In vivo aggregation of bovine β-lactoglobulin is affected by Cys at position 121

Bovine β-lactoglobulin (BLG) has been widely used as a model system to study protein folding and aggregation and for biotechnology applications. Native BLG contains two disulfide bonds and one free cysteine at position 121. This free thiol group has been shown to be responsible for the irreversibility of BLG denaturation in vitro, but nothing is known about its relevance during protein folding inside the cell. Here, we report the expression of soluble wild type recombinant BGL in Escherichia coli cells at about 109 mg rBLG/g wet weight cells and a comparison between the aggregation of wt BLG and its variant C121S upon intracellular expression. We show that in E. coli C121SBLG is more prone to aggregation than the wild type protein and that their different behavior depends on the oxidation of disulfide bonds. Our results underline the key contribution of the unpaired cysteine residue during the oxidative folding pathway and indicate BLG as a useful tool for the study of protein aggregation in vivo.