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-Huckel screening by monovalent ions but not the enhanced compaction driven by divalent ion binding. 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. TOC GRAPHIC O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=108 SRC="FIGDIR/small/663830v1_ufig1.gif" ALT="Figure 1"> View larger version (43K): org.highwire.dtl.DTLVardef@84fba8org.highwire.dtl.DTLVardef@a5f3aforg.highwire.dtl.DTLVardef@12194a7org.highwire.dtl.DTLVardef@1a68366_HPS_FORMAT_FIGEXP M_FIG C_FIG