Crystal structure of BamB bound to a periplasmic domain fragment of BamA, the central component of the ?-barrel assembly machine.

ABSTRACT: The ?-barrel assembly machinery (BAM) mediates folding and insertion of ?-barrel outer membrane proteins (OMPs) into the outer membrane of Gram-negative bacteria. BAM is a five-protein complex consisting of the ?-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 ?-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 ?-propeller contact the face of the POTRA3 ?-sheet in BamA. The interface is stabilized by several hydrophobic contacts, a network of hydrogen bonds, and a cation-? 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.

SUBMITTER: Jansen KB 

PROVIDER: S-EPMC4303665 | biostudies-literature | 2015 Jan

REPOSITORIES: biostudies-literature