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==''' Cytoplasmic Polyadenylation Element-Binding Protein (CPEB) ''' ==

CPEB protein regulates the length of the polyA tail which allows to control the translation. It binds to mRNA and in association with some factors, can act as a translational repressor or activator, depending on these factors.

* A N-terminal region which is a regulatory region with phosphorylation and dephosphorylation sites. This region is variable in length and composition.

* A C-terminal region, composed of 2 recognition patterns :  RRMs domains and zinc finger domains.

<table><tr><td colspan='3'><br>

<StructureSection load='2m13' size='340' side='left' caption='Zinc finger domain' scene=''>

***A 3₁₀ <scene name='86/868187/310/1'> helical turn</scene> (residues 550-552).

***2 zinc binding sites, the first one is composed of <scene name='86/868187/Z1/3'>Cys515, Cys518, Cys537, Cys540</scene> and the second is composed of <scene name='86/868187/Z2/2'>Cys527, Cys532, His545 and His553</scene>.

<StructureSection load='2MKK' size='350' side='right' caption='RNA binding to RRMs' scene=''>

</StructureSection>

== ''' Function ''' ==

CPEB controls the balance between [https://en.wikipedia.org/wiki/Senescence senescence] and proliferation. Indeed, due to its two structural domains in its C-terminal region, it has the capacity to modify mRNA<ref name="publi" />.

RRMs domains allow binding to the [https://en.wikipedia.org/wiki/Cytoplasmic_polyadenylation_element CPE] sequence of mRNA (pyrimidine rich : UUUUUAU), thus ensuring good RNA positioning and high fidelity.

Zing finger domains allow binding to different proteins, which play a role in affinity but not in specificity. It's therefore its ability to recruit different proteins that will determine its action, activation or repression of translation.

A specific arrangement of CPEs in mRNA can lead to the repression of the translation. In this case, the CPEB can form a dimer which could avoid the bound of the polyA polymerase complex in 2 different ways. It could prevent the association of ePAB with the polyA tail because CPEB recruits the deadenylase [https://en.wikipedia.org/wiki/Poly(A)-specific_ribonuclease PARN] which reduce the length of the polyA tail. It could disrupt the interaction between the binding factors of the translation [https://en.wikipedia.org/wiki/EIF4E eIF4E] and [https://en.wikipedia.org/wiki/EIF4G eIF4G] too, because CPEB recruits the protein Maskin which blocks eIF4G recruitment too<ref name="publi" />. This prevents the bound of the cap machinery to the mRNA and therefore inhibits the translation.

On the other hand, the CPEB can activate the translation. Indeed, in the cytoplasm, there are some repressed or silenced mRNA with a short polyA tail. They can be activated by cytoplasmic polyadenylation thanks to a hormonal stimulation. This stimulation can lead to the phosphorylation of CPEB which increase its affinity with the [https://en.m.wikipedia.org/wiki/Cleavage_and_polyadenylation_specificity_factor CPSF] (Cleavage and Polyadenylation Specificity Factor) and decrease the binding between CPEB and PARN. CPSF binds to the mRNA at the sequence 3’ of the tail of the mRNA (AAUAAA) and recruits the [https://en.wikipedia.org/wiki/Polynucleotide_adenylyltransferase poly(A) polymerase] which leads to the elongation of the polyA tail and therefore to the activation of the translation<ref name="publi" />.  

== ''' Diseases ''' ==

CPEB proteins play a key role in some diseases, especially in cancers. Indeed, in some humans tumors the level of CPEB 1 is lower than in healthy cells and this leads to the growth of these tumors. An overexpression of CPEB 4 can lead to tumor growth too. Some researches try to find a cancer treatment thanks to CPEB mutations<ref name="publi" />.