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Counterion valency significantly impacts intrinsically disordered proteins (IDPs). Divalent cations cause greater compaction of the AGARP protein than monovalent cations, revealing a specific ion-binding mechanism beyond simple charge screening.

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Area of Science:

  • Biophysics
  • Protein Chemistry
  • Computational Biology

Background:

  • Intrinsically disordered proteins (IDPs) are sensitive to ionic conditions, but the mechanisms of ion-induced conformational changes are not fully understood.
  • Understanding these changes is crucial for comprehending IDP function in diverse biological environments.

Purpose of the Study:

  • To investigate how counterion valency affects the dimensions of a highly charged IDP, the AGARP protein.
  • To elucidate the mechanisms behind ion-induced compaction in IDPs.

Main Methods:

  • Utilized fluorescence correlation spectroscopy and size exclusion chromatography to measure changes in hydrodynamic radius.
  • Employed molecular dynamics simulations and polyampholyte theory for theoretical analysis.
  • Used circular dichroism spectroscopy to assess secondary structure formation.

Main Results:

  • Observed a pronounced, valency-dependent reduction in AGARP's hydrodynamic radius.
  • Divalent cations (Ca2+, Mg2+) induced significant collapse at lower concentrations compared to monovalent cations (Na+, K+).
  • Simulations captured Debye-Hückel screening for monovalent ions but not the enhanced compaction by divalent ions, indicating specific binding interactions.

Conclusions:

  • IDP compaction can occur via structure-free electrostatic collapse, independent of secondary structure formation.
  • Specific chelation of divalent ions by polyanionic IDP chains is a key mechanism regulating compaction.
  • Findings have implications for understanding IDP behavior in complex biological ionic milieus.