Bam A
| OMP insertion (BamComplex) porin | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| Symbol | BamA | ||||||||
| Pfam | PF01103 | ||||||||
| InterPro | IPR023707 | ||||||||
| TCDB | 1.B.33 | ||||||||
| OPM superfamily | 179 | ||||||||
| OPM protein | 5ayw | ||||||||
| |||||||||
BamA is a β-barrel, outer membrane protein found in Gram-negative bacteria and it is the main and vital component of the β-barrel assembly machinery (BAM) complex in those bacteria.[1] BAM Complex consists of five components; BamB, BamC, BamD, BamE (all are lipoproteins) and BamA (Outer membrane protein).[2][3][4][5] This complex is responsible in catalyzing folding and insertion of β-barrel proteins into the outer membrane of Gram-negative bacteria.[6][7]
β-barrel membrane proteins can only be found in the outer membrane of Gram-negative bacteria and in organelles such as mitochondria and chloroplasts which were evolved from bacteria.[8][9] In Gram-negative bacteria, outer membrane proteins are synthesized in the cytoplasm and then exported into the periplasm by Sec translocon machinery.[10] Then they are escorted to the inner surface of the outer membrane by molecular chaperons. Finally those nascent proteins interact with BAM Complex and insert into the outer membrane as β-barrel proteins.[11]
Structure and function
According to the fully resolved BamA structure of N. gonorrhoeae, BamA has a large periplasmic domain connected to a transmembrane β-barrel domain which is made of 16 antiparallel β strands.[12] There are five polypeptide translocation-associated (POTRA) domains extending from the barrel at the periplasmic domain of BamA. Current studies suggest that the four lipoproteins in the BAM Complex (BamB, BamC, BamD, BamE ) assemble on to the POTRA domains of BamA, making it the vital component of BAM Complex. The first and the last or 16th β-strands associate in closing the barrel. Extracellular loops (eL) eL4, eL6 and eL7 of the barrel forms a dome over the barrel by isolating the interior of the barrel from the extracellular space and interior of the BamA barrel is completely empty.
The external rim of the β-barrel has a narrow, reduced hydrophobic surface and it reduces lipid order and thickness of the membrane around the barrel. Transient separation of 1st and 16th β-strands which are associated in closing the barrel causes lateral opening of the barrel making a route from interior cavity of the BamA into the outer membrane. POTRA 5 domain of BamA sits close to the β–barrel and interacts with periplasmic loops (pL) pL3, pL4, pL5, pL7 and stabilize the closed conformation of the barrel. Swing movements of POTRA 5 domain and having no interactions with pLs make the opening of the barrel. Thus POTRA domains act as a gate to regulate the access into the interior of β–barrel. Hence there are three structural features associate with BamA that regulates the entry of β-barrel proteins into the outer membrane. First, open and closed conformation of BamA β-barrel. Second, the narrow and reduced hydrophobic rim on the surface of the β –barrel causes local destabilization of the outer membrane. Third, ability to undergo lateral opening of the barrel by transient separation of 1st and 16th β–barrel strands.
References
- ^ Walther, D. M., Rapaport, D. & Tommassen, J. Biogenesis of β-barrel membrane proteins in bacteria and eukaryotes: evolutionary conservation and divergence. Cell. Mol. Life Sci. 66, 2789–2804 (2009)
- ^ Habib, S. J. et al. The N-terminal domain of Tob55 has a receptor-like function in the biogenesis of mitochondrial beta-barrel proteins. J Cell Biol 176, 77-88 (2007)
- ^ Knowles, T. J., Scott-Tucker, A., Overduin, M. & Henderson, I. R.Membrane protein architects: the role of the BAM Complex in outer membrane protein assembly. Nature Rev. Microbiol. 7, 206–214 (2009)
- ^ Hagan, C. L., Silhavy, T. J. & Kahne, D. β-Barrel membrane protein assembly by the BAM Complex. Annu. Rev. Biochem. 80, 189–210 (2011)
- ^ Rigel, N. W. & Silhavy, T. J. Making a beta-barrel: assembly of outer membrane proteins in Gram-negative bacteria. Curr. Opin. Microbiol. 15, 189–193 (2012)
- ^ Jiang, J. H., Tong, J., Tan, K. S. & Gabriel, K. From evolution to pathogenesis: the link between β-barrel assembly machineries in the outer membrane of mitochondria and Gram-negative bacteria. Int. J. Mol. Sci. 13, 8038–8050 (2012)
- ^ Tommassen, J. Assembly of outer-membrane proteins in bacteria and mitochondria. Microbiology 156, 2587–2596 (2010)
- ^ Chacinska, A., Koehler, C. M., Milenkovic, D., Lithgow, T. & Pfanner, N. Importing mitochondrial proteins: machineries and mechanisms. Cell 138, 628–644 (2009)
- ^ Webb, C. T., Heinz, E. & Lithgow, T. Evolution of the β-barrel assembly machinery. Trends Microbiol. 20, 612–620 (2012)
- ^ Goujon, M. et al. A new bioinformatics analysis tools framework at EMBL-EBI. Nucleic Acids Res 38 Suppl, W695-9 (2010)
- ^ Wu, T. et al. Identification of a multicomponent complex required for outer membrane biogenesis in Escherichia coli. Cell 121, 235–245 (2005)
- ^ N. Noinaj, A. J. Kuszak, J. C. Gumbart, P. Lukacik, H. Chang, N. C. Easley, T. Lithgow, and S. K. Buchanan, “Structural insight into the biogenesis of β-barrel membrane proteins,” Nature, vol. 501, no. 7467, pp. 385–390(2013)