## 3 LPS transport to the Outer Membrane
### Molecular Mechanism of Lpt Transport: Towards the Transenvelope Bridge Model  
LPS is an amphipatic molecule, this implies that its transport across the periplasm cannot occurs by simple diffusion and requires at least two energy inputs: one for its extraction from the lipid environment of the IM, and one to facilitate the transit of the hydrophobic lipid A portion through the aqueous environment of the periplasm. Since the discovery of the first Lpt factors, this notion induced researchers to postulate different working models to describe the mechanism of LPS transport. Based on the fact that every envelope compartment has at least one factor dedicated to LPS transport [56, 61], two main models have been considered: the chaperone-mediated transit across the periplasm and the transport through a transenvelope proteinaceous bridge spanning both IM and OM. According to the first model, LptA would be the soluble carrier that accepts LPS from LptBFGC, forms a soluble complex shielding its hydrophobic moiety during the diffusion across the periplasm and, ultimately, delivers it to the OM complex LptDE. As such, LptA would function in the same fashion of LoIA, the periplasmic chaperone involved in OM lipoprotein trafficking (see above) [20]. By analogy with LoIA, the three-dimensional structure of LptA reveals a novel fold resembling a semiclosed \(\beta\)-jellyroll with an internal hydrophobic cavity [66], compatible with a role in hosting and shielding hydrophobic molecule portions. In fact, LptA binds LPS _in vitro_ and _in vivo_[67, 68] and can displace it from LptC, in line with their subcellular locations and the unidirectionality of the transport (from IM to OM) [67, 68, 69]. However, alongside the analogies there are also some substantial differences between LoI and Lpt system: first of all, a recent work from [PERSON]'s group demonstrated that, unlike LoIA, physiologically expressed LptA is not a soluble periplasmic protein but fractionates with both IM and OM in sucrose density gradient centrifugation [65]. Moreover, _in vitro_, LptA shows the propensity to form oligomeric fibrils [66, 70]. LptA association to both IM and OM thus suggests that _in vivo_ the protein does not function as a soluble carrier but form oligomeric structures spanning the width of the periplasm. This is in agreement with previous evidence that LPS transport occurs in spheroplast, where the periplasmic soluble content has been drained, and that soluble LPS-protein complexes have never been isolated from periplasm [71]. Another remarkable difference between LoI and Lpt system lies in the atypical LptC subunit of the ABC transporter LptBFG. LptC is a bitopic LPS binding protein whose three-dimensional structure closely resembles the LptA fold, despite the low sequence similarity between the two proteins [69]. LptC stably associates to the LptBFG complex, but _in vitro_ does not affect the ATPase activity of the transporter [60], making obscure its role in LPS export. Overall, the balance between differences and analogies with the LoI mechanism of transport makes the chaperone-assisted LPS export model unlikely. The second model for LPS transport suggests the existence of a molecular machine made up of individual protein components located in each cellular compartment thus connecting IM and OM. These molecular machines are not unusual structures in Gram-negative bacteria. Examples are the efflux pumps, responsible for antibiotic resistance in pathogenic bacteria [72], or the Type III secretion systems, whose function is to inject toxins into the infected host cells [73]. The most important evidence supporting this model is that depletion of any Lpt factor leads to accumulation of _de novo_ synthesized LPS at the periplasmic side of the IM [61]. This suggests that the Lpt proteins constitute a molecular machine that operates as a single device downstream of MsbA-mediated LPS flipping across the IM (see above).