CPEB
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 is present at postsynaptic sites and dendrites where it stimulates polyadenylation and translation in response to synaptic activity. CPEB most commonly activates the target RNA for translation, but can also act as a repressor, dependent on its phosphorylation state. As a repressor, CPEB interacts with the deadenylation complex and shortens the polyadenine tail of mRNAs. 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.
| Cytoplasmic polyadenylation element binding protein | |||||||||
|---|---|---|---|---|---|---|---|---|---|
Properties of CPEBs and their interactions with other proteins in RNP complexes | |||||||||
| Identifiers | |||||||||
| Symbol | CPEB | ||||||||
| Pfam | PF16366 | ||||||||
| InterPro | IPR034819 | ||||||||
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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, is important to note that this mechanism has been under great scrutiny.
CPEB has been shown to shuttle between the nucleus and cytoplasm. In the nuclei of different organisms, it was found that CPEB helps guide the path of mRNA in the cytoplasm. CPEB was found to be almost exclusively in the c plasm in stage VI Xenopus oocytes. However, a further study on this topic found that there is a substantial amount of CPEB in the nucleus. CPEB can bind with CPE-containing mRNAs in the nucleus, which forces tight translational regulation in the cytoplasm. CPEBs bound to these mRNAs were found to have wer translation efficiency, which is indicative of the translation regulation.