1.B.33.1.3
Outer membrane biogenesis complex (Wu et al., 2005). YaeT (BamA) may serve as an outer membrane ""receptor"" for the CdiA/CdiB 2-partner secretion system that mediates direct cell-cell contact-dependent growth inhibition (Aoki et al., 2008). High-resolution structures of crystal forms of BamA POTRA4-5 from E. coli has been reported (Zhang et al., 2011; Sinnige et al. 2014). Solid-state NMR on BamA, a large multidomain integral membrane protein, revealed dynamic conformational states (Renault et al., 2011). In contrast to the N-terminal periplasmic polypeptide-transport-associated (POTRA) domains, the C-terminal transmembrane β-barrel domain of BamA is mechanically much more stable. Exposed to mechanical stress, this β-barrel stepwise unfolds β-hairpins until unfolding has been completed. The mechanical stabilities of β-barrel and β-hairpins are thereby modulated by the POTRA domains, the membrane composition and the extracellular lid closing the β-barrel. The NMR structure of SmpA (OmlA) is also known (Vanini et al. 2006).  The periplasmic region of BamA is firmly attached to the β-barrel and does not experience fast global motion around the angle between POTRA 2 and 3, but the barrel is flexible (Sinnige et al. 2014).  It appears that the BAM complex does not catalyze insertion and assembly of all out membrane (α- and β-)porins (Dunstan et al. 2015).  YfgL shows significant sequence similarity (e-9) with YxaL/K of Bacillus subtilis. The E. coli periplasmic chaperones, Skp and SurA, and BamA, the central subunit of the BAM complex, have been examined with respect to the folding kinetics of a model OMP (tOmpA) (Schiffrin et al. 2017), showing that prefolded BamA promotes the release of tOmpA from Skp, despite the nM affinity of the Skp for tOmpA. This activity is located in the BamA β-barrel domain, but is greater when full-length BamA is present, indicating that both the beta-barrel and POTRA domains are required for maximal activity. By contrast, SurA is unable to release tOmpA from Skp, providing direct evidence against a sequential chaperone model. BamA has a greater catalytic effect on tOmpA folding in thicker bilayers, suggesting that BAM catalysis involves lowering the kinetic barrier imposed by the hydrophobic thickness of the membrane (Schiffrin et al. 2017). While BamA is the primary translocator, TamB is involved in folding and maturation of autotransporters (Babu et al. 2018). The TAM complex is a "Translocation and Assembly Module" for protein assembly and potential conduit for phospholipid transfer (Goh et al. 2024).