OMP insertion (BamComplex) porin
Identifiers
SymbolBamA
PfamPF01103
InterProIPR023707
TCDB1.B.33
OPM superfamily179
OPM protein5ayw
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary

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

  1. ↑ Walther DM, Rapaport D, Tommassen J (September 2009). "Biogenesis of beta-barrel membrane proteins in bacteria and eukaryotes: evolutionary conservation and divergence". Cellular and Molecular Life Sciences. 66 (17): 2789–804. doi:10.1007/s00018-009-0029-z. PMC 2724633. PMID 19399587.
  2. ↑ Habib SJ, Waizenegger T, Niewienda A, Paschen SA, Neupert W, Rapaport D (January 2007). "The N-terminal domain of Tob55 has a receptor-like function in the biogenesis of mitochondrial beta-barrel proteins". The Journal of Cell Biology. 176 (1): 77–88. doi:10.1083/jcb.200602050. PMC 2063629. PMID 17190789.
  3. ↑ Knowles TJ, Scott-Tucker A, Overduin M, Henderson IR (March 2009). "Membrane protein architects: the role of the BAM complex in outer membrane protein assembly". Nature Reviews. Microbiology. 7 (3): 206–14. doi:10.1038/nrmicro2069. PMID 19182809.
  4. ↑ Hagan CL, Silhavy TJ, Kahne D (2011). "β-Barrel membrane protein assembly by the Bam complex". Annual Review of Biochemistry. 80: 189–210. doi:10.1146/annurev-biochem-061408-144611. PMID 21370981.
  5. ↑ Rigel NW, Silhavy TJ (April 2012). "Making a beta-barrel: assembly of outer membrane proteins in Gram-negative bacteria". Current Opinion in Microbiology. 15 (2): 189–93. doi:10.1016/j.mib.2011.12.007. PMC 3320693. PMID 22221898.
  6. ↑ Jiang JH, Tong J, Tan KS, Gabriel K (2012). "From evolution to pathogenesis: the link between β-barrel assembly machineries in the outer membrane of mitochondria and gram-negative bacteria". International Journal of Molecular Sciences. 13 (7): 8038–50. doi:10.3390/ijms13078038. PMC 3430219. PMID 22942688.
  7. ↑ Tommassen J (September 2010). "Assembly of outer-membrane proteins in bacteria and mitochondria". Microbiology. Reading, England. 156 (Pt 9): 2587–2596. doi:10.1099/mic.0.042689-0. PMID 20616105.
  8. ↑ Chacinska A, Koehler CM, Milenkovic D, Lithgow T, Pfanner N (August 2009). "Importing mitochondrial proteins: machineries and mechanisms". Cell. 138 (4): 628–44. doi:10.1016/j.cell.2009.08.005. PMC 4099469. PMID 19703392.
  9. ↑ Webb CT, Heinz E, Lithgow T (December 2012). "Evolution of the β-barrel assembly machinery". Trends in Microbiology. 20 (12): 612–20. doi:10.1016/j.tim.2012.08.006. PMID 22959613.
  10. ↑ Goujon M, McWilliam H, Li W, Valentin F, Squizzato S, Paern J, Lopez R (July 2010). "A new bioinformatics analysis tools framework at EMBL-EBI". Nucleic Acids Research. 38 (Web Server issue): W695–9. doi:10.1093/nar/gkq313. PMC 2896090. PMID 20439314.
  11. ↑ Wu T, Malinverni J, Ruiz N, Kim S, Silhavy TJ, Kahne D (April 2005). "Identification of a multicomponent complex required for outer membrane biogenesis in Escherichia coli". Cell. 121 (2): 235–45. doi:10.1016/j.cell.2005.02.015. PMID 15851030.
  12. ↑ Noinaj N, Kuszak AJ, Gumbart JC, Lukacik P, Chang H, Easley NC, Lithgow T, Buchanan SK (September 2013). "Structural insight into the biogenesis of β-barrel membrane proteins". Nature. 501 (7467): 385–90. doi:10.1038/nature12521. PMC 3779476. PMID 23995689.
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