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• W2024283568 abstract "Translational activation of dormant cyclin B1 mRNA stored in oocytes is a prerequisite for the initiation or promotion of oocyte maturation in many vertebrates. Using a monoclonal antibody against the domain highly homologous to that ofDrosophila Pumilio, we have shown for the first time in any vertebrate that a homolog of Pumilio is expressed inXenopus oocytes. This 137-kDa protein binds to the region including the sequence UGUA at nucleotides 1335–1338 in the 3′-untranslated region of cyclin B1 mRNA, which is close to but does not overlap the cytoplasmic polyadenylation elements (CPEs). Physical in vitro association of XenopusPumilio with a Xenopus homolog of Nanos (Xcat-2) was demonstrated by a protein pull-down assay. The results of immunoprecipitation experiments showed in vivo interaction between Xenopus Pumilio and CPE-binding protein (CPEB), a key regulator of translational repression and activation of mRNAs stored in oocytes. This evidence provides a new insight into the mechanism of translational regulation through the 3′-end of mRNA during oocyte maturation. These results also suggest the generality of the function of Pumilio as a translational regulator of dormant mRNAs in both invertebrates and vertebratesAB045628. Translational activation of dormant cyclin B1 mRNA stored in oocytes is a prerequisite for the initiation or promotion of oocyte maturation in many vertebrates. Using a monoclonal antibody against the domain highly homologous to that ofDrosophila Pumilio, we have shown for the first time in any vertebrate that a homolog of Pumilio is expressed inXenopus oocytes. This 137-kDa protein binds to the region including the sequence UGUA at nucleotides 1335–1338 in the 3′-untranslated region of cyclin B1 mRNA, which is close to but does not overlap the cytoplasmic polyadenylation elements (CPEs). Physical in vitro association of XenopusPumilio with a Xenopus homolog of Nanos (Xcat-2) was demonstrated by a protein pull-down assay. The results of immunoprecipitation experiments showed in vivo interaction between Xenopus Pumilio and CPE-binding protein (CPEB), a key regulator of translational repression and activation of mRNAs stored in oocytes. This evidence provides a new insight into the mechanism of translational regulation through the 3′-end of mRNA during oocyte maturation. These results also suggest the generality of the function of Pumilio as a translational regulator of dormant mRNAs in both invertebrates and vertebratesAB045628. maturation-promoting factor mitogen-activated protein kinase germinal vesicle breakdown cytoplasmic polyadenylation element CPE-binding protein untranslated region hunchback nanos response element modified Barth's saline buffered with Hepes reverse transcription polymerase chain reaction glutathioneS-transferase open reading frame base pair(s) kilobase(s) rapid amplification of cDNA ends Xenopus Pumilio The final inducer of oocyte maturation is the maturation-promoting factor (MPF),1 which consists of Cdc2 and cyclin B. MPF is stored in immature oocytes as an inactive form (called pre-MPF), although its amount differs from species to species (1Yamashita M. Mita K. Yoshida N. Kondo T. Prog. Cell Cycle Res. 2000; 4: 115-129Crossref PubMed Scopus (101) Google Scholar, 2Taieb F. Thibier C. Jessus C. Mol. Reprod. Dev. 1997; 48: 397-411Crossref PubMed Scopus (77) Google Scholar). In Xenopus (as well as fish and mammals except for mice), the initiation of oocyte maturation requires proteins (called “initiators”) newly synthesized by translational activation of dormant mRNAs stored in oocytes (masked mRNAs). Mos functions as an initiator with the aid of mitogen-activated protein kinase (MAPK) (3Yew N. Mellini M.L. Vande Woude G.F. Nature. 