The polyadenylation of mRNAs is usually thought of as a process that occurs in the nucleus, and indeed this is the cellular compartment in which pre-mRNA processing and polyadenylation does occur. However, mRNA polyadenylation is not restricted to the nucleus. Indeed, one of the more fascinating and important mechanisms that control gene expression during oogenesis and early development, stages in some organisms (such as animals) when the nucleus is not “active”, is mRNA polyadenylation. In these cases, the process occurs in the cytoplasm.
During oocyte development, a large population of maternally-encoded mRNAs are synthesized and stored for “use” in particular stages of development. These maternal mRNAs typically possess short poly(A) tails (20-40 nts) and are not available for translation (being “masked” by a complex of RNA-binding proteins). During oocyte maturation or following fertilization, these masked mRNAs become polyadenylated and thus activated for translation. This activation is a regulated process that helps to coordinate the ballet of gene expression attendant with meiotic maturation and early development. As such, it touches on many tangential phenomena (such as movement of stored mRNAs within the cell).
What is of particular interest for this blog is the nature of the mechanism that mediates polyadenylation in the cytoplasm. As indicated in the following figure, this mechanism includes some familiar players as well as some equally-intriguing partners.
The most striking commonality of stored mRNAs that are activated by polyadenylation is that they possess a conserved sequence motif (called, appropriately enough, the Cytoplasmic Polyadenylation Element, or CPE). This element is recognized by an RNA binding protein called the Cytplasmc Polyadenylation Element Binding protein (CPEB).
This combination of components acts in concert with CPSF (related to the CPSF that is part of the nuclear polyadenylation complex) and novel poly(A) polymerases to polyadenylate stored mRNAs. The cytoplasmic CPSF seems to consist of all but one of the (four) subunits that are found in the nuclear complex, the absent member being CPSF73 (one of the nucleases found in the nuclear CPSF). Curiously, while the cytoplasmic CPSF subunits are identical to their nuclear counterparts, the cytoplasmic PAPs are distinct from their nuclear relatives; indeed, they form a distinct clade when related nucleotidyltransferases are compared. (In the figure, the PAP is called Gld2.) The cytoplasmic polyadenylation complex also includes a protein called symplekin; this protein is also a part of the nuclear polyadenylation complex. Finally, there is a poly(A) binding protein, distinct from the nuclear PAB, that is involved in the process.
(In the figure, components of the translational machinery that mediate the link between the poly(A) tail and the mRNA 5′-cap are shown; this is to illustrate the means by which cytoplasmic polyadenylation turns on gene expression.)
There are many fascinating things about this system. In keeping with one of the themes that I am developing in this series, namely flexibility and adaptability of the “irreducible” polyadenylation complex, it bears mentioning that cytoplasmic polyadenylation is yet another example of the borrowing, or co-opting, of a part of the nuclear complex for other functions. It is interesting that CPSF is again the sub-complex that works with other cytoplasmic factors. It is also interesting that the cytoplasmic CPSF lacks one of the proteins seen in the nuclear CPSF. This brings to mind the even more remarkable reduction of the polyadenylation complex that is seen in Giardia. Clarifying the specifics about the functioning of CPSF in these various systems promises to reveal some interesting new facets about both the mechanisms and evolution of polyadenylation in eukaryotes.
References (a smattering of representative ones – a more complete list is in the first citation, a review article):
HE Radford, HA Meijer, CH de Moor. 2008. Translational control by cytoplasmic polyadenylation in Xenopus oocytes. Biochim Biophys Acta 1779, 217-229.
KS Dickson, A Bilger, S Ballantyne, MP Wickens. 1999. The cleavage and polyadenylation specificity factor in Xenopus laevis oocytes is a cytoplasmic factor involved in regulated polyadenylation. Mol Cell Biol 19, 5707-5717. (This is among the first reports describing the cytoplasmic CPSF)
G Martin, W Keller. 2007. RNA-specific ribonucleotidyl transferases. RNA 13, 1834-1849. (A recent review of nucleotidyl transferases, a family of enzymes that includes the cytoplasmic and canonical PAPs)