The role of site-2-proteases in bacteria: a review on physiology, virulence, and therapeutic potential

Abstract

Site-2-proteases are a class of intramembrane proteases involved in regulated intramembrane proteolysis. Regulated intramembrane proteolysis is a highly conserved signaling mechanism that commonly involves sequential digestion of an anti-sigma factor by a site-1- and site-2-protease in response to external stimuli, resulting in an adaptive transcriptional response. Variation of this signaling cascade continues to emerge as the role of site-2-proteases in bacteria continues to be explored. Site-2-proteases are highly conserved among bacteria and play a key role in multiple processes, including iron uptake, stress response, and pheromone production. Additionally, an increasing number of site-2-proteases have been found to play a pivotal role in the virulence properties of multiple human pathogens, such as alginate production in Pseudomonas aeruginosa, toxin production in Vibrio cholerae, resistance to lysozyme in enterococci and antimicrobials in several Bacillus spp, and cell-envelope lipid composition in Mycobacterium tuberculosis. The prominent role of site-2-proteases in bacterial pathogenicity highlights the potential of site-2-proteases as novel targets for therapeutic intervention. In this review, we summarize the role of site-2-proteases in bacterial physiology and virulence, as well as evaluate the therapeutic potential of site-2-proteases.

Figure 1.

The general S2P-mediated RIP paradigm. In the absence of environmental cues (left panel), the cleavage of the anti-sigma factor is inhibited, leaving the bound σ-factor in an inactive state. When extracytoplasmic stimuli are detected (right panel), a site-1-protease (S1P) cleaves a membrane-bound anti-sigma factor on the periplasmic side. The primary cleavage triggers a secondary cleavage by a site-2-protease (S2P), resulting in release of the σ-factor (orange) from the membrane, and activation of genes involved in adaptive response.

Figure 2.

The role of S2P in RIP signaling mechanism in selected bacteria. (A) The Escherichia coli RseP activation of the extracytoplasmic stress response serves as a blueprint for S2P-mediated signaling. Accumulation of unfolded outer membrane proteins (OMPs) is a known stress cue sensed by the PDZ domain of the S1P DegS. The accumulation of OMPs results in sequential cleavage of the anti-sigma factor RseA by DegS (site-1-cleavage) and RseP (site-2-cleavage). The RseP cut reveals a degron in the anti-sigma domain which is subsequently cleaved by ClpXP in the cytosol. The resulting release of σ^E activates genes involved in the adaptive stress response. RseB, a periplasmic protein, negatively regulates σ^E activity by blocking DegS/RseP cleavage under non-stress conditions. (B–F) Variations of RIP involving S2P are shown for various species. In all panels: The suggested stimuli signal is shown in yellow, the substrate in blue, presumed S1P in red, S2P in green, sigma factors (A–D) or lipoprotein (E) in active form in orange, and additional components in grey. See the text for more details.