1992; 355: 649-652Crossref PubMed Scopus (202) Google Scholar, 4Sagata N. Oskarsson M. Copeland T. Brumbaugh J. Vande Woude G.F. Nature. 1988; 335: 519-525Crossref PubMed Scopus (462) Google Scholar, 5Sagata N. Daar I. Oskarsson M. Showalter S.D. Vande Woude G.F. Science. 1989; 245: 643-646Crossref PubMed Scopus (249) Google Scholar, 6Kosako H. Gotoh Y. Nishida E. EMBO J. 1994; 13: 2131-2138Crossref PubMed Scopus (189) Google Scholar, 7Huang W. Kessler D.S. Erikson R.L. Mol. Biol. Cell. 1995; 6: 237-245Crossref PubMed Scopus (110) Google Scholar, 8Haccard O. Lewellyn A. Hartley R.S. Erikson E. Maller J.L. Dev. Biol. 1995; 168: 677-682Crossref PubMed Scopus (147) Google Scholar, 9Gotoh Y. Masuyama N. Dell K. Shirakabe K. Nishida E. J. Biol. Chem. 1995; 270: 25898-25904Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar). The Cdc2 molecule of pre-MPF is phosphorylated on threonine 14/tyrosine 15 by Myt1 and threonine 161 by cyclin-dependent kinase-activating kinase. The Mos/MAPK pathway probably leads to the activation of Cdc25, which dephosphorylates threonine 14/tyrosine 15 via the polo-like kinase (10Abrieu A. Brassac T. Galas S. Fisher D. Labbe J.C. Doree M. J. Cell Sci. 1998; 111: 1751-1757Crossref PubMed Google Scholar, 11Karaı̈skou A. Jessus C. Brassac T. Ozon R. J. Cell Sci. 1999; 112: 3747-3756PubMed Google Scholar, 12Qian Y.-W. Erikson E. Li C. Maller J.L. Mol. Cell. Biol. 1998; 18: 4262-4271Crossref PubMed Scopus (212) Google Scholar), and to the inhibition of Myt1 through the activation of p90rsk (13Palmer A. Gavin A.-C. Nebreda A.R. EMBO J. 1998; 17: 5037-5047Crossref PubMed Scopus (288) Google Scholar, 14Gavin A.-C. Ainle A.N. Chierici E. Jones M. Nebreda A.R. Mol. Biol. Cell. 1999; 10: 2971-2986Crossref PubMed Scopus (35) Google Scholar).Enhanced cyclin B synthesis is a ubiquitous event that occurs during oocyte maturation in all species examined so far, and it is indispensable for initiating oocyte maturation in fish and amphibians except Xenopus, in which pre-MPF is absent in immature oocytes (15Yamashita M. Semin. Cell Dev. Biol. 1998; 9: 569-579Crossref PubMed Scopus (76) Google Scholar). The neosynthesized cyclin B is also required for leading oocyte maturation beyond germinal vesicle breakdown (GVBD) in mammals, in which pre-MPF in immature oocytes is only sufficient for inducing GVBD (16Polanski Z. Ledan E. Brunet S. Louvet S. Verlhac M.-H. Kubiak J.Z. Maro B. Development. 1998; 125: 4989-4997PubMed Google Scholar, 17Hampl A. Eppig J.J. Mol. Reprod. Dev. 1995; 40: 9-15Crossref PubMed Scopus (76) Google Scholar). In contrast, it has been shown that newly synthesized cyclins upon progesterone stimulation are not required for initiating oocyte maturation in Xenopus (18Minshull J. Murray A. Colman A. Hunt T. J. Cell Biol. 1991; 114: 767-772Crossref PubMed Scopus (83) Google Scholar, 19Ferby I. Blazquez M. Palmer A. Eritja R. Nebreda A.R. Genes Dev. 1999; 13: 2177-2189Crossref PubMed Scopus (146) Google Scholar). Like c-mosmRNA, however, translation of cyclin B1 mRNA is activated during Xenopus oocyte maturation (20Kobayashi H. Minshull J. Ford C. Golsteyn R. Poon R. Hunt T. J. Cell Biol. 1991; 114: 755-765Crossref PubMed Scopus (223) Google Scholar, 21de Moor C.H. Richter J.D. Mol. Cell. Biol. 1997; 17: 6419-6426Crossref PubMed Scopus (129) Google Scholar). Artificial translational activation of endogenous cyclin B1 mRNA brings about GVBD without progesterone and Mos (22de Moor C.H. Richter J.D. EMBO J. 1999; 18: 2294-2303Crossref PubMed Scopus (173) Google Scholar, 23Barkoff A.F. Dickson