Phosphorylation Pushes Floppy Protein To Fold

Intrinsically disordered proteins, which lack a well-defined three-dimensional structure, typically fold up only when they bind their targets. But appending phosphate groups, a common protein modification, can be enough to make one of these floppy proteins fold on its own, a team led by Julie D. Forman-Kay of the University of Toronto reports (Nature 2014, DOI: 10.1038/nature13999).

Forman-Kay’s team had studied phosphorylation in other disordered proteins but had never seen the modification cause a protein to fold. This time they focused on a disordered protein called 4E binding protein-2 (4E-BP2), which binds to a protein involved in translation and suppresses initiation of protein synthesis.

When 4E-BP2 binds its target, part of it folds up into a small helix. Phosphorylation of two amino acids in 4E-BP2, however, causes the surrounding region to fold up into a β-sheet domain, blocking the formation of the helix. Phosphorylation at three other amino acids stabilizes the β-sheet domain, holding it in place. The resulting nonfunctional 4E-BP2 can’t bind its target.

“People like to think that you need a fold for function, and if it’s disordered, a protein can’t be functional,” Forman-Kay says. In this case, the opposite is true: Only floppy, unphosphorylated 4E-BP2 can bind its target and influence translation. “That’s kind of fun because it turns the paradigm on its head.”

The team analyzed 4E-BP2’s structure using nuclear magnetic resonance spectroscopy. Phosphorylation caused significant shifts in the NMR spectra of amide protons that are diagnostic of folding.

Forman-Kay and her collaborators, particularly Nahum Sonenberg of McGill University, in Montreal, are now studying 4E-BP2 as a potential drug target for cancer, autism, and other neurological disorders. The researchers are screening for molecules that can stabilize or destabilize the β-sheet. “While 4E-BP2 was known to have a very powerful effect on the regulation of translation initiation, the fact that it can fold presents a completely new target,” Forman-Kay says.

“This work represents a very important contribution to the intrinsically disordered protein field, providing a well-characterized example of a phosphorylation-based regulatory switch,” says Vladimir Uversky, an expert in intrinsically disordered proteins at the University of South Florida, in Tampa. “Since intrinsically disordered proteins are very promiscuous binders that are commonly phosphorylated, this mechanism could be widespread.”