## 3 LPS transport to the Outer Membrane  
### The Lpt Machinery: Structure and Organization of the Components across IM and OM  
Following MsbA dependent translocation [40, 41] the mature LPS molecule assembled at the periplasmic face of the IM by WaaL ligase must traverse the aqueous periplasmic compartment before being inserted and correctly assembled at the OM.  
The transport of LPS across the periplasm has been studied mainly in _E. coli_ and _N. meningitidis_. In _E. coli_ the Lpt (lipopolysaccharide transport) complex is composed of seven essential proteins (LptABCDEFG) [53, 54, 55, 56] that are located in every cellular compartment: cytoplasm, IM, periplasm, and OM (Figure 2). The Lpt complex provides energy for LPS extraction from the IM and mediates its transport across the aqueous periplasm and its insertion and assembly at the OM [3, 57, 58].  
The Lpt machinery may be divided in three subassemblies: LptBFGC, LptA, and LptDE, which are located at the IM, in the periplasm, and at the OM, respectively. LptBFG [56, 59, 60] constitute an IM ABC transporter that is associated to an atypical subunit, the bitopic IM protein LptC [61], whose function in the ABC transporter has not yet been clarified [60]. LptB is the ATP binding domain of this transporter [62] and LptF and LptG represent the transmembrane subunits. At the OM the LptDE translocon composed by the \(\beta\)-barrel protein LptD and the lipoprotein LptE is responsible of the final stage of LPS assembly at the cell surface [55, 63]. The periplasmic protein LptA has the role of connecting the two sub-complexes, somehow coordinating their functions [59, 64].  
The Lpt machinery appears to work as a singe device as depletion of any components leads to similar phenotypes namely block of LPS transport and its accumulation to the periplasmic side of the IM where it is decorated by colanic acid by the WaaL ligase [56, 61]; such LPS modification appears to be diagnostic of defects in the LPS export pathway [57]. Indeed all the seven proteins physically interact and form a transenvelope complex spanning IM and OM [65].  
Figure 2: Overview of OMPs and LPS biogenesis pathways and extracytoplasmic stress response. (**a**) OMPs are synthesized in the cytoplasm and translocated across the IM by the Sec translocon. After translocation, the signal sequence (indicated in red) is cleaved. In the periplasm, chaperone proteins as SurA assist OMPs folding and deliver them to the BamABCDE complex for assembly at the OM. Blue arrows show the sequence of events occurring during OMPs biogenesis. (**b**) LptD is an OMP and follows the chaperone/Bam folding pathway. Correct LptD folding requires its association with the lipoprotein LptE and interdomain disulfide bridges isomerization. LptD-LptE interaction at the Bam complex is favored by the chaperone/protease BepA. Red dashed arrows show the sequence of events occurring during LptDE complex assembly. (**c**) LptDE complex is the LPS OM translocon. LPS is synthesized in the cytoplasm by the Lpx pathway, flipped to the periplasmic face of the IM by MsbA and transported through the periplasm to the outer leaflet of the OM by the Lpt machinery. Continuous red arrows show the sequence of events occurring during LPS biogenesis. (**d**) Mislocalized or misfolded OMPs and defects in the LPS export pathway trigger the \(\sigma^{E}\) envelope stress response (black arrows). Misassembled porins bind to DegS, cleaving RseA and initiating the \(\sigma^{E}\) stress response. The current model suggests that defects in Lpt assembly/function result in RseB binding to LPS possibly released by the Lpt machinery (question mark). RseB bound to LPS frees RseA that can then be cleaved by OMP-activated DegS and by RseP thus activating the \(\sigma^{E}\) stress response.