K.S. Gray N.K. Wickens M. Dev. Biol. 2000; 220: 97-109Crossref PubMed Scopus (76) Google Scholar). Moreover, the finding that progesterone induces cyclin B1 synthesis without any activities of Mos, MAPK, and MPF strongly suggests that progesterone-induced cyclin B1 synthesis is a physiological trigger of pre-MPF activation (24Frank-Vaillant M. Jessus C. Ozon R. Maller J.L. Haccard O. Mol. Biol. Cell. 1999; 10: 3279-3288Crossref PubMed Scopus (76) Google Scholar). Taken together, the results of past studies strongly suggest that the translational activation of cyclin B1 mRNA universally plays a key role in initiation of oocyte maturation in lower vertebrates, includingXenopus (25Yamashita M. Zool. Sci. 2000; 17: 841-851Crossref Scopus (29) Google Scholar), and an understanding of its control mechanisms during oocyte maturation is therefore of particular importance.Over the past decade, the mystery of how translationally repressed mRNAs in immature oocytes are released from masking at the appropriate time during oocyte maturation has been partially solved. Translational control of masked mRNAs stored in oocytes is mediated by the U-rich motif U4–6A1–2U, named cytoplasmic polyadenylation element (CPE), present in their 3′-untranslated region (3′-UTR) (for review see Ref. 26Hake L.E. Richter J.D. Biochim. Biophys. Acta. 1997; 1332: M31-M38PubMed Google Scholar). The CPE sequence is the target site for CPE-binding protein (CPEB), which facilitates polyadenylation of the target mRNA during oocyte maturation (27Hake L.E. Richter J.D. Cell. 1994; 79: 617-627Abstract Full Text PDF PubMed Scopus (361) Google Scholar). In addition to the involvement in translational activation, CPEB has recently been shown to mediate the masking of cyclin B1 mRNA (22de Moor C.H. Richter J.D. EMBO J. 1999; 18: 2294-2303Crossref PubMed Scopus (173) Google Scholar, 23Barkoff A.F. Dickson K.S. Gray N.K. Wickens M. Dev. Biol. 2000; 220: 97-109Crossref PubMed Scopus (76) Google Scholar, 28Tay J. Hodgman R. Richter J.D. Dev. Biol. 2000; 221: 1-9Crossref PubMed Scopus (142) Google Scholar). Thus, CPEB seems to have a dual function: to maintain maternal mRNAs, including those of cyclin B1, in a dormant state in immature oocytes and to activate their translation via polyadenylation during oocyte maturation. Nevertheless, many issues still remain to be elucidated. For example, although both c-mos and cyclin B1 mRNAs hold the CPEs in their 3′-UTR, the following findings indicate that their translational activation is controlled by different mechanisms: 1) translational activation of c-mos mRNA precedes that of cyclin B1 mRNA (29Ballantyne S. Daniel D.L.J. Wickens M. Mol. Biol. Cell. 1997; 8: 1633-1648Crossref PubMed Scopus (88) Google Scholar); 2) in contrast to c-mos mRNA, the protein level of cyclin B1 can increase in the absence of polyadenylation or cap-ribose methylation of mRNA and is independent of MAPK activity (24Frank-Vaillant M. Jessus C. Ozon R. Maller J.L. Haccard O. Mol. Biol. Cell. 1999; 10: 3279-3288Crossref PubMed Scopus (76) Google Scholar, 30Kuge H. Brownlee G.G. Gershon P.D. Richter J.D. Nucleic Acids Res. 1998; 26: 3208-3214Crossref PubMed Scopus (69) Google Scholar,31Howard E.L. Charlesworth A. Welk J. MacNicol A.M. Mol. Cell. Biol. 1999; 19: 1990-1999Crossref PubMed Scopus (74) Google Scholar); 3) a reporter mRNA carrying the cyclin B1 3′-UTR is translationally repressed when injected into oocytes, whereas that carrying the c-mos 3′-UTR is not (23Barkoff A.F. Dickson K.S. Gray N.K. Wickens M. Dev. Biol. 2000; 220: 97-109Crossref PubMed Scopus (76) Google Scholar); and 4) the injection of high concentrations of the CPE induces unmasking of cyclin B1 mRNA but not of c-mos mRNA (22de Moor C.H. Richter J.D. EMBO J. 1999; 18: 2294-2303Crossref PubMed Scopus (173) Google Scholar). Therefore, regulatory elements other than CPEs and CPEB must operate for the coordinated activation of masked mRNAs to promote oocyte maturation.In early Drosophila embryos, Pumilio represses translation of maternal hunchback (hb) mRNA, in conjunction with Nanos (32Tautz D. Nature. 