## 3 LPS transport to the Outer Membrane
### Regulation of Lpt Complex Assembly and LPS Transport Process
Interaction with LptE to form the \"plug and barrel\" architecture triggers the conversion of this non-functional intermediate to mature LptD possessing native disulfide bonds between non-consecutive cysteines. Therefore interaction of LptE with LptD ultimately controls the activation of the LptDE traslocon. Interestingly, LptD folds its \(\beta\)-barrel structure before disulfide bonds rearrangement and this folding is rate-limiting for its maturation [96]. Such non-functional folding intermediate is crucial to avoid LPS mistargeting: LptDE translocon becomes active only if both components are correctly synthesized, targeted and assembled together in the OM. This kind of regulatory mechanism is crucial to coordinate the assembly of a translocon whose components must follow different transport pathways (chaperones/BAM for LptD and Lol for LptE) to reach the final destination at the OM. Very recently, an additional quality control system that assists LptDE translocon folding/oxidation has been identified: it relies on the chaperone/protease BpA. BpA has been proposed to promote efficient LptD-LptE interaction at the Bam complex, as overexpression of LptE suppresses the delay in LptD disulfide bonds rearrangement observed in \(\Delta\)_bepA_ mutant and as BpA itself physically and genetically interacts with BamA [97]. Interestingly, in \(\Delta\)_bepA_ mutant the LptD non-functional intermediate with non native disulfide bonds is stabilized and overexpression of protease-dead BpA mutants does not completely rescue LptDE translocon functionality, suggesting that the protease activity of BpA is required to degrade LptD when it fails to be correctly inserted into the OM [97]. There is a strong link between LptDE translocon folding and Lpt transenvelope complex assembly. It has been demonstrated that the _N_-terminal domain of LptD has the essential function of connecting the OM translocon to the transenvelope bridge assembly by interacting with the _C_-terminal region of LptA [64]. However, _in vivo_ this interaction can occurs only when at least one native disulfide bond is formed in LptD [64] that ultimately depends on the proper interaction between LptD and LptE [96]. This suggests the existence of a regulatory mechanism where the \(C\)-terminal \(\beta\)-barrel domain of LptD prevents the \(N\)-terminal periplasmic domain from interacting with LptA when its plug domain (LptE) is not properly inserted. Accordingly, it has been shown that mutations impairing LptE plug function result in defective LPS transport and, as a consequence, in OM barrier perturbations [98]. The existence of such regulatory mechanism is in line with the observation that upon LptC, LptD, or LptE depletion LptA is degraded and LPS accumulates at the IM [61, 74]. Moreover it is known that mutations in LptC impairing interaction with LptBFG fail to recruit LptA and therefore to assemble the Lpt transenvelope complex [82]. Overall these data suggest that the cell cannot tolerate the formation of defective transenvelope bridges or the leakage of LPS molecules within the periplasm. The importance of mechanisms that monitor the proper delivery of LPS to the cell surface is also outlined by a recently identified dedicated surveillance system that detects directly LPS mistargeting and triggers the envelope stress response [99]. In _E. coli_ OM integrity is monitored by an envelope stress response system that relies on activation of the essential alternative \(\sigma\) factor (\(\sigma^{\mathrm{E}}\)) which controls the expression of genes required for damage-repair pathways [100]. Notably, LptA [101] and LptD [102] belong to the \(\sigma^{\mathrm{E}}\) regulon. Under non-stress conditions, \(\sigma^{\mathrm{E}}\) is kept inactive at the cytoplasmic site of the IM by the negative regulator RseA through binding to its cytoplasmic domain. RseA is a single-pass IM protein whose periplasmic domain binds the regulatory protein RseB and this binding enhances \(\sigma^{\mathrm{E}}\) inhibition [103, 104].