Certain highly soluble proteins, such as Escherichia coli maltose-binding protein (MBP), have the ability to enhance the solubility of their fusion partners, making them attractive vehicles for the production of recombinant proteins, yet the mechanism of solubility enhancement remains poorly understood. Here, we report that the solubility-enhancing properties of MBP are dramatically affected by amino acid substitutions that alter the equilibrium between its "open" and "closed" conformations. Our findings indicate that the solubility-enhancing activity of MBP is mediated by its open conformation and point to a likely role for the ligand-binding cleft in the mechanism of solubility enhancement. Published by Elsevier Inc.