1988; 327: 383-389Crossref Scopus (282) Google Scholar, 33Irish V. Lehmann R. Akam M. Nature. 1989; 338: 646-648Crossref PubMed Scopus (215) Google Scholar, 34Barker D.D. Wang C. Moore J. Dickinson L.K. Lehmann R. Genes Dev. 1992; 6: 2312-2326Crossref PubMed Scopus (167) Google Scholar). The translational repression is due to the direct binding of Pumilio to two copies of a bipartite sequence, the nanos response elements (NREs), located in the 3′-UTR ofhb mRNA (35Wharton R.P. Struhl G. Cell. 1991; 67: 955-967Abstract Full Text PDF PubMed Scopus (305) Google Scholar, 36Wharton R.P. Sonoda J. Lee T. Patterson M. Murata Y. Mol. Cell. 1998; 1: 863-872Abstract Full Text Full Text PDF PubMed Scopus (196) Google Scholar, 37Sonoda J. Wharton R.P. Genes Dev. 1999; 13: 2704-2712Crossref PubMed Scopus (267) Google Scholar, 38Murata Y. Wharton R.P. Cell. 1995; 80: 747-756Abstract Full Text PDF PubMed Scopus (332) Google Scholar). Loss-of-function mutations ofpumilio and nanos induce the premature expression of cyclin B protein in pole cells (39Asaoka-Taguchi M. Yamada M. Nakamura A. Hanyu K. Kobayashi S. Nat. Cell Biol. 1999; 1: 431-437Crossref PubMed Scopus (247) Google Scholar). Deletion from cyclin B mRNA of cis-elements (named TCEs) resembling the NREs inhb mRNA results in a phenotype similar to that caused bypumilio and nanos mutations (40Dalby B. Glover D.M. EMBO J. 1993; 12: 1219-1227Crossref PubMed Scopus (99) Google Scholar). These results strongly suggest that Pumilio, in cooperation with Nanos, is involved in translational repression of cyclin B mRNA through the NRE-like sequence TCE, although it remains to be elucidated biochemically whether or not the direct binding of Pumilio to cyclin B mRNA brings about its translational repression.Our goal is to elucidate the molecular mechanism of translational activation of dormant cyclin B1 mRNA upon hormonal stimulation, which is prerequisite for initiating oocyte maturation in fish and amphibians, probably inclusive of Xenopus (25Yamashita M. Zool. Sci. 2000; 17: 841-851Crossref Scopus (29) Google Scholar). NRE-like motifs are found in the 3′-UTR of cyclin B1 mRNA in fish and amphibians, including goldfish (41Hirai T. Yamashita M. Yoshikuni M. Lou Y.-H. Nagahama Y. Mol. Reprod. Dev. 1992; 33: 131-140Crossref PubMed Scopus (88) Google Scholar), medaka (42Mita K. Ohbayashi T. Tomita K. Shimizu Y. Kondo T. Yamashita M. Zool. Sci. 2000; 17: 365-374PubMed Google Scholar), zebrafish, 2M. Yamashita (2000) GenBank™ accession numberAB040435. 2M. Yamashita (2000) GenBank™ accession numberAB040435. Xenopus (43Minshull J. Blow J.J. Hunt T. Cell. 1989; 56: 947-956Abstract Full Text PDF PubMed Scopus (288) Google Scholar), and Rana (44Ihara J. Yoshida N. Tanaka T. Mita K. Yamashita M. Mol. Reprod. Dev. 1998; 50: 499-509Crossref PubMed Scopus (39) Google Scholar). It is therefore expected that a homolog of Pumilio or its related protein(s) binds directly to cyclin B1 mRNA to control its translation. To date, however, there has been no report on protein expression of Pumilio homologs in vertebrates, although cDNAs probably encoding human homologs have been isolated in a cDNA project (45Nagase T. Seki N. Ishikawa K. Ohira M. Kawarabayasi Y. Ohara O. Tanaka A. Kotani H. Miyajima N. Nomura N. DNA Res. 1996; 3: 321-329Crossref PubMed Scopus (205) Google Scholar, 46Nagase T. Miyajima N. Tanaka A. Sazuka T. Seki N. Sato S. Tabata S. Ishikawa K.