title: Crystal structure of BamB bound to a periplasmic domain fragment of BamA, the central component of the β-barrel assembly machine.
abstract: The β-barrel assembly machinery (BAM) mediates folding and insertion of β-barrel outer membrane proteins (OMPs) into the outer membrane of Gram-negative bacteria. BAM is a five-protein complex consisting of the β-barrel OMP BamA and lipoproteins BamB, -C, -D, and -E. High resolution structures of all the individual BAM subunits and a BamD-BamC complex have been determined. However, the overall complex architecture remains elusive. BamA is the central component of BAM and consists of a membrane-embedded β-barrel and a periplasmic domain with five polypeptide translocation-associated (POTRA) motifs thought to interact with the accessory lipoproteins. Here we report the crystal structure of a fusion between BamB and a POTRA3-5 fragment of BamA. Extended loops 13 and 17 protruding from one end of the BamB β-propeller contact the face of the POTRA3 β-sheet in BamA. The interface is stabilized by several hydrophobic contacts, a network of hydrogen bonds, and a cation-π interaction between BamA Tyr-255 and BamB Arg-195. Disruption of BamA-BamB binding by BamA Y255A and probing of the interface by disulfide bond cross-linking validate the physiological relevance of the observed interface. Furthermore, the structure is consistent with previously published mutagenesis studies. The periplasmic five-POTRA domain of BamA is flexible in solution due to hinge motions in the POTRA2-3 linker. Modeling BamB in complex with full-length BamA shows BamB binding at the POTRA2-3 hinge, suggesting a role in modulation of BamA flexibility and the conformational changes associated with OMP folding and insertion.
authors.affiliation: From the Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309.
authors.forename: Katarina Bartoš
authors.initials: KB
authors.lastname: Jansen
authors.affiliation: From the Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309.
authors.forename: Susan Lynn
authors.initials: SL
authors.lastname: Baker
authors.affiliation: From the Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 Marcelo.Sousa@colorado.edu.
authors.forename: Marcelo Carlos
authors.initials: MC
authors.lastname: Sousa
mesh_terms: D020816:Amino Acid Motifs; D001120:Arginine / Q000737:chemistry; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry*; D002412:Cations; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000737:chemistry; D029968:Escherichia coli Proteins / Q000737:chemistry*; D006860:Hydrogen Bonding; D008074:Lipoproteins / Q000737:chemistry; D016296:Mutagenesis; D010455:Peptides / Q000737:chemistry; D019897:Periplasm / Q000737:chemistry; D011485:Protein Binding; D017510:Protein Folding; D025941:Protein Interaction Mapping; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
chemical_list: D001425:Bacterial Outer Membrane Proteins; C503914:BamA protein, E coli; C557407:BamB protein, E coli; D002412:Cations; D029968:Escherichia coli Proteins; D008074:Lipoproteins; D010455:Peptides; D001120:Arginine
keywords: BAM; Crystal Structure; Lipoprotein Structure; Membrane Biogenesis; Outer Membrane; Protein-Protein Interaction