Assembly of Outer Membrane β-Barrel Proteins: the Bam Complex

EcoSal ◽  
2011 ◽  
Author(s):  
Juliana C. Malinverni ◽  
Thomas J. Silhavy
Keyword(s):  
Outer Membrane ◽  
Bam Complex

scholarly journals The role of BamA in the biogenesis of beta-barrel membrane proteins

2014 ◽  
Vol 70 (a1) ◽  
pp. C578-C578
Author(s):  
Nicholas Noinaj ◽  
Adam Kuszak ◽  
Curtis Balusek ◽  
JC Gumbart ◽  
Petra Lukacik ◽  
...  
Keyword(s):  
Membrane Proteins ◽  
Outer Membrane ◽  
Crystal Structures ◽  
Outer Membrane Proteins ◽  
Hydrophobic Surface ◽  
Md Simulations ◽  
Structural Features ◽  
Bam Complex ◽  
Beta Barrel

Beta-barrel membrane proteins are essential for nutrient import, signaling, motility, and survival. In Gram-negative bacteria, the beta-barrel assembly machinery (BAM) complex is responsible for the biogenesis of beta-barrel outer membrane proteins (OMPs), with homologous complexes found in mitochondria and chloroplasts. Despite their essential roles, exactly how these OMPs are formed remains unknown. The BAM complex consists of a central and essential component called BamA (an OMP itself) and four lipoproteins called BamB-E. While the structure of the lipoproteins have been reported, the structure of full length BamA has been elusive. Recently though, we described the structure of BamA from two species of bacteria: Neisseria gonorrhoeae and Haemophilus ducreyi. BamA consists of a large periplasmic domain attached to a 16-strand transmembrane beta-barrel domain. Together, our crystal structures and molecule dynamics (MD) simulations revealed several structural features which gave clues to the mechanism by which BamA catalyzes beta-barrel assembly. The first is that the interior cavity is accessible in one BamA structure and conformationally closed in the other. Second, an exterior rim of the beta-barrel has a distinctly narrowed hydrophobic surface, locally destabilizing the outer membrane. Third, the beta-barrel can undergo lateral opening, suggesting a route from the interior cavity in BamA into the outer membrane. And fourth, a surface exposed exit pore positioned above the lateral opening site which may play a role in the biogenesis of extracellular loops. In this presentation, the crystal structures and MD simulations of BamA will be presented along with our work looking at the role of these four structural features in the role of BamA within the BAM complex.

scholarly journals A small-molecule inhibitor of BamA impervious to efflux and the outer membrane permeability barrier

2019 ◽  
Vol 116 (43) ◽  
pp. 21748-21757 ◽  
Author(s):  
Elizabeth M. Hart ◽  
Angela M. Mitchell ◽  
Anna Konovalova ◽  
Marcin Grabowicz ◽  
Jessica Sheng ◽  
...  
Keyword(s):  
Outer Membrane ◽  
Small Molecule ◽  
Cytoplasmic Membrane ◽  
Multidrug Resistant ◽  
Resistant Bacteria ◽  
Permeability Barrier ◽  
Gram Negative Bacteria ◽  
Gram Negative ◽  
Bam Complex ◽  
Induced Aggregation

The development of new antimicrobial drugs is a priority to combat the increasing spread of multidrug-resistant bacteria. This development is especially problematic in gram-negative bacteria due to the outer membrane (OM) permeability barrier and multidrug efflux pumps. Therefore, we screened for compounds that target essential, nonredundant, surface-exposed processes in gram-negative bacteria. We identified a compound, MRL-494, that inhibits assembly of OM proteins (OMPs) by the β-barrel assembly machine (BAM complex). The BAM complex contains one essential surface-exposed protein, BamA. We constructed a bamA mutagenesis library, screened for resistance to MRL-494, and identified the mutation bamAE470K. BamAE470K restores OMP biogenesis in the presence of MRL-494. The mutant protein has both altered conformation and activity, suggesting it could either inhibit MRL-494 binding or allow BamA to function in the presence of MRL-494. By cellular thermal shift assay (CETSA), we determined that MRL-494 stabilizes BamA and BamAE470K from thermally induced aggregation, indicating direct or proximal binding to both BamA and BamAE470K. Thus, it is the altered activity of BamAE470K responsible for resistance to MRL-494. Strikingly, MRL-494 possesses a second mechanism of action that kills gram-positive organisms. In microbes lacking an OM, MRL-494 lethally disrupts the cytoplasmic membrane. We suggest that the compound cannot disrupt the cytoplasmic membrane of gram-negative bacteria because it cannot penetrate the OM. Instead, MRL-494 inhibits OMP biogenesis from outside the OM by targeting BamA. The identification of a small molecule that inhibits OMP biogenesis at the cell surface represents a distinct class of antibacterial agents.