-I. Kawarabayasi Y. Kotani H. Nomura N. DNA Res. 1995; 2: 37-43Crossref PubMed Scopus (114) Google Scholar). In the present study, we biochemically characterized a XenopusPumilio protein, and we investigated its involvement in the translational control of cyclin B1 mRNA. We isolated a cDNA clone encoding a protein homologous (78% identity) toDrosophila Pumilio in the domain that defines the Pumilio family. A monoclonal antibody raised against this domain has revealed for the first time in any vertebrate that Xenopus oocytes have a protein homologous to Pumilio (named XPum for XenopusPumilio), which has an apparent molecular mass of 137 kDa and binds to the NRE of Drosophila hb mRNA, as in the case of theDrosophila counterpart. Using UV cross-linking assays with various RNA probes, we have also provided the first biochemical evidence for the sequence-specific direct binding of Pumilio to the 3′-UTR of cyclin B1 mRNA. Moreover, the results of protein pull-down and co-immunoprecipitation assays indicated that XPum associates with a Xenopus homolog of Nanos, Xcat-2 (47Mosquera L. Forristall C. Zhou Y. King M.L. Development. 1993; 117: 377-386PubMed Google Scholar), and CPEB.DISCUSSIONWe have focused on Pumilio as one of the possible translational regulators of cyclin B1, because the 3′-UTR of cyclin B1 mRNA has sequences resembling those required for translational repression by Pumilio in Drosophila. We report here the following main findings: 1) A 137-kDa Xenopus homolog of Pumilio (XPum) exists in oocytes and eggs, the first demonstration of Pumilio protein expression in vertebrates; 2) XPum binds to the cyclin B1 3′-UTR containing the UGUA at nucleotides 1335–1338; 3) The sequence responsible for masking cyclin B1 mRNA, as determined by translational activation of endogenous cyclin B1 mRNA by injection of various cis-elements, is located around CPE1 (nucleotides 1353–1360), close to but different from the XPum-binding site; 4) XPum physically associates with a Xenopus Nanos homolog, Xcat-2, at least in vitro and with CPEB in vivo.The amino acid sequence alignment of Pumilio homologs exhibits a high degree of conservation in the Pum-HD (Fig. 1), which is known to have a dual function, not only recognizing the NREs but also interacting with some component of the translational machinery (36Wharton R.P. Sonoda J. Lee T. Patterson M. Murata Y. Mol. Cell. 1998; 1: 863-872Abstract Full Text Full Text PDF PubMed Scopus (196) Google Scholar, 37Sonoda J. Wharton R.P. Genes Dev. 1999; 13: 2704-2712Crossref PubMed Scopus (267) Google Scholar, 65Kraemer B. Crittenden S. Gallegos M. Moulder G. Barstead R. Kimble J. Wickens M. Curr. Biol. 1999; 9: 1009-1018Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar). The sequence similarity is suggestive of a common function of Pumilio. Indeed, our results showed that XPum can bind to the NRE ofhb mRNA, the sequence recognized by theDrosophila counterpart. Moreover, the present UV cross-linking assays using endogenous Pumilio protein and various mutant probes indicated that the Pumilio-binding site contains the sequence UGUA, which is identical to the sequence determined by gel-shift experiments with recombinant Pumilio proteins from human andDrosophila (60Zamore P.D. Williamson J.R. Lehmann R. RNA ( N. Y. ). 