Stress-responsive systems set specific limits to the overproduction of membrane proteins in Bacillus subtilis

Essential membrane proteins are generally recognized as relevant potential drug targets due to their exposed localization in the cell envelope. Unfortunately, high-level production of membrane proteins for functional and structural analyses is often problematic. This is mainly due to their high overall hydrophobicity. To develop new concepts for membrane protein overproduction, we investigated whether the biogenesis of overproduced membrane proteins is affected by stress response-related proteolytic systems in the membrane. For this purpose, the well-established expression host Bacillus subtilis was used to overproduce eight essential membrane proteins from B. subtilis and Staphylococcus aureus. The results show that the sigma(W) regulon (responding to cell envelope perturbations) and the CssRS two-component regulatory system (responding to unfolded exported proteins) set critical limits to membrane protein production in large quantities. The identified sigW or cssRS mutant B. subtilis strains with significantly improved capacity for membrane protein production are interesting candidate expression hosts for fundamental research and biotechnological applications. Importantly, our results pinpoint the interdependent expression and function of membrane-associated proteases as key parameters in bacterial membrane protein production.