Electrostatic Collapse of Intrinsically Disordered Acid-Rich Protein Is Sensitive to Counterion Valency

Intrinsically disordered proteins (IDPs) respond sensitively to their ionic environment, yet the mechanisms driving ion-induced conformational changes remain incompletely understood. Here, we investigate how counterion valency modulates the dimensions of an extremely charged model IDP, the aspartic and glutamic acid-rich protein AGARP. Fluorescence correlation spectroscopy and size exclusion chromatography reveal a pronounced, valency-dependent reduction in its hydrodynamic radius, with divalent cations (Ca²⁺, Mg²⁺) inducing collapse at much lower activities than monovalent cations (Na⁺, K⁺). Molecular dynamics simulations, direct sampling, and polyampholyte theory quantitatively capture the Debye-Hückel screening by monovalent ions but not the enhanced compaction driven by divalent ion binding thereby suggesting that, beyond differences in screening strength, a valency-sensitive mode of interaction is at play. Circular dichroism spectroscopy shows that compaction occurs without secondary structure formation. Our results demonstrate a structure-free electrostatic collapse and suggest that specific chelation of divalent ions by disordered polyanionic protein chains is a key mechanism regulating IDP compaction, with implications for understanding their behavior in biologically relevant ionic environments.