Mapping the per-residue surface electrostatic potential of CAPRIN1 along its phase-separation trajectory

Area of Science:

• Biochemistry
• Molecular Biology
• Biophysics

Background:

• Biomolecular condensates are crucial for cellular organization, involving proteins and nucleic acids.
• Electrostatic interactions and charge balance govern condensate formation.
• A detailed atomistic understanding of protein charge distribution during phase separation is needed.

Purpose of the Study:

• To investigate the role of electrostatic potential in adenosine triphosphate (ATP)-induced phase separation of the RNA-binding protein CAPRIN1.
• To map residue-specific near-surface electrostatic potentials (ϕENS) of CAPRIN1 during phase separation.

Main Methods:

• Solution NMR spectroscopy was used to measure residue-specific near-surface electrostatic potentials (ϕENS) of CAPRIN1.
• The study focused on the positively charged carboxyl-terminal intrinsically disordered 103 residues of CAPRIN1.
• ϕENS values were mapped along the ATP-induced phase-separation trajectory.

Main Results:

• In the absence of ATP, CAPRIN1 exhibits large positive ϕENS, decreasing with ATP addition.
• ATP addition promotes interchain interactions between aromatic-rich and arginine-rich regions.
• Upon phase separation, CAPRIN1 becomes neutral (ϕENS ≈ 0 mV) with associated ATP, and further ATP addition leads to negative charge and re-entrance into a mixed phase.

Conclusions:

• A delicate balance of electrostatic repulsion and interchain attraction regulates CAPRIN1 phase separation.
• Nucleotides like ATP can reversibly control the formation and dissolution of protein condensates.
• This provides insight into the dynamic regulation of membraneless organelles.