Gram-negative bacteria, mitochondria, and chloroplasts contain an inner and outer membrane. The outer membrane contains a host of beta-barrel proteins commonly called outer membrane proteins (OMPs), which serve essential functions in cargo transport and signaling and are also vital for membrane biogenesis. In Gram-negative bacteria, it is known that OMPs are synthesized in the cytoplasm and then transported across the inner membrane into the periplasm via a Sec translocon. Once in the periplasm, chaperones guide the nascent OMPs across the periplasm and peptidoglycan to the inner surface of the outer membrane. Here, the nascent OMPs are recognized by a complex known as the beta-barrel assembly machinery (BAM) complex which folds and inserts the new OMPs into the outer membrane. Exactly how the BAM complex is able to accomplish its function remains unknown. However, we do know that the BAM complex consists of five components named BamA (an OMP itself) and BamB, BamC, BamD, and BamE, which are all accessory lipoproteins. Studies have shown that BamA and BamD are absolutely essential for cell viability and OMP biogenesis. Similar mechanisms for OMP biogenesis exist for both mitochondria and chloroplasts, further evidence of the evolution of these organelles. Recently, we solved the structure of BamB, while other groups solved BamC, BamD, BamE and a large portion of the periplasmic domain of BamA. Together these structures provided insight into how the BAM complex may recognize nascent OMPs. However, even with these structures being known, the mechanism for how the BAM complex recognizes, folds, and inserts nascent OMPs into the outer membrane remains elusive. To understand the mechanism of the BAM complex, we have determined crystal structures of the core membrane component called BamA, a β-barrel membrane protein itself, from two different species (Neisseria gonorrhoeae and Haemophilus ducreyi). The structure of BamA contains a large N-terminal periplasmic domain and a C-terminal 16-stranded β-barrel domain. The periplasmic domain was found in two different conformations representing open and closed states, which may serve as a gating mechanism to allow substrate access to the internal barrel cavity. Interestingly, the closed state was accompanied by a significant destabilization of the terminal strand, which was found tucked inside the barrel domain. MD simulations revealed that BamA could destabilize the local membrane along the terminal strand, thinning the membrane by as much as 16 . In addition, these MD simulations also revealed that the barrel domain of BamA may undergo a lateral opening to create a portal from the periplasm directly into the outer membrane. This work is in press at Nature. Future experiments will investigate the roles of the 4 BAM lipoproteins and how they assemble and function together.

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Bakelar, Jeremy; Buchanan, Susan K; Noinaj, Nicholas (2016) The structure of the β-barrel assembly machinery complex. Science 351:180-6
Kuszak, Adam J; Jacobs, Daniel; Gurnev, Philip A et al. (2015) Evidence of Distinct Channel Conformations and Substrate Binding Affinities for the Mitochondrial Outer Membrane Protein Translocase Pore Tom40. J Biol Chem 290:26204-17
Noinaj, Nicholas; Kuszak, Adam J; Buchanan, Susan K (2015) Heat Modifiability of Outer Membrane Proteins from Gram-Negative Bacteria. Methods Mol Biol 1329:51-6
Noinaj, Nicholas; Buchanan, Susan K (2015) Summary and Future Directions. Methods Mol Biol 1329:279-80
Noinaj, Nicholas; Rollauer, Sarah E; Buchanan, Susan K (2015) The β-barrel membrane protein insertase machinery from Gram-negative bacteria. Curr Opin Struct Biol 31:35-42
Kuszak, Adam J; Noinaj, Nicholas; Buchanan, Susan K (2015) Methods to Characterize Folding and Function of BamA Cross-Link Mutants. Methods Mol Biol 1329:137-47
O'Neil, Patrick K; Rollauer, Sarah E; Noinaj, Nicholas et al. (2015) Fitting the Pieces of the β-Barrel Assembly Machinery Complex. Biochemistry 54:6303-11
Noinaj, Nicholas; Kuszak, Adam J; Balusek, Curtis et al. (2014) Lateral opening and exit pore formation are required for BamA function. Structure 22:1055-62
Noinaj, Nicholas; Kuszak, Adam J; Gumbart, James C et al. (2013) Structural insight into the biogenesis of β-barrel membrane proteins. Nature 501:385-90
Madden, Jeremy T; Toth, Scott J; Dettmar, Christopher M et al. (2013) Integrated nonlinear optical imaging microscope for on-axis crystal detection and centering at a synchrotron beamline. J Synchrotron Radiat 20:531-40

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