User:Amdavies2/sandbox

CPEB, or cytoplasmic polyadenylation element binding protein, is a highly conserved RNA-binding protein that promotes the elongation of the polyadenine tail of messenger RNA. CPEB most commonly activates the target RNA for translation, but can also act as a repressor, dependent on its phosphorylation state. In animals, CPEB is expressed in several alternative splicing isoforms that are specific to particular tissues and functions, including the self-cleaving Mammalian CPEB3 ribozyme. CPEB was first identified in Xenopus oocytes and associated with meiosis; a role has also been identified in the spermatogenesis of Caenorhabditis elegans.

CPEB is involved in closed-loop regulation of mRNAs that keeps them inactive. The closed-loop structure between the 3'UTR and 5'UTR inhibits translation. This has been observed in Xenopus laevis in which eIF4E bound to the 5' cap interacts with Maskin bound to CPEB on the 3' UTR creating translationally inactive transcripts. This translational inhibition is lifted once CPEB is phosphorylated, displacing the Maskin binding site, allowing for the polymerization of the PolyA tail, which can recruit the translational machinery by means of PABP. However, it is important to note that this mechanism has been under great scrutiny.

Role in memory

Drosophila Orb2 binds to genes implicated in long-term memory. An isoform of CPEB found in the neurons of the sea slug Aplysia californica, as well as in Drosophila, mice, and humans, contains an N-terminal domain not found in other isoforms that shows high sequence similarity to prion proteins. Experiments with the Aplysia isoform expressed in yeast reveal that CPEB has a key property associated with prions: it can cause other proteins to assume alternate protein conformations that are heritable in successive generations of yeast cells. Furthermore, the functional RNA-binding form of the CPEB protein may be the prion-like state. These observations have led to the suggestion that long-lasting bistable prionlike proteins play a role in the formation of long-term memory.

Interactions

CPEB has been shown to interact with the following proteins:

• PUM2
• PARN
• GLD-2
• symplekin
• eIF4E binding protein

Structure

There are two regions on all CPEB proteins that have the ability to bind RNA. CPEB proteins also contain two zinc fingers that maintain the protein's structure and interact with RNA. In vertebrates and Caenorhabditis elegans, there are four types of CPEB proteins. These include CPEB 1-4 in vertebrates, and CPB 1-3 and FOG-1 in C. elegans. Drosophila melanogaster only contains two types of CPEB proteins, Orb and Orb2. The four vertebrate CPEB proteins each have different RNA binding regions, and do not function in the same way.

Mechanism

CPEB proteins are sequence specific and bind the cytoplasmic polyadenylation element, or CPE, in the 3' untranslated region (UTR) of messenger RNA (mRNA). They most commonly recognise the CPE RNA sequence UUUUUAU, though there are variations to this sequence that they can recognise. Binding of CPEB proteins and CPSF to the CPE occurs in the nucleus, and then they are exported into the cytoplasm of the cell. While in the cytoplasm, the RNA-protein complex associates with the assembly factor symplekin, and the proteins PARN and Gld2. While Gld2 acts to elongate the polyadenine tail, PARN acts to remove it. Because PARN is more active than Gld2, the result is a shortened polyadenine tail. At this time, CPEB is bound by the protein Maskin associated with EIF4E. When Maskin is bound to EIF4E, it masks the mRNA and prevents translation. Once CPEB is phosphorylated by adding a phosphate to the amino acid serine 174, translational inhibition is lifted by removing PARN from the RNA-protein complex, allowing Gld2 to elongate the polyadenine tail. At the same time, the protein PABP binds EIF4E, causing Maskin to be removed. Removal of Maskin unmasks the mRNA, and PABP causes the recruitment of the translation initiation complex, resulting in translation.

Role in the Cell Cycle

CPEB proteins regulate mitosis in somatic cells by regulating poly(A) tail length in mRNA. CPEB proteins are important for progression through the cell cycle in the Xenopus laevis embryo. In particular, phosphorylated CPEB1 promotes elongation of the Cyclin B mRNA polyadenine tail, initiating translation. Cyclin B is required for X. laevis to enter into mitosis, and is subsequently destroyed as the cell cycle enters S phase of the cell cycle. In S phase, the phosphate is removed from CPEB1 causing association of the inhibitory RNA-protein complex, and consequently, Cyclin B is not translated. CPEB1 behaves similarly in some mammalian cell cycles, however it is important to note that mice without CPEB1 are still viable. The cell cycle of X. laevis may depend more heavily on CPEB proteins than cell cycles of other organisms, such as mice.

Role in Senescence and Cancer

CPEB proteins are important for proper senescence, in which normal cells stop dividing once they have undergone a certain number of divisions. Mice that do not contain CPEB stop undergoing senescence and become immortal. Human cells without CPEB also experience this. Inability to undergo senescence results in uncontrollable cell growth and cancer. There are two types of CPEB proteins that have been related to cancer: CPEB1 and CPEB4.

CPEB1 mediates the immortality of cells. Cells that undergo proper senescence have proper expression of the tumor suppressor gene p53, which prevents tumor formation. p53 contains a CPE that CPEB1 binds to initiate polyadenylation and translation. When CPEB1 expression is reduced, p53 translation reduces, and cells do not undergo senescence. Reduced expression of CPEB1 is mediated by the production of stable microRNA by Gld2. Stable microRNA silences CPEB1 mRNA causing reduced cytoplasmic polyadenylation. This particularly occurs in the reproductive system and brain tissues.

Overexpression of CPEB4 is associated with increased translation of tumor plasminogen activator (TPA) mRNA in pancreatic cancerous tissue. Increased TPA promotes increased cell growth, blood vessel formation in tumors, and tumor migration to other tissues in the body.