Credit: Nature Publishing Group

Intrinsically disordered proteins do not possess stable three-dimensional structures, but they can fold into ordered states upon interacting with partners such as ligands or other macromolecules. Although post-translational modifications have been known to regulate many intrinsically disordered proteins by stabilizing or destabilizing short localized secondary-structure segments, evidence of post-translational modifications mediating conformational transitions of intrinsically disordered proteins into folded states was lacking. Now Forman-Kay and colleagues show that phosphorylation of the intrinsically disordered protein 4E-BP2 at multiple sites induces a structured state with important functional implications. 4E-BP2 binds the translation initiation factor eIF4E to suppress cap-dependent translation initiation. Interaction of the nonphosphorylated form of 4E-BP2 with eIF4E causes a disorder-to-helix transition of its canonical YXXXXLΦ eIF4E-binding motif. Using a combination of NMR spectroscopy and isothermal titration calorimetry, the authors show that phosphorylation of 4E-BP2 at two sites, Thr37 and Thr46, induces folding of the region encompassing residues Pro18 to Arg62, which also includes the YXXXXLΦ motif, into a four-stranded β-domain while the rest of the protein remains disordered. The folding involves recruitment of the YXXXXLΦ motif into the fourth β-strand, and it results in reduced affinity for eIF4E as well as potential inhibition of the degradation-inducing ubiquitination of Lys57. This folded state is weakly stable, and the interaction with eIF4E—albeit diminished—leads to unfolding of the β-domain. Additional phosphorylation at three more sites (Ser65, Thr70 and Ser83) is required to stabilize the β-domain, with the fully phosphorylated 4E-BP2 having a 4,000-fold-reduced affinity for eIF4E. Thus, the transition into a folded state driven by phosphorylation provides a mechanism to regulate the biological activity of 4E-BP2 and suggests that post-translational modifications may have a role in mediating structural changes of other intrinsically disordered proteins into functionally relevant ordered states. (Nature doi:10.1038/nature13999, 22 December 2014) CD