Fig. 7: Model of OMP assembly by TAM.

Nascent OMPs are transported into the periplasm through the Sec complex and bind to periplasmic chaperones (e.g., SurA) which maintain them in an assembly competent conformation. Possibly due to partial folding and the exposure of a hydrophobic surface, a subset of OMPs might subsequently bind to TamB, which could then use the hydrophobic interior of its β-taco structure to escort them to TamA. Alternatively, chaperones might target specific OMPs directly to TamA and thereby trigger conformational changes in TamB. In either scenario TamB applies a force to the TamA POTRA domains that alters their position and thereby promotes the release of the C-terminus of TamB from TamA β1. Like BamA, TamA subsequently forms a hybrid barrel with OMPs through a strong interaction between β1 and the β signal of the substrate. After the substrate curves inward to form a barrel-like structure, its first and last β strands form a closed seam via a strand exchange reaction and the fully folded OMP is released into the lipid bilayer. Conformational changes in the TamA POTRA domains and TamB that occur during the OMP assembly cycle are denoted by green and blue arrows, respectively. It should be noted that available evidence suggests that TamB also mediates the anterograde transport of phospholipids, presumably through the interior of the β-taco. It is conceivable that the lipids are transported in association with OMPs. This figure was created with BioRender.com, released under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International license.