## 3 LPS transport to the Outer Membrane
### Regulation of Lpt Complex Assembly and LPS Transport Process  
As discussed above, LPS is a crucial structural component of the bacterial OM that it is responsible of the permeability barrier proprieties of this membrane [12]. Moreover, this complex glycolipid is also believed to function as a molecular chaperone for OMPs assembly: for example, LPS has an established role in trimerization of OmpC and OmpF porins [84; 85; 86], and it is required for maintaining their proper channel gating function [87]. Due to the important roles played by LPS at the OM and the dangerous consequences of impaired transport of this molecule to the cell surface (for instance, accumulation of LPS in the IM could influence the proper assembly of essential IM proteins that can be lethal for cells), it is not surprising that the bacteria have evolved a fine tuned quality control system to avoid LPS mistargeting during OM biogenesis. In _E. coli_ this system comprises a panoply of chaperones assisting the assembly of the LPS OM translocon (LptDE) and molecular mechanisms to ensure that only functional OM translocons are coupled to active Lpt transenvelope bridges allowing LPS translocation from IM to OM. LptD is an OM protein with a large periplasmic \(N\)-terminal domain, which is essential for its function _in vivo_[63], and contains four cysteine residues, two in the \(N\)-terminal and two in the \(C\)-terminal domain of the protein [54,88], implicated in the formation of two non consecutive interdomain disulfide bonds. Being an OMP, LptD is synthesized in the cytoplasm with an \(N\)-terminal cleavable signal peptide that allows its transport across the SEC translocon [89]. After secretion across the IM, the signal peptide-processed LptD is transported along the aqueous and oxidizing environment of the periplasm with the aid of chaperones to be finally delivered to the BAM machinery that inserts it into the OM (see above), [22]. The main chaperone involved in LptD folding is SurA [90]. \(\Delta surA\) mutant displays lower LptD level and severe OM perturbations, typical phenotypes of mutants impaired in LPS transport [21]. It has been recently demonstrated that the chaperone Skp, belonging to a secondary periplasmic folding pathway together with the chaperone/protease DegP [21,91], acts in concert with SurA to efficiently assemble LptD. The role performed by Skp is masked when another chaperone protein, FkpA, is present in the cell. Interestingly, Skp has a role different from that played by SurA, as overexpression of SurA cannot restore LptD biogenesis in \(\Delta\)_skp_\(\Delta\)_fkpA_ double mutants [92]. The unique role of Skp/FkpA in LptD biogenesis seems to be correlated with the presence of the large periplasmic _N_-terminal domain, which is the first to emerge into the periplasm during LptD translocation. This would fit well with the observation that Skp association to OMPs occurs at the IM [93]. Based on these results, it is not surprising that FhuA, a protein with a similar structure of LptD, depends on the same group of chaperones for its folding [92]. However, the complex folding process of LptD must take into account two additional proprieties of the mature LPS OM translocon: the interaction with LptE and the presence of disulfide bonds. First of all, in the mature translocon the lipoprotein LptE resides inside the lumen of the _C_-terminal domain of LptD and this interaction is required for the correct folding of LptD \(\beta\)-barrel into the OM by the Bam complex [81,94]. LptDE complex folding is further challenged by the fact that LptD contains two disulfide bonds whose correct formation is required to build a functional translocon [88]. Periplasmic oxidase DsbA is involved in the formation of disulfide bonds in LptD [95], however the first oxidized folding intermediate of LptD is not active as it presents a disulfide bond between the first two _N_-terminal consecutive cysteines [96].