1997; 3: 1421-1433PubMed Google Scholar). Recent studies have also demonstrated that yeast Pumilio-related proteins play a role in negative control of translation by recognizing a cis-element similar to that forDrosophila, Xenopus, and human Pumilio homologs (66Olivas W. Parker R. EMBO J. 2000; 19: 6602-6611Crossref PubMed Scopus (216) Google Scholar, 67Tadauchi T. Matsumoto K. Herskowitz I. Irie K. EMBO J. 2001; 20: 552-561Crossref PubMed Scopus (121) Google Scholar). Taken together, the results strongly suggest that the function of Pumilio as a translational regulator and itscis-element of the target RNAs are widely conserved in evolution.Drosophila Pumilio represses translation of maternalhb mRNA by direct binding to the NRE in its 3′-UTR (36Wharton R.P. Sonoda J. Lee T. Patterson M. Murata Y. Mol. Cell. 1998; 1: 863-872Abstract Full Text Full Text PDF PubMed Scopus (196) Google Scholar,38Murata Y. Wharton R.P. Cell. 1995; 80: 747-756Abstract Full Text PDF PubMed Scopus (332) Google Scholar). The results of genetic analyses also suggest that Pumilio, in cooperation with Nanos, controls translation of cyclin B mRNA in the migrating pole cells (39Asaoka-Taguchi M. Yamada M. Nakamura A. Hanyu K. Kobayashi S. Nat. Cell Biol. 1999; 1: 431-437Crossref PubMed Scopus (247) Google Scholar), although there has been no direct biochemical evidence for the binding of Pumilio to cyclin B mRNA in any species to date. The results of the present study have provided the first biochemical evidence of a Pum-binding site in cyclin B1 mRNA as well as evidence of the binding of endogenous Pumilio to cyclin B1 mRNA in a sequence-specific manner. These findings led us to verify the possibility that, as in Drosophila, XPum plays an essential role in translational control of cyclin B1 mRNA during oocyte maturation. To this end, we injected variouscis-elements into oocytes in the expectation that, if the injected sequence is responsible for masking cyclin B1 mRNA, it will titrate the masking proteins and thereby induce translational activation of endogenous cyclin B1 as already demonstrated (22de Moor C.H. Richter J.D. EMBO J. 1999; 18: 2294-2303Crossref PubMed Scopus (173) Google Scholar, 23Barkoff A.F. Dickson K.S. Gray N.K. Wickens M. Dev. Biol. 2000; 220: 97-109Crossref PubMed Scopus (76) Google Scholar). We confirmed that the injection of large amounts of cyclin B1 3′-UTR intoXenopus oocytes induced cyclin B1 synthesis and GVBD. We also demonstrated that the cis-element necessary for translational repression of cyclin B1 mRNA exists in the neighborhood of CPE1 (nucleotides 1353–1360) but not in CPE2 (nucleotides 1371–1377). Injection of the XPum-binding sequence, however, induced neither translational activation of endogenous cyclin B1 mRNA nor GVBD (Fig. 5). These results seem to imply that the binding of Pumilio to cyclin B1 mRNA is not responsible for masking cyclin B1 mRNA. Nevertheless, this suggestion needs to be proved by experimental approaches other than the injection ofcis-elements, because it has been reported that injection of large amounts of CPE-containing RNA into mouse oocytes does not induce translational activation of cyclin B1 mRNA despite the involvement of CPEB in both the repression and the stimulation of cyclin B1 mRNA in this species, as in the case of Xenopus (28Tay J. Hodgman R. Richter J.D. Dev. Biol. 2000; 221: 1-9Crossref PubMed Scopus (142) Google Scholar). In cooperation with CPEB as a major control element, XPum might contribute as a fine tuner of cyclin B1 mRNA translation. This notion is also supported by the present finding that XPum physically interacts with CPEB both in vivo and in vitro (Fig. 6,B–D).The function of Drosophila Pumilio in control of the translation of mRNAs is dependent on Nanos (68Wreden C. Verrotti A.C. Schisa J.A. Lieberfarb M.E. Strickland S. Development. 1997; 124: 3015-3023Crossref PubMed Google Scholar, 69Forbes A. Lehmann R. Development. 