPEB1 (mCPEB1) results in abnormal follicle growth suggesting a role for mCPEB1 before its canonical role in meiotic maturation (Racki and Richter, 2006). Additionally, there are four CPEB paralogs in mam- mals, and three are expressed in mouse oocytes (Evsikov and Marin de Evsikova, 2009b), complicating analysis in this system. It is also puzzling that mGld2/Papd4 knockout fe- males are fertile, leaving the identity of the cytoplasmic polyadenylase unclear in mice (Nakanishi et al., 2007). Fi- nally, sequence comparisons have failed to identify a mam- malian Maskin homologue. Transforming acidic coiled- coiled domain proteins (TACCs) share some sequence homology with Maskin but lack the eIF4E binding region, and it is unclear whether they participate in translational repression (de Moor et al., 2005). The discovery of both 3137