Post-liberation cleavage of signal peptides is catalyzed by the site-2 protease (S2P) in bacteria

Membrane proteases in the bacterial protein secretion and quality control pathway

Proteolytic cleavage of proteins that are permanently or transiently associated with the cytoplasmic membrane is crucially important for a wide range of essential processes in bacteria. This applies in particular to the secretion of proteins and to membrane protein quality control. Major progress has been made in elucidating the structure-function relationships of many of the responsible membrane proteases, including signal peptidases, signal peptide hydrolases, FtsH, the rhomboid protease GlpG, and the site 1 protease DegS. These enzymes employ very different mechanisms to cleave substrates at the cytoplasmic and extracytoplasmic membrane surfaces or within the plane of the membrane. This review highlights the different ways that bacterial membrane proteases degrade their substrates, with special emphasis on catalytic mechanisms and substrate delivery to the respective active sites.

The twin-arginine translocation (Tat) protein export pathway

The twin-arginine translocation (Tat) protein export system is present in the cytoplasmic membranes of most bacteria and archaea and has the highly unusual property of transporting fully folded proteins. The system must therefore provide a transmembrane pathway that is large enough to allow the passage of structured macromolecular substrates of different sizes but that maintains the impermeability of the membrane to ions. In the Gram-negative bacterium Escherichia coli, this complex task can be achieved by using only three small membrane proteins: TatA, TatB and TatC. In this Review, we summarize recent advances in our understanding of how this remarkable machine operates.

Extra cytoplasmic function σ factor activation

The bacterial cell envelope is essential for cell viability and is a target for numerous antibiotics and host immune defenses. Thus bacteria must sense and respond to damage to the cell envelope. Many bacteria utilize alternative σ factors such as extracytoplasmic function (ECF) σ factors to respond to cell envelope stress. Although ECF σ factors are utilized by both Gram negative and Gram positive bacteria to respond to cell envelope stress, the mechanisms of sensing differ. In this review, we examine the events and proteins that are required for activation of two model extracytoplasmic function σ factors, σ(E) in E. coli and σ(W) in B. subtilis.