scholarly journals Functions of the BamBCDE Lipoproteins Revealed by Bypass Mutations in BamA

2020 ◽  
Vol 202 (21) ◽  
Author(s):  
Elizabeth M. Hart ◽  
Thomas J. Silhavy
Keyword(s):  
Membrane Proteins ◽  
Outer Membrane ◽  
Outer Membrane Proteins ◽  
Gram Negative Bacteria ◽  
Wild Type ◽  
Gram Negative ◽  
Bam Complex ◽  
Essential Function ◽  
The Individual

ABSTRACT The heteropentomeric β-barrel assembly machine (BAM complex) is responsible for folding and inserting a diverse array of β-barrel outer membrane proteins (OMPs) into the outer membrane (OM) of Gram-negative bacteria. The BAM complex contains two essential proteins, the β-barrel OMP BamA and a lipoprotein BamD, whereas the auxiliary lipoproteins BamBCE are individually nonessential. Here, we identify and characterize three bamA mutations, the E-to-K change at position 470 (bamAE470K), the A-to-P change at position 496 (bamAA496P), and the A-to-S change at position 499 (bamAA499S), that suppress the otherwise lethal ΔbamD, ΔbamB ΔbamC ΔbamE, and ΔbamC ΔbamD ΔbamE mutations. The viability of cells lacking different combinations of BAM complex lipoproteins provides the opportunity to examine the role of the individual proteins in OMP assembly. Results show that, in wild-type cells, BamBCE share a redundant function; at least one of these lipoproteins must be present to allow BamD to coordinate productively with BamA. Besides BamA regulation, BamD shares an additional essential function that is redundant with a second function of BamB. Remarkably, bamAE470K suppresses both, allowing the construction of a BAM complex composed solely of BamAE470K that is able to assemble OMPs in the absence of BamBCDE. This work demonstrates that the BAM complex lipoproteins do not participate in the catalytic folding of OMP substrates but rather function to increase the efficiency of the assembly process by coordinating and regulating the assembly of diverse OMP substrates. IMPORTANCE The folding and insertion of β-barrel outer membrane proteins (OMPs) are conserved processes in mitochondria, chloroplasts, and Gram-negative bacteria. In Gram-negative bacteria, OMPs are assembled into the outer membrane (OM) by the heteropentomeric β-barrel assembly machine (BAM complex). In this study, we probe the function of the individual BAM proteins and how they coordinate assembly of a diverse family of OMPs. Furthermore, we identify a gain-of-function bamA mutant capable of assembling OMPs independently of all four other BAM proteins. This work advances our understanding of OMP assembly and sheds light on how this process is distinct in Gram-negative bacteria.

Structural basis for the interaction of BamB with the POTRA3–4 domains of BamA

2016 ◽  
Vol 72 (2) ◽  
pp. 236-244 ◽  
Author(s):  
Zhen Chen ◽  
Li-Hong Zhan ◽  
Hai-Feng Hou ◽  
Zeng-Qiang Gao ◽  
Jian-Hua Xu ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Escherichia Coli ◽  
Outer Membrane ◽  
Essential Role ◽  
Biochemical Analysis ◽  
Hydrophobic Interactions ◽  
Outer Membrane Proteins ◽  
Structural Basis ◽  
Conserved Residues ◽  
Bam Complex

InEscherichia coli, the Omp85 protein BamA and four lipoproteins (BamBCDE) constitute the BAM complex, which is essential for the assembly and insertion of outer membrane proteins into the outer membrane. Here, the crystal structure of BamB in complex with the POTRA3–4 domains of BamA is reported at 2.1 Å resolution. Based on this structure, the POTRA3 domain is associated with BamBviahydrogen-bonding and hydrophobic interactions. Structural and biochemical analysis revealed that the conserved residues Arg77, Glu127, Glu150, Ser167, Leu192, Leu194 and Arg195 of BamB play an essential role in interaction with the POTRA3 domain.

