Selective Domain Stabilization as a Strategy to Reduce Fusion Protein Aggregation

A human serum albumin-human growth hormone (HSA-hGH) fusion protein was used as a model to understand the contributions of individual domains to the aggregation behavior of the overall fusion protein. Aggregation of HSA-hGH was studied at two different pH conditions, pH 5 and pH 7. Conformational stability of the HSA domain was modulated by addition of octanoic acid, a binding ligand. Conformational stability of the fusion protein and the HSA domain were determined from experimentally measured values for free energies of unfolding (ΔGunf) with midpoint of apparent unfolding temperatures (Tm) used as surrogate in some cases. Apparent Tms of both HSA and HSA-hGH were increased by octanoic acid binding. Osmotic second virial coefficients were measured to monitor protein-protein interactions in solution. Reductions in rates of aggregation were observed under solution conditions that increased protein-protein repulsive interactions even when no changes in conformational stability were detected. The results indicate that colloidal instabilities are responsible for HSA-hGH aggregation and that conformational stability of the HSA domain does not play a dominant role in the aggregation of HSA-hGH.