1998; 125: 679-690Crossref PubMed Google Scholar). FBF, aC. elegans Pumilio-related protein, also controls the sperm/oocyte switch in a hermaphrodite germ line by regulating the translation of fem-3 mRNA with interaction with one of the C. elegans Nanos homologs (61Zhang B. Gallegos M. Puoti A. Durkin E. Fields S. Kimble J. Wickens M.P. Nature. 1997; 390: 477-484Crossref PubMed Scopus (431) Google Scholar, 65Kraemer B. Crittenden S. Gallegos M. Moulder G. Barstead R. Kimble J. Wickens M. Curr. Biol. 1999; 9: 1009-1018Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar). XPum may likewise collaborate with one or more partners to regulate the translation of specific maternal mRNAs (for review see Ref. 70Parisi M. Lin H. Curr. Biol. 2000; 10: R81-R83Abstract Full Text Full Text PDF PubMed Google Scholar). The most likely candidate for the XPum partner is a Xenopus homolog of Nanos, Xcat-2 protein (71Zhou Y. King M.L. Development. 1996; 122: 2947-2953PubMed Google Scholar, 72Forristall C. Pondel M. Chen L. King M.L. Development. 1995; 121: 201-208PubMed Google Scholar). In fact, we have provided evidence of association between XPum and Xcat-2, at least in vitro (Fig.6 A). However, Xcat-2 protein is reported to be absent in oocytes and eggs despite the presence of its mRNA. The protein appears in accordance with the movement of germ plasm during early embryogenesis (73MacArthur H. Bubunenko M. Houston D.W. King M.L. Mech. Dev. 1999; 84: 75-88Crossref PubMed Scopus (69) Google Scholar). Consistent with this, Xcat-2 is undetectable in anti-XPum immunoprecipitates from oocyte extracts. 3S. Nakahata and M. Yamashita, unpublished data. Although we cannot exclude the possibility that one or more Nanos homologs other than Xcat-2 collaborate with XPum in oocytes, it remains a mystery whether XPum acts together with a Nanos homolog to govern the translation of mRNAs in oocytes.The actual biological roles of XPum are completely unknown at present, but we speculate that XPum plays an important role in translational control of cyclin B1 mRNA like in Drosophila (39Asaoka-Taguchi M. Yamada M. Nakamura A. Hanyu K. Kobayashi S. Nat. Cell Biol. 1999; 1: 431-437Crossref PubMed Scopus (247) Google Scholar, 40Dalby B. Glover D.M. EMBO J. 1993; 12: 1219-1227Crossref PubMed Scopus (99) Google Scholar). CPEB directly binds to maskin, a protein that can also bind directly to the cap-binding translation initiation factor elF-4E, which leads to translational repression. The dissociation of maskin from elF-4E allows elF-4G to bind to elF-4E, which brings elF-3 and the 40 S ribosomal subunit to the mRNA to initiate translation via cap-ribose methylation (30Kuge H. Brownlee G.G. Gershon P.D. Richter J.D. Nucleic Acids Res. 1998; 26: 3208-3214Crossref PubMed Scopus (69) Google Scholar, 74Stebbins-Boaz B. Cao Q. de Moor C.H. Mendez R. Richter J.D. Mol. Cell. 1999; 4: 1017-1027Abstract Full Text Full Text PDF PubMed Scopus (314) Google Scholar, 75Keiper B.D. Rhoads R.E. Dev. Biol. 1999; 206: 1-14Crossref PubMed Scopus (24) Google Scholar). Recent studies have also shown that a progesterone-induced early phosphorylation of CPEB at serine 174 is catalyzed by Eg2 (76Mendez R. Hake L.E. Andresson T. Littlepage L.E. Ruderman J.V. Richter J.D. Nature. 2000; 404: 302-307Crossref PubMed Scopus (290) Google Scholar) and that this phosphorylation recruits cleavage and polyadenylation specificity factor into an active cytoplasmic polyadenylation complex (77Mendez R. Murthy K.G.K. Ryan K. Manley J.L. Richter J.D. Mol. Cell. 2000; 6: 1253-1259Abstract Full Text Full Text PDF PubMed Scopus (191) Google Scholar). Thus, CPEB plays a key role in both translational repression and activation of mRNAs stored in oocytes (for review see Ref. 78Richter J.D. Hershey J.W.B. Mathews M. Sonenberg N. Translational Control of Gene Expression. