Extracytoplasmic Proteases Determining the Cleavage and Release of Secreted Proteins, Lipoproteins, and Membrane Proteins in <i>Bacillus subtilis</i>

Gram-positive bacteria are known to export many proteins to the cell wall and growth medium, and accordingly, many studies have addressed the respective protein export mechanisms. In contrast, very little is known about the subsequent fate of these proteins. The present studies were therefore aimed at determining the fate of native exported proteins in the model organism <i>Bacillus subtilis</i>. Specifically, we employed a gel electrophoresis-based liquid chromatography–mass spectrometry approach to distinguish the roles of the membrane-associated quality control proteases HtrA and HtrB from those of eight other proteases that are present in the cell wall and/or growth medium of <i>B. subtilis</i>. Notably, HtrA and HtrB were previously shown to counteract potentially detrimental “protein export stresses” upon overproduction of membrane or secreted proteins. Our results show that many secreted proteins, lipoproteins, and membrane proteins of <i>B. subtilis</i> are potential substrates of extracytoplasmic proteases. Moreover, potentially important roles of HtrA and HtrB in the folding of native secreted proteins into a protease-resistant conformation, the liberation of lipoproteins from the membrane–cell wall interface, and the degradation of membrane proteins are uncovered. Altogether, our observations show that HtrA and HtrB are crucial for maintaining the integrity of the <i>B. subtilis</i> cell even under nonstress conditions