Polyadenylation of c-mos mRNA in Xenopus oocytes (p. 460/461)
Stage-specific protein synthesis in oocytes and eggs (p. 464/465)
Translational Control during Spermiogenesis (p. 467/468)
A class of small regulatory RNAs known as microRNAs (miRNAs) controls mRNA survival and/or translation similar to small interfering RNA (siRNA). Both siRNA and miRNA are cut by dicer from larger double-styranded RNA precursors (see Comments to Chapter 15). And both miRNAs and siRNAs act either by causing the destruction of their target RNAs or by preventing their transl;ation. However, while siRNA come from the same genes that they ultimately silence, miRNAs come from different genes whose sole function is to produce the small regulatory RNAs. One of the first miRNAs to be discovered is the one encoded by the lin-4 gene of C. elegans (see p. 680). Since then, dozens if not hundreds of miRNAs have been identified in C. elegans, Drosophila, plants, and the human (Carrington and Ambros, 2003).
p. 458, Fig. 18.6. The two yellow proteins should be labeled "4E-BP" insead of "F4-BP".
Carrington J.C. and Ambros V. (2003) Role of microRNAs in plant and animal development. Science 301: 336-338.
Cooperstock R.L., Lipshitz H.D. (2001) RNA localization and translational regulation during axis specification in the Drosophila oocyte. Int. Rev. Cytol. 203: 541-566
Leatherman J.L. and Jongens T.A. (2003) Transcriptional silencing and translational control: key features of early germ line development. BioEssays 25: 326-335
Mendez R., Richter J.D. (2001) Translational control by CPEB: a means to the end. Nature Rev. Mol. Cell Biol. 2: 521-529
Preiss T. and Hentze M.W. (2003) Starting the protein synthesis machine: eukaryotic translation initiation. BioEssays 25: 1201-1211
Hodgman R., Tay J., Mendez R., Richter J.D. (2001) CPEB phosphorylation and cytoplasmic polyadenylation are catalyzed by the kinase IAK1/Eg2 in maturing mouse oocytes. Development 128: 2815-2822.
Mendez R., Hake L.E., Andresson T., Littlepage L.E., Ruderman J.V., Richter J.D. (2000) Phosphorylation of CPE binding factor by Eg2 regulates translation of c-mos mRNA. Nature 404: 302-307.
Mendez R., Murthy K.G., Ryan K., Manley J.L., Richter J.D. (2000) Phosphorylation of CPEB by Eg2 mediates the recruitment of CPSF into an active cytoplasmic polyadenylation complex. Mol Cell 6: 1253-1259.
Stebbins-Boaz B., Cao Q., de Moor C.H., Mendez R., Richter J.D. (1999) Maskin is a CPEB-associated factor that transiently interacts with elF-4E. Mol Cell. 4: 1017-1027.
These studies address the mechanism that connects the cytoplasmic polyadenylation of mRNAs with their translational activation in the oocytes of frogs and mice. Cytoplasmic polyadenylation depends on two regulatory sequences in the 3' UTR: the AAUAAA polyadenylation signal and an adjacent cytoplasmic polyadenylation element (CPE). The CPE is bound by CPEB, a sequence-specific RNA binding protein. In the dormant oocyte, CPEB is associated with the protein Maskin, which also interacts with the cap-associated initiation factor eIF4E. This interaction prevents eIF4G from binding to eIF4E and recruiting the ribosomal initiation complex to the cap site. Polyadenylation is initiated by the phosphorylation of CPEB by a kinase known as Eg2 in Xenopus and IAK1 in mice. The phosphorylated CPEB recruits a group of proteins collectively called the cleavage and polyadenylation-specific factor (CPSF) and poly(A) polymerase to the AAUAAA sequence, with poly(A) tail formation ensuing. At the same time, eIF4E dissociates from Maskin and allows eIF4G to bind, so that translation is initiated.
Groisman I., Jung M.Y., Sarkissian M., Cao Q., Richter J.D. (2002) Translational control of the embryonic cell cycle. Cell 109: 473-483.
Cell cycle progression is regulated at the level of cyclin B synthesis and destruction. The investigators prepared cycling extracts from Xenopus embryos and found that progression into M phase requires the polyadenylation-induced translation of cyclin B1 mRNA. Polyadenylation requires the binding of cytoplasmic polyadenylation elements (CPEs) by a CPE-binding protein (CPEB). This step is mediated by the phosphorylation of CPEB by Aurora, a kinase whose activity oscillates with the cell cycle. Exit from M phase seems to require deadenylation and subsequent translational silencing of cyclin B1 mRNA by Maskin, a CPEB and eIF4E binding factor, whose expression is cell cycle regulated. These observations suggest that regulated cyclin B1 mRNA translation is essential for the embryonic cell cycle. Mammalian cells also display a cell cycle-dependent cytoplasmic polyadenylation, suggesting that translational control by polyadenylation might be a general feature of mitosis in animal cells.
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