EIF4EBP1 (Eukaryotic translation initiation factor 4E binding protein 1)

	The diagram illustrates key regulatory features of the human 4E-BP1 protein, including the RAIP and TOS motifs that are important for the phosphorylation of the protein at Thr37, Thr46, Ser65, Thr70 and Ser101 by the Raptor/mTOR complex (Eguchi et al., 2006; Lee et al., 2008). Additional phosphorylation sites have been identified at Ser83 and Ser112. The region required for binding of 4E-BP1 to initiation factor eIF4E and a site of cleavage of the protein by caspases in apoptotic cells are also shown (diagram adapted from an original prepared by Dr C. Constantinou).
Description	Human 4E-BP1 is a 118 amino acid protein (119 amino acids including the initiating methionine) and is encoded by an mRNA containing 877 nucleotides (including a short poly(A) tail). The mRNA has a 72 nucleotide 5' untranslated region and a 448 nucleotide 3' untranslated region. The coding region comprises nucleotides 73-429. The protein can be reversibly phosphorylated at Thr37, Thr46, Ser65, Thr70, Ser83, Ser101 and Ser112 in response to a variety of physiological stimuli. The key enzyme involved in these phosphorylations is the protein kinase mTOR, but other kinases may also be involved (Yonezawa et al., 2004).
Expression	4E-BP1 is ubiquitously expressed, although its presence is not essential to the viability of cells or the organism as a whole (Le Bacquer et al., 2007). The protein is stable (half-life more than 16h) but can be ubiquitinated and targeted for degradation by a mechanism that responds to its state of phosphorylation (Elia et al., 2008). The level of expression and state of phosphorylation of the protein may influence cellular phenotype, with high levels of phosphorylated 4E-BP1 in breast, ovary, and prostate tumours being associated with malignant progression and an adverse prognosis (Armengol et al., 2007). Conversely, hypophosphorylated 4E-BP1 may have an anti-oncogenic role due to its inhibitory effect on eIF4E and its potential pro-apoptotic properties (Li et al., 2002).
Localisation	4E-BP1 is present in both cytoplasm and nucleus. The hypophosphorylated protein in the latter compartment can sequester eIF4E within the nucleus under conditions of physiological stress (Rong et al., 2008).
Function	The members of the 4E-BP family of proteins act by binding to the mRNA cap-binding protein eukaryotic initiation factor 4E (eIF4E), in competition with another initiation factor, eIF4G, that is essential for polypeptide chain initiation. Thus the availability of eIF4E for translation of cap-dependent mRNAs is limited by the extent to which this factor is sequestered by the 4E-BPs. 4E-BP1 is reversibly phosphorylated at multiple sites (see diagram above), in response to several physiological signals that promote translation (Proud, 2004; Wang et al., 2005; Proud, 2006). Such phosphorylations lower the affinity of 4E-BP1 for eIF4E and result in the dissociation of the two proteins, thereby enhancing the level of active eIF4E and promoting the translation of capped mRNAs, most likely in a selective manner (Averous et al., 2008). Conversely, physiological stresses and other conditions that inhibit translation - e.g. exposure of cells to cytokines of the TNFalpha family (Lang et al., 2007; Jeffrey et al., 2006) or activation of the tumour suppressor protein p53 (Tilleray et al., 2006; Constantinou and Clemens, 2007) - cause dephosphorylation of 4E-BP1 and increase binding of the latter to eIF4E. 4E-BP1 is also susceptible to other post-translational modifications, notably specific proteolytic cleavages (Tee and Proud, 2002; Constantinou et al., 2008) and phosphorylation-dependent ubiquitination (Elia et al., 2008). There is good evidence for involvement of 4E-BP1 in malignant transformation. The protein can negatively regulate cell growth, block cell cycle progression and revert the transformed phenotype of cells over-expressing eIF4E (Rousseau et al., 1996; Jiang et al., 2003; Barnhart et al., 2008). It has been shown that 4E-BP1 is a key regulator of the oncogenic Akt (protein kinase B) and ERK (extracellular-regulated kinase) signalling pathways and it integrates the function of these pathways in tumours (She et al., 2010). Consistent with this, high levels of phosphorylated (inactive) 4E-BP1 indicate poor prognosis in some cancer patients (Castellvi et al., 2006; Frederick et al., 2011). Although 4E-BP1 is not essential to viability the protein (together with its homologue 4E-BP2) is important for regulation of adipogenesis and insulin resistance (Le Bacquer et al., 2007). The 4E-BPs have also been reported to play a role in myelopoiesis (Olson et al., 2009). There is a major role for 4E-BP1 in the responses of cells to hypoxia, which promotes dephosphorylation of the protein (Koritzinsky et al., 2006; Connolly et al., 2006 ; Barnhart et al., 2008). It is likely that this response implements hypoxia-induced changes in gene expression at the translational level (Magagnin et al., 2008; Barnhart et al., 2008).
Homology	4E-BP1 was identified alongside another member of the eIF4E-binding protein family designated 4E-BP2 (Pause et al., 1994). A further homologue has also been identified, 4E-BP3 (Poulin et al., 1998), and these proteins respectively share 55,7% identity (82,0% similarity) and 50,8% identity (66,9% similarity) with 4E-BP1. All share the central eIF4E binding motif and are capable of competing with the eIF4G proteins for binding to eIF4E.

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