The inner membrane protein DsbD (Slamti & Waldor, 2009), part of an enzyme system involved in ensuring proper disulphide bond formation of secreted proteins (Kadokura & Beckwith, 2010), activates the Cpx system in Vibrio cholerae, suggesting that incorrect disulphide bond formation of proteins might act as a trigger of the Cpx-TCS (Slamti & Waldor, 2009).

Likewise, incorrect disulphide bond formation of a variant find more of the periplasmic LolA protein (I93C/F149C) might induce the Cpx-TCS in a similar way (Tao et al., 2010). However, LolA acts as a periplasmic chaperone for the lipid tail of outer membrane lipoproteins (Tokuda, 2009). For this process, a hydrophobic cavity of LolA is essential (Tokuda, 2009). Under oxidizing conditions, the hydrophobic IDH tumor cavity of LolA (I93C/F149C) is closed owing to disulphide bond formation between the two introduced

cysteine residues (Watanabe et al., 2008). Consequently, outer membrane sorting of lipoproteins is defective for LolA (I93C/F149C; Watanabe et al., 2008) and might be the trigger for the Cpx-TCS (Tao et al., 2010). Outer membrane lipoproteins are a well-known stimulus for the Cpx system (Snyder et al., 1995; Miyadai et al., 2004; Fadl et al., 2006). NlpE induces the Cpx-TCS, resulting in additional expression of the periplasmic protease DegP (Snyder et al., 1995) and the periplasmic folding factors FkpA and of DsbA (Danese & Silhavy, 1997). Notably, overproduction of NlpE, referred as a specific Cpx stimulus, has been used to identify the Cpx-dependent expression

of proposed Amobarbital regulon members (Vogt et al., 2010). Activation of the Cpx-TCS by NlpE depends on lipidation but is independent of anchoring either in the outer or the inner membrane (Miyadai et al., 2004). The structure of the soluble region of NlpE suggests that conformational changes in NlpE might result in direct interaction with CpxA (Hirano et al., 2007). However, although it is clear that NlpE activates the Cpx-TCS in an CpxP-independent manner (Buelow & Raivio, 2010), the mechanism of Cpx-TCS activation by NlpE with respect to the impact of NlpE in sensing surface attachment and copper is unknown. The Cpx-TCS has also been linked to the sensing of β-barrel outer membrane proteins (OMPs; Gerken et al., 2010). Assembly-defective OMP variants and a defective OMP assembly machinery (Bam-complex) induce the Cpx regulon (Gerken et al., 2010). However, CpxP appears not to be involved in the degradation of misfolded OMPs by DegP nor in the activation of the Cpx-TCS by misfolded OMPs (Gerken et al., 2010). The impact of the Cpx-TCS in sensing defects during the assembly of adhesive surface structures has been established for type IV bundle-forming pili (BFP) of enteropathogenic E. coli (EPEC; Nevesinjac & Raivio, 2005), the curli fimbriae of E.