Further analysis of cytoplasmic polyadenylation in Xenopus embryos and identification of embryonic cytoplasmic polyadenylation element-binding proteins.

Abstract

Early development in Xenopus laevis is programmed in part by maternally inherited mRNAs that are synthesized and stored in the growing oocyte. During oocyte maturation, several of these messages are translationally activated by poly(A) elongation, which in turn is regulated by two cis elements in the 3' untranslated region, the hexanucleotide AAUAAA and a cytoplasmic polyadenylation element (CPE) consisting of UUUUUAU or similar sequence. In the early embryo, a different set of maternal mRNAs is translationally activated. We have shown previously that one of these, C12, requires a CPE consisting of at least 12 uridine residues, in addition to the hexanucleotide, for its cytoplasmic polyadenylation and subsequent translation (R. Simon, J.-P. Tassan, and J. D. Richter, Genes Dev. 6:2580Dev. 6: -2591Dev. 6: , 1992 for cytoplasmic polyadenylation at times up to the gastrula stage. This RNA also required a poly(U) CPE for cytoplasmic polyadenylation in embryos, but in this case the CPE consisted of 18 uridine residues. In addition, the timing and extent of cytoplasmic poly(A) elongation during early embryogenesis were dependent upon the distance between the CPE and the hexanucleotide. Further, as was the case with C12 RNA, Cli RNA contains a large masking element that prevents premature cytoplasmic polyadenylation during oocyte maturation. To examine the factors that may be involved in the cytoplasmic polyadenylation of both C12 and Cli RNAs, we performed UV cross-linking experiments in egg extracts. Two proteins with sizes of -36 and -45 kDa interacted specifically with the CPEs of both RNAs, although they bound preferentially to the C12 CPE. The role that these proteins might play in cytoplasmic polyadenylation is discussed.

The 36-Kilodalton Embryonic-Type Cytoplasmic Polyadenylation Element-Binding Protein in Xenopus laevis Is ElrA, a Member of the ELAV Family of RNA-Binding Proteins

“…In previous experiments, we identified two Xenopus proteins with molecular sizes of 36 and 45 kDa that were UV crosslinked to the eCPEs of Cl1, Cl2, and ActR mRNAs (35,36). However, only the 36-kDa protein was cytoplasmic and therefore seemed the most likely one to be involved in mRNA polyadenylation in embryos.…”

“…Members of this protein family do interact with the AUUUA degradation sequence (2,4,24). However, we should point out that the minimum number of uridine residues that can promote cytoplasmic polyadenylation in embryos is 12, which strongly correlates with the number of uridines required for binding of the 36-kDa protein/ElrA (34,35). Thus, if ElrA has a function related to mRNA degradation in early development, it would probably have to do so via this sequence, which has not previously been identified with this activity.…”

“…The maturation-type CPE is UUUUUAU (consensus), whereas the embryonic CPE (eCPE) is oligo (U) [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] (reviewed in reference 31). At least two mRNAs that are polyadenylated in embryos also contain a third cis-acting sequence in the 3Ј UTR: the masking element that prevents precocious polyadenylation during maturation (34,35). Because deletion of the masking element results in premature polyadenylation during oocyte maturation, it is likely that the factors that support embryonic polyadenylation are present and active in maturing oocytes.…”

“…Some mRNAs, which encode such proteins as c-Mos, cyclin B1, cyclin-dependent kinase 2 (Cdk2), and the earlydevelopment-specific histone B4, are polyadenylated and translated during oocyte maturation (27-29, 33, 39). Other mRNAs, however, undergo these processes sometime after fertilization; they encode such proteins as polypeptide chain release factor (Cl1), Cl2 (function unknown), and activin receptor (28,(34)(35)(36).…”

East of EDEN was a poly(A) tail

“…It was first shown that Cl1/eRF1, Cl2 and activin receptor mRNAs were polyadenylated after fertilization due to a novel type of CPE-a poly(U) sequence (Simon and Richter, 1994;Simon et al, 1993Simon et al, , 1996. As mRNAs that contain this element are polyadenylated only after fertilization, this U-rich CPE is often called an embryonic CPE-in contrast to the UA-rich CPEs that are present within mRNAs that are polyadenylated during maturation.…”