In this study, we assessed the potential of arginine and lysine solubility-enhancing peptide (SEP) tags to control the solubility of a model protein, anti-EGFR VHH-7D12, in a thermally denatured state at a high temperature. We produced VHH-7D12 antibodies attached with a C-terminal SEP tag made of either five or nine arginines or lysines (7D12-C5R, 7D12-C9R, 7D12-C5K and 7D12-C9K, respectively). The 5-arginine and 5-lysine SEP tags increased the E. coli expression of VHH-7D12 by over 80%. Biophysical and biochemical analysis confirmed the native-like secondary and tertiary structural properties and the monomeric nature of all VHH-7D12 variants. Moreover, all VHH-7D12 variants retained a full binding activity to the EGFR extracellular domain. Finally, thermal stress with 45-minute incubation at 60 and 75 °C, where VHH-7D12 variants are unfolded, showed that the untagged VHH-7D12 formed aggregates in all of the four buffers, and the supernatant protein concentration was reduced by up to 35%. 7D12-C5R and 7D12-C9R did not aggregate in Na-acetate (pH 4.7) and Tris-HCl (pH 8.5) but formed aggregates in phosphate buffer (PB, pH 7.4) and phosphate buffer saline (PBS, pH 7.4). The lysine tags (either C5K or C9K) had the strongest solubilization effect, and both 7D12-C5K and 7D12-C9K remained in the supernatant. Altogether, our results indicate that, under a thermal stress condition, the lysine SEP tags solubilization effect is more potent than that of an arginine SEP tags, and the SEP tags did not affect the structural and functional properties of the protein.
Keywords: SEP tag; VHH; aggregation; arginine SEP tag; lysine SEP tag; protein solubility; single domain antibody.