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY2000: 785-806Google Scholar). We demonstrated in this study that XPum is physically associated with CPEB in oocytes. In cooperation with CPEB, XPum may control the CPEB/maskin-mediated translational masking and unmasking to assure the highly coordinated successive translational activation of masked mRNAs during oocyte maturation. Further studies are required to understand the biological significance of the interactions among XPum, CPEB, and cyclin B1 mRNA, as well as to elucidate the functions of XPum in oocytes. The final inducer of oocyte maturation is the maturation-promoting factor (MPF),1 which consists of Cdc2 and cyclin B. MPF is stored in immature oocytes as an inactive form (called pre-MPF), although its amount differs from species to species (1Yamashita M. Mita K. Yoshida N. Kondo T. Prog. Cell Cycle Res. 2000; 4: 115-129Crossref PubMed Scopus (101) Google Scholar, 2Taieb F. Thibier C. Jessus C. Mol. Reprod. Dev. 1997; 48: 397-411Crossref PubMed Scopus (77) Google Scholar). In Xenopus (as well as fish and mammals except for mice), the initiation of oocyte maturation requires proteins (called “initiators”) newly synthesized by translational activation of dormant mRNAs stored in oocytes (masked mRNAs). Mos functions as an initiator with the aid of mitogen-activated protein kinase (MAPK) (3Yew N. Mellini M.L. Vande Woude G.F. Nature. 1992; 355: 649-652Crossref PubMed Scopus (202) Google Scholar, 4Sagata N. Oskarsson M. Copeland T. Brumbaugh J. Vande Woude G.F. Nature. 1988; 335: 519-525Crossref PubMed Scopus (462) Google Scholar, 5Sagata N. Daar I. Oskarsson M. Showalter S.D. Vande Woude G.F. Science. 1989; 245: 643-646Crossref PubMed Scopus (249) Google Scholar, 6Kosako H. Gotoh Y. Nishida E. EMBO J. 1994; 13: 2131-2138Crossref PubMed Scopus (189) Google Scholar, 7Huang W. Kessler D.S. Erikson R.L. Mol. Biol. Cell. 1995; 6: 237-245Crossref PubMed Scopus (110) Google Scholar, 8Haccard O. Lewellyn A. Hartley R.S. Erikson E. Maller J.L. Dev. Biol. 1995; 168: 677-682Crossref PubMed Scopus (147) Google Scholar, 9Gotoh Y. Masuyama N. Dell K. Shirakabe K. Nishida E. J. Biol. Chem. 1995; 270: 25898-25904Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar). The Cdc2 molecule of pre-MPF is phosphorylated on threonine 14/tyrosine 15 by Myt1 and threonine 161 by cyclin-dependent kinase-activating kinase. The Mos/MAPK pathway probably leads to the activation of Cdc25, which dephosphorylates threonine 14/tyrosine 15 via the polo-like kinase (10Abrieu A. Brassac T. Galas S. Fisher D. Labbe J.C. Doree M. J. Cell Sci. 1998; 111: 1751-1757Crossref PubMed Google Scholar, 11Karaı̈skou A. Jessus C. Brassac T. Ozon R. J. Cell Sci. 1999; 112: 3747-3756PubMed Google Scholar, 12Qian Y.-W. Erikson E. Li C. Maller J.L. Mol. Cell. Biol. 1998; 18: 4262-4271Crossref PubMed Scopus (212) Google Scholar), and to the inhibition of Myt1 through the activation of p90rsk (13Palmer A. Gavin A.-C. Nebreda A.R. EMBO J. 1998; 17: 5037-5047Crossref PubMed Scopus (288) Google Scholar, 14Gavin A.-C. Ainle A.N. Chierici E. Jones M. Nebreda A.R. Mol. Biol. Cell. 1999; 10: 2971-2986Crossref PubMed Scopus (35) Google Scholar). 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• W2024283568 title "Biochemical Identification of Xenopus Pumilio as a Sequence-specific Cyclin B1 mRNA-binding Protein That Physically Interacts with a Nanos Homolog, Xcat-2, and a Cytoplasmic Polyadenylation Element-binding Protein" @default.
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