Structural insight into the biogenesis of b-barrel membrane proteins

Nicholas Noinaj, Adam J. Kuszak, James C. Gumbart, Petra Lukacik, Hoshing Chang, Nicole C. Easley, Trevor Lithgow & Susan K. Buchanan

Nature. 501 (2013) 385-390

Speaker: Yong-Siang Cao (曹詠翔)                     Time: 14:00~15:00, Feb.19,2014

Commentator: Dr. Shang-Rung Wu (吳尚蓉 老師)            Place: Room 601

Abstract

Membrane proteins perform diverse set functions such as electron carriers, ion channels, transporters. The mechanism for membrane integration for a-helical membrane proteins had been well studied, but little is known for b-barrel membrane proteins (1). b-barrel membrane proteins exist in the outer membranes of Gram-negative bacteria and of eukaryotic organelles that evolved from bacteria (2). In Gram-negative bacteria, nascent outer membrane proteins (OMPs) are synthesized in the cytoplasm, nascent OMPs are folded and inserted into the outer membrane by BAM complex which are composed of BamA, BamB, BamC, BamD, BamE and five polypeptide translocation-associated (POTRA) domains (3). Structures have been determined for these protein exceptBamA. Therefore, it is not clear that how BAM complex work. In this study, the authors solved crystal structures of BamA from two bacterial species, Neisseria gonorrhoeae and Haemophilusducreyi, to reveal the molecular mechanism of BAM complex. There are large periplasmic domain attached to a 16-strand transmembrane b-barrel domain in BamA. BamA can catalyse the entry ofb-barrels into the outer membrane by conformational switch of loop 6, the POTRA gating motion, and the lateral barrel opening.The nascent OMPs was transferd through the b-barrel of BamA directly into the outer membrane. This work advances in our understanding of how bacterial nascent OMPs inserts into outer membrane. Future work is necessary to determine whether there are similar mechanisms in eukaryotic systems.

References

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2.     Webb, C. T., Heinz, E. & Lithgow, T. Evolution of the b-barrel assembly machinery.Trends Microbiol. 20, 612–620 (2012).

3.     Tommassen, J. Assembly of outer-membrane proteins in bacteria and mitochondria. Microbiology 156, 2587–2596 (2010).