- Ccaat-enhancer-binding proteins
CCAAT-enhancer-binding proteins (or C/EBPs) are a family of
transcription factorsthat are composed of six members C/EBP α to C/EBP ζ. They promote the expression of certain proteins through interaction with DNA.
C/EBP proteins interact with the CCAAT (
cytidine-cytidine- adenosine-adenosine- thymidine) box motif which is present in several gene promoters. They are characterized by a highly conserved basic-leucine zipper (bZIP) domain at the C-terminus. This domain is involved in dimerization and DNA binding like other transcription factors of the leucine zipperfamily like c-Fos and Jun. C/EBPs bZIP domain structure is composed of an α-helixthat forms a coiled coil structure when it dimerizes. The different members of C/EBP family can form homodimers, heterodimers with another form of the C/EBPs and with other transcription factors that may or may not contain the leucine zipper domain. The dimerization is required for the activity of C/EBPs to bind specifically to DNA through a palindromic sequence in the major groove of the DNA. The C/EBP proteins also contain activation domains at the N-terminus and regulatory domains.
These proteins are found in
hepatocytes, adipocytes, hematopoietic cells, spleen, kidney, brainand many others organs. C/EBPs proteins are involved in different cellular responses like in the control of cellular proliferation, growth and differentiation, metabolism, immunologyand many others. All the members of the C/EBP family, except C/EBPγ, can induce transcription, through their activation domains, by interacting with components of the basal transcription apparatus. Their expression is regulated at multiple levels through hormones, mitogens, cytokines, nutrients, etc.
The C/EBPα, -β, -γ and -δ genes are
intronless and C/EBPε and -ζ have respectively two and four exonsthat lead in the case of C/EBP ε to four isoformsdue to an alternative use of promoters and splicing. For C/EBPα and -β, different sizes of polypeptidescan be produced by alternative use of initiation codonsdue to weak ribosomescanning mechanisms. The mRNAof C/EBPα can lead to two polypeptides and for C/EBPβ three different polypeptides are made: LAP* (38 kDa), LAP (35 kDa) and LIP (20 kDa). The most translated isoform is LAP, then LAP* and LIP; the latter can act as an inhibitorof the other C/EBPs by forming non-functional heterodimers.
This protein is expressed in the mammalian
nervous systemand has many implications in the nerve cells. C/EBPβ plays a role in neuronal differentiation, in learning and memory process, glial or neuronal cell functions and neurotrophic factory expression.
The regulation of C/EBPβ is exerted in many manners,
phosphorylation, acetylation, activation and repression via others transcription factors, oncogenicelements or chemokines, autoregulation, etc. C/EBPβ can interact with different proteins like CREB, NF-κBand others that lead to a trans-activation potential. Or phosphorylation can have an activation or a repression effect. For example, phosphorylation of the Threonine 235 in human or of the Threonine 188 in mouse and rat is important for its trans-activation capacity or phosphorylation(s) in its regulatory domain modulate its function.
* Ramji, D. P. & Foka P., CCAAT/enhancer-binding proteins: structure, function and regulation, Biochem. J. 365:561-575 (2002).
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