The pH-responsive precipitation–redissolution of the CspB fusion protein, CspB50TEV-Teriparatide, triggered by changes in secondary structure

Cell surface protein B (CspB) fusion proteins can undergo reversible pH-responsive precipitation–redissolution. A pH-responsive precipitation–redissolution of CspB tag purification (pPRCP) method was established for protein purification using this property. However, the mechanism of the pH-responsive precipitation of CspB fusion proteins is unknown, which has made it difficult to set process parameters for pPRCP. In this study, we investigated the mechanism of the pH-responsive precipitation of CspB fusion proteins using CspB50TEV-Teriparatide (CspB-teri) as a model. As expected, CspB-Teri was reversibly precipitated at acidic pH. By contrast, CspB-Teri was not precipitated under unfolding conditions induced by trifluoroethanol, urea, or guanidine hydrochloride, even at acidic pH. The conformation of CspB-Teri changed to a β-sheet-rich structure as the pH decreased, followed by the formation of intermolecular interactions, which caused precipitation. The particle size of the CspB-Teri precipitate increased in a protein concentration-dependent manner. These results indicated that the pH-responsive precipitation of CspB-Teri is triggered by the formation of a β-sheet structure in response to decreasing pH, and the growth of the precipitate particles occurred through intermolecular interactions.