To fold or expand—a charged question

It is no secret anymore that many proteins “defy” the common paradigm and do not fold to a well-defined 3D structure under native conditions. These are the so-called intrinsically disordered proteins (IDPs) (1). Some of these proteins do fold upon binding to a target (2), whereas others do not seem to fold under any known conditions. Much has been written in recent years about the connection between the folding behavior of IDPs and their activity. Furthermore, it was recognized that proteins belonging to this group are in general characterized by low hydrophobicity and high charge density (3); but are there any structural characteristics that might help us to understand the differences among various IDPs? An article by Muller-Spath, Soranno et al. (4) in PNAS proposes a correlation between the charge density and the overall dimensions of IDPs. The authors perform single-molecule fluorescence resonance energy transfer (FRET) spectroscopy on diffusing molecules. From the measured mean FRET efficiency they are able to compute the radius of gyration (Rg) of the molecules as a function of chemical denaturant concentration. The proteins studied include one stably folded protein (the globular cold shock protein CspTm) and two IDPs (the N-terminal domain of HIV-1 integrase, which folds upon binding of a zinc ion, and human prothymosin α). As is now well established (5), all three proteins gradually collapse when denaturant concentration is lowered. Surprisingly, the authors find that in the case of the two IDPs, Rg grows again as the concentration of the ionic denaturant guanidinium chloride is lowered below 1 M. They conclude that this is due to “release” of the proteins from electrostatic screening, because the extent of the observed expansion in buffer correlates well with the mean net charge on each chain.