Bam A

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]

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

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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