Once Go6983 in the Fedratinib chemical structure periplasm, the unfolded OMP is bound by chaperones that help direct the OMP to the OM for proper folding and membrane insertion [6–8]. Until recently, these latter steps of periplasmic OMP trafficking and OM assembly have remained largely uncharacterized. In 2003, however, Tommassen and coworkers identified an essential β-barrel OMP whose function is dedicated to the proper OM-assembly of most known OMPs [9]. This protein, now known as BamA [10, 11], is evolutionarily well-conserved since putative orthologs can be found in all known diderm bacteria, as well as in dual-membraned eukaryotic organelles, such as mitochondria and

chloroplasts [7, 12–15]. The functional importance of BamA was illustrated when researchers discovered that BamA was essential for the viability of both N. meningitidis and E. coli, and that its depletion resulted in dramatically decreased levels of properly-inserted OMPs in the OM of both organisms [9, 16, 17]. In E. coli, combined genetic and biochemical studies have now revealed that BamA exists in a multiprotein OM complex, termed the beta-barrel Sirolimus in vitro assembly machine (BAM) [10, 11]. This complex is

composed of the OM-imbedded BamA protein and four OM-anchored accessory lipoproteins, termed BamB, BamC, BamD, and BamE (previously known as YfgL, NlpB, YfiO, and SmpA respectively) [10, 18–20]. More recent studies have revealed that all of the BAM components are important at some level for OMP assembly and/or for the stability of the BAM complex. The BamB lipoprotein interacts directly with BamA within the complex, and this association is independent of the other BAM lipoproteins [19, 21]. BamB is thought to be an important scaffolding protein for

the BAM complex, and although BamB deletion mutants are viable, they have reduced levels of various OMPs [20, 22–26]. bamC- and bamE-null strains have relatively mild OMP assembly defects; however, they both show moderate OM permeability defects, and biochemical second studies show that their presence in the complex is important for the BamA-BamD interaction [18, 19, 21, 25]. The BamD protein, however, is essential for cell viability, and depletion of BamD causes a phenotype similar to that observed in BamA mutants [21, 25]. Additionally, BamD is the most evolutionarily conserved lipoprotein in the BAM complex. Like BamA, BamD orthologs are predicted to be present in all diderm bacteria [6, 15, 21], and they are proposed to contain conserved tetratricopeptide repeat (TPR) domains which have been shown to function in protein-protein interactions [27–29]. BAM complexes have now been characterized from other Gram-negative bacteria, such as N. meningitidis and Caulobacter crescentus [30, 31]. In N.