TABLE 1.

Overview of envelope stress response pathways.

Pathways	Envelope stresses	Activation mechanisms	Targets	References
σE system	Unfolded OMPs	The unfolded OMP in the periplasm binds to the IM protease DegS, causing the conformational change of DegS and enabling it to cleave anti-sigma factor RseA, thereby removing the inhibitory effect of RseA on the σE system	OMP folding pathways	Mecsas et al., 1993; Walsh et al., 2003; Rhodius et al., 2006
Cpx system	Defects in IM protein secretion; misfolding of IM or periplasmic protein; lipoprotein export defect	Upon receiving a stimulus, CpxA autophosphorylates at the histidine kinase (HK) domain, and the phosphoryl group is transferred to the phosphoryl receiver (PR) domain of response regulator CpxR to activate the Cpx system	IM protein complexes; protein folding and degradation; peptidoglycan modification; efflux; metal and redox homeostasis	Danese and Silhavy, 1998; Otto and Silhavy, 2002; Jubelin et al., 2005; Yamamoto and Ishihama, 2006; Evans et al., 2013; López et al., 2018; Delhaye et al., 2019; May et al., 2019
Bae system	Toxic molecules	Upon receiving a stimulus, the BaeS autophosphorylates at its HK domain, and the phosphoryl group is transferred to the PR domain of the response regulator BaeR to activate the Bae system	Periplasmic chaperone; efflux	Nagakubo et al., 2002; Raffa and Raivio, 2002; Zhou et al., 2003; Nishino et al., 2005; Bury-Moné et al., 2009; Leblanc et al., 2011
Psp system	Severe damage to the IM	Upon receiving a stimulus, the IM proteins PspB and PspC interact with PspA, which releases PspF to activate the Psp system	psp genes	Brissette et al., 1990; Carlson and Silhavy, 1993; Jovanovic et al., 1996; Kobayashi et al., 1998; Jones et al., 2003
Rcs system	OM damage; LPS synthesis defects; peptidoglycan perturbation; lipoprotein mislocalization	Upon receiving a stimulus, RcsC autophosphorylates at its HK domain. The phosphoryl group is finally transferred to the PR domain of response regulator RcsB through multiple transfer steps to activate the Rcs system	Capsule; motility; biofilm formation; virulence	Girgis et al., 2007; Callewaert et al., 2009; Farris et al., 2010; Tao et al., 2012; Konovalova et al., 2016; Meng et al., 2020a