scholarly journals Assembly of Outer Membrane β-Barrel Proteins: the Bam Complex

EcoSal Plus ◽  
2011 ◽  
Vol 4 (2) ◽  
Author(s):  
Juliana C. Malinverni ◽  
Thomas J. Silhavy
Keyword(s):  
Outer Membrane ◽  
Bam Complex

scholarly journals Conformational Changes That Coordinate the Activity of BamA and BamD Allowing β-Barrel Assembly

2017 ◽  
Vol 199 (20) ◽  
Author(s):  
Anne L. McCabe ◽  
Dante Ricci ◽  
Modupe Adetunji ◽  
Thomas J. Silhavy
Keyword(s):  
Outer Membrane ◽  
Conformational Changes ◽  
Signal Sequence ◽  
Outer Membrane Proteins ◽  
Gram Negative Bacteria ◽  
Complex Stability ◽  
Essential Proteins ◽  
Bam Complex ◽  
Sequence Recognition ◽  
Charged Residues

ABSTRACT Most integral outer membrane proteins (OMPs) of Gram-negative bacteria, such as Escherichia coli, assume a β-barrel structure. The β-barrel assembly machine (Bam), a five-member complex composed of β-barrel OMP BamA and four associated lipoproteins, BamB, BamC, BamD, and BamE, folds and inserts OMPs into the outer membrane. The two essential proteins BamA and BamD interact to stabilize two subcomplexes, BamAB and BamCDE, and genetic and structural evidence suggests that interactions between BamA and BamD occur via an electrostatic interaction between a conserved aspartate residue in a periplasmic domain of BamA and a conserved arginine in BamD. In this work, we characterize charge-change mutations at these key BamA and BamD residues and nearby charged residues in BamA with respect to OMP assembly and Bam complex stability. We show that Bam complex stability does not correlate with function, that BamA and BamD must adopt at least two active conformational states during OMP assembly, and that these charged residues are not required for function. Rather, these charged residues are important for coordinating the activities of BamA and BamD to allow efficient OMP assembly. We present a model of OMP assembly wherein recognition and binding of unfolded OMP substrate by BamA and BamD induce a signaling interaction between the two proteins, causing conformational changes necessary for the assembly reaction to proceed. By analogy to signal sequence recognition by SecYEG, we believe these BamA-BamD interactions ensure that both substrate and complex are competent for OMP assembly before the assembly reaction commences. IMPORTANCE Conformational changes in the proteins of the β-barrel assembly machine (Bam complex) are associated with the folding and assembly of outer membrane proteins (OMPs) in Gram-negative bacteria. We show that electrostatic interactions between the two essential proteins BamA and BamD coordinate conformational changes upon binding of unfolded substrate that allow the assembly reaction to proceed. Mutations affecting this interaction are lethal not because they destabilize the Bam complex but rather because they disrupt this coordination. Our model of BamA-BamD interactions regulating conformation in response to proper substrate interaction is reminiscent of conformational changes the secretory (Sec) machinery undergoes after signal sequence recognition that ensure protein quality control.

scholarly journals The CpxQ sRNA Negatively Regulates Skp To Prevent Mistargeting of β-Barrel Outer Membrane Proteins into the Cytoplasmic Membrane

mBio ◽  
2016 ◽  
Vol 7 (2) ◽  
Author(s):  
Marcin Grabowicz ◽  
Daria Koren ◽  
Thomas J. Silhavy
Keyword(s):  
Stress Response ◽  
Membrane Proteins ◽  
Outer Membrane ◽  
Protein Misfolding ◽  
Outer Membrane Proteins ◽  
Inner Membrane ◽  
Bam Complex ◽  
Envelope Stress ◽  
Periplasmic Chaperone

