9.B.217.  The Transmembrane PrsW Protease (PrsW) Family  

The activation of the RNA polymerase sigma factor sigmaW in Bacillus subtilis is regulated by intramembrane proteolysis by a PrsW protease (Ellermeier and Losick 2006). SigmaW is activated by proteolytic destruction of the membrane-bound anti-sigmaW factor RsiW in response to antimicrobial peptides and other agents that damage the cell envelope. RsiW is destroyed by successive proteolytic events known as Site-1 and Site-2 cleavage. Site-2 cleavage is mediated by a member of the SpoIVFB-S2P family of intramembrane-acting metalloproteases, but the protease responsible for Site-1 cleavage is by PrsW (annotated YpdC) that is both necessary and sufficient for Site-1 cleavage of RsiW. Ellermeier and Losick 2006 identified residues important for proteolysis and a cluster of acidic residues involved in sensing antimicrobial peptides and cell envelope stress. Another membrane protease, RasP, when defective, causes defects in competence development, protein secretion and membrane protein production (Zweers et al. 2012). σ^V activation in B. subtilis is controlled by regulated intramembrane proteolysis and requires the site 2 protease RasP (Hastie et al. 2013). RasP, an intramembrane metalloprotease of Bacillus subtilis, cleaves both the stress response anti-sigma factors RsiW and RsiV as well as the cell division protein FtsL, and remnant signal peptides, within their transmembrane segments (Parrell et al. 2017). PrsW likely regulates the activation of the ECF σ factor CsfT in Clostridium difficile and controls the resistance of C. difficile to antimicrobial peptides (Ho and Ellermeier 2011). In Staphylococcus aureus, PrsS, a homologue of PrsW, and σ^S together mediate virulence and resistance to DNA damage and cell wall-targeting antibiotics via a common pathway (Krute et al. 2015).

PrsW proteases, the DUF2324 family and the γ-secretase subunit APH-1 proteins share four predicted core transmembrane segments and possess similar yet distinct sets of sequence motifs (Pei et al. 2011). Remote similarity between APH-1 and membrane proteases sheds light on APH-1's evolutionary origin and raises the possibility that APH-1 may possess proteolytic activity in the current or ancestral form of γ-secretase. 

The second sub-family in this family consists of DUF368 proteins and is only distantly related to the first sub-family. Precursors of peptidoglycan (PG) and other cell surface glycopolymers are synthesized in the cytoplasm and then delivered across the cell membrane bound to the recyclable lipid carrier undecaprenyl phosphate (C55-P). The transporter protein(s) that return C55-P to the cytoplasmic face of the cell membrane have been elusive. Sit et al. 2022 identified the DUF368-containing and DedA transmembrane protein families as candidate C55-P translocases. Gram-negative and -positive bacteria lacking their cognate DUF368-containing protein exhibited alkaline-dependent cell wall and viability defects, along with increased cell surface C55-P levels. pH-dependent synthetic genetic interactions between DUF368-containing proteins and DedA family members suggested that the putative C55-P transporter usage is dynamic and modulated by environmental inputs. C55-P transporter activity was required by the cholera pathogen for growth and cell shape maintenance in the intestine. Sit et al. 2022 proposed that conditional transporter reliance provides resilience in lipid carrier recycling, bolstering microbial fitness within and outside of the host.