Contrasting Assemblies of Oppositely Charged Proteins

Area of Science:

• Biophysics
• Protein interactions
• Phase separation

Background:

• Oppositely charged proteins can form soluble assemblies, leading to liquid-liquid phase separation (heteroprotein coacervation).
• Surface charge anisotropy is increasingly recognized as crucial for heteroprotein complexation and coacervation.

Purpose of the Study:

• Investigate protein complexation and phase separation between acidic beta-lactoglobulin (BLG) and basic napin (NAP) or lysozyme (LYS).
• Compare the roles of surface charge distribution versus net charge in protein assembly.

Main Methods:

• Turbidity measurements
• Isothermal titration calorimetry
• Dynamic light scattering (DLS)
• Brownian dynamics simulations

Main Results:

• Lysozyme (LYS) binding to BLG induced liquid-solid or liquid-liquid phase separation depending on pH.
• Napin (NAP) interacted with BLG, forming nanometer-sized assemblies detected by DLS.
• Brownian dynamics simulations confirmed assembly formation for both BLG+NAP and BLG+LYS.
• BLG+NAP assemblies were smaller than BLG+LYS assemblies, suggesting molecular details are important.

Conclusions:

• Protein surface charge distribution, beyond net charge, significantly impacts heteroprotein assembly and phase separation.
• Discrepancies between DLS and simulations highlight the complexities of modeling protein interactions accurately.