Salt Induced Collapse

IDPs enriched in charged residues can be classified as either polyelectrolytes or polyampholytes. In polyelectrolytes, one type of charged residues either negative (Aspartate and Glutamate) or positive (Lysine and Arginine) dominates over other whereas in polyapmpholytes both positively and negatively charged residues are present almost in an equal proportion. Salts can modulate the stability of a configuration of an IDP by modulating the electrostatics interactions among the charged residues or through salt specific interactions known as Hofmeister effects. To investigate the effects of salts on the properties of IDPs, we simulated five IDPs, prothymosin, Sic1, ERMTADn, IN, and nucleoporin with varying chain length, composition in charged residues, and primary sequence in presence of a strong destabilizing salt (guanidine hydrochloride, (GuHCl)), a weak stabilizing salt (KCl) and a strongly stabilizing salt (KGlu). The electrostatic interactions play important role in low salt concentration ([salt] < 1 M) regime whereas Hofmeister effects are important in high salt concentration regime ([salt] > 8 M).

Average radius of gyration computed in presence of [GuHCl] (red squares), [KCl] (black circles) and [KGlu] (blue triangles) for prothymosin and Sic1 in (A) and (C), respectively. Rg as a function of [salt] in low [salt] regime is shown in inset. (B) Normalized structure factor, S(q) as a function of wave vector, q for prothymosin at [GuHCl] = 0.01, 1 and 8 M, [KCl] = 8 M and [KGlu] = 8.0 M are in red, yellow, blue, green and black, respectively. (D) Normalised structure factor, S(q) as a function of wave vector, q for Sic1 at [GuHCl] = 0.01, 0.5 and 8 M, [KCl] = 8 M and [KGlu] = 8.0 M are in red, yellow, blue, green and black, respectively. This plot shows collapse of polyelectrolytes in low [salt] regime depends on fraction of charge in IDPs.

Average radius of gyration computed in presence of [GuHCl] (red squares), [KCl] (black circles) and [KGlu] (blue triangles) for ERM TADn and IN in (A) and (C). Rg as a function of [salt] in low [salt] regime is shown in inset. (B) Normalised S(q) for for ERM TADn in the presence of [GuHCl] = 0.01 M (red), 1 M (yellow), 8 M (blue), [KCl] = 8 M (green) and [KGlu] = 8 M (black) is plotted for ERM TADn. (D) S(q) for IN computed in the presence of [GuHCl] = 0.01 M (red), 0.15 M (yellow) and 8 M (blue), [KCl] = 8 M (green) and [KGlu] = 8 M (black). Compaction/Expansion in low [salt] regime depends on charge asymmtry i.e. the ratio of net charge to total charged residues.