Charge Shielding Prevents Aggregation of Supercharged GFP Variants at High Protein Concentration

Understanding protein stability is central to combatting protein aggregation diseases and developing new protein therapeutics. At the high concentrations often present in biological systems, purified proteins can exhibit undesirable high solution viscosities and poor solubilities mediated by short-range electrostatic and hydrophobic protein-protein interactions. The interplay between protein amino acid sequence, protein structure, and solvent conditions to minimize protein-protein interactions is key to designing well-behaved pharmaceutical proteins. However, theoretical approaches have yet to yield a general framework to address these problems. Here, we analyzed the high concentration behavior of superfolder GFP (sfGFP) and two supercharged sfGFP variants engineered to have formal charges of -18 or +15. Under low cosolute conditions, sfGFP and the -18 variant formed a gel or phase separated at ∼10 mg/mL. Under conditions that screen surface charges, including formulations with high histidine or high NaCl concentrations, all three variants attained concentrations up to 250 mg/mL with moderate viscosities. Moreover, all three variants exhibited very similar viscosity-concentration profiles over this range. This effect was not mimicked by high sugar concentrations that exert excluded-volume effects without shielding charge. Collectively, these data demonstrate that charge shielding neutralizes not only long-range electrostatic interactions but also, surprisingly, short-range electrostatic effects due to surface charge anisotropy. This work shows that supercharged sfGFP behavior under high ionic strength is largely determined by particle geometry, a conclusion that is supported by colloid models and may be applicable to pharmaceutically relevant proteins.