ABSTRACT The promoter most strongly induced upon activation of the Cpx two-component envelope stress response is the cpxP promoter. The 3′ untranscribed region (UTR) of the cpxP transcript is shown to produce a small RNA (sRNA), CpxQ. We investigated the role of CpxQ in combating envelope stress. Remarkably, the two effectors specified by the transcript are deployed to combat distinct stresses in different cellular compartments. CpxP acts in both a regulatory negative-feedback loop and as an effector that combats periplasmic protein misfolding. We find that CpxQ combats toxicity at the inner membrane (IM) by downregulating the synthesis of the periplasmic chaperone Skp. Our data indicate that this regulation prevents Skp from inserting β-barrel outer membrane proteins (OMPs) into the IM, a lethal event that likely collapses the proton motive force. Our findings suggest that Skp can fold and directly insert OMPs into a lipid bilayer in vivo without the aid of the Bam complex. IMPORTANCE Skp is a well-characterized periplasmic chaperone that binds unfolded OMPs. Surprisingly, we find that Skp can catalyze the folding and mistargeting of OMPs into the inner membrane without the aid of the other cellular proteins that normally assemble OMPs. Several OMPs function as diffusion pores. Accordingly, their mistargeting is lethal because it depolarizes the inner membrane. We show that the most highly expressed transcript of the Cpx stress response produces an sRNA from the 3′ UTR, CpxQ, which combats this potential toxicity by downregulating Skp production. Defects in OMP assembly trigger the σ E response to upregulate factors, including Skp, that promote OMP folding. The Cpx response downregulates σ E . Our findings reveal that this heretofore puzzling hierarchy exists to protect the inner membrane.

scholarly journals Lipoprotein DolP supports proper folding of BamA in the bacterial outer membrane promoting fitness upon envelope stress

eLife ◽  
2021 ◽  
Vol 10 ◽  
Author(s):  
David Ranava ◽  
Yiying Yang ◽  
Luis Orenday-Tapia ◽  
François Rousset ◽  
Catherine Turlan ◽  
...  
Keyword(s):  
Outer Membrane ◽  
Outer Membrane Proteins ◽  
Transcriptional Response ◽  
Permeability Barrier ◽  
Cell Recruitment ◽  
Bam Complex ◽  
Outer Membrane Lipoprotein ◽  
Envelope Stress ◽  
Late Step ◽  
Membrane Lipoprotein

In Proteobacteria, integral outer membrane proteins (OMPs) are crucial for the maintenance of the envelope permeability barrier to some antibiotics and detergents. In Enterobacteria, envelope stress caused by unfolded OMPs activates the sigmaE (σE) transcriptional response. σE upregulates OMP biogenesis factors, including the β-barrel assembly machinery (BAM) that catalyses OMP folding. Here we report that DolP (formerly YraP), a σE-upregulated and poorly understood outer membrane lipoprotein, is crucial for fitness in cells that undergo envelope stress. We demonstrate that DolP interacts with the BAM complex by associating with outer membrane-assembled BamA. We provide evidence that DolP is important for proper folding of BamA that overaccumulates in the outer membrane, thus supporting OMP biogenesis and envelope integrity. Notably, mid-cell recruitment of DolP had been linked to regulation of septal peptidoglycan remodelling by an unknown mechanism. We now reveal that, during envelope stress, DolP loses its association with the mid-cell, thereby suggesting a mechanistic link between envelope stress caused by impaired OMP biogenesis and the regulation of a late step of cell division.

scholarly journals Formation of a β-barrel membrane protein is catalyzed by the interior surface of the assembly machine protein BamA

eLife ◽  
2019 ◽  
Vol 8 ◽  
Author(s):  
James Lee ◽  
David Tomasek ◽  
Thiago MA Santos ◽  
Mary D May ◽  
Ina Meuskens ◽  
...  
Keyword(s):  
Membrane Protein ◽  
Outer Membrane ◽  
Molecular Mechanisms ◽  
Gram Negative Bacteria ◽  
Site Specific ◽  
Gram Negative ◽  
Interior Surface ◽  
Essential Component ◽  
Bam Complex ◽  
Β Sheet

The β-barrel assembly machine (Bam) complex in Gram-negative bacteria and its counterparts in mitochondria and chloroplasts fold and insert outer membrane β-barrel proteins. BamA, an essential component of the complex, is itself a β-barrel and is proposed to play a central role in assembling other barrel substrates. Here, we map the path of substrate insertion by the Bam complex using site-specific crosslinking to understand the molecular mechanisms that control β-barrel folding and release. We find that the C-terminal strand of the substrate is stably held by BamA and that the N-terminal strands of the substrate are assembled inside the BamA β-barrel. Importantly, we identify contacts between the assembling β-sheet and the BamA interior surface that determine the rate of substrate folding. Our results support a model in which the interior wall of BamA acts as a chaperone to catalyze β-barrel assembly.

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