Crystal Structure of BamB Bound to a Periplasmic Domain Fragment of BamA, the Central Component of the {beta}-Barrel Assembly Machine [Membrane Biology]

December 2nd, 2014 by Jansen, K. B., Baker, S. L., Sousa, M. C.

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 Y255 and BamB R195. Disruption of BamA:BamB binding by BamA Y255A and probing of the interface by disulfide bond crosslinking 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.
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