National Center for
3PRW: Crystal Structure Of The Lipoprotein Bamb
Augmenting beta-augmentation: structural basis of how BamB binds BamA and may support folding of outer membrane proteins
J. Mol. Biol. (2011) 406 p.659-666
beta-Barrel proteins are frequently found in the outer membrane of mitochondria, chloroplasts and Gram-negative bacteria. In Escherichia coli, these proteins are inserted in the outer membrane by the Bam (beta-barrel assembly machinery) complex, a multiprotein machinery formed by the beta-barrel protein BamA and the four peripheral membrane proteins BamB, BamC, BamD and BamE. The periplasmic part of BamA binds prefolded beta-barrel proteins by a beta-augmentation mechanism, thereby stabilizing the precursors prior to their membrane insertion. However, the role of the associated proteins within the Bam complex remains unknown. Here, we describe the crystal structure of BamB, a nonessential component of the Bam complex. The structure shows a typical eight-bladed beta-propeller fold. Two sequence stretches of BamB were previously identified to be important for interaction with BamA. In our structure, both motifs are located in close proximity to each other and contribute to a conserved region forming a narrow groove on the top of the propeller. Moreover, crystal contacts reveal two interaction modes of how BamB might bind unfolded beta-barrel proteins. In the crystal lattice, BamB binds to exposed beta-strands by beta-augmentation, whereas peptide stretches rich in aromatic residues can be accommodated in hydrophobic pockets located at the bottom of the propeller. Thus, BamB could simultaneously bind to BamA and prefolded beta-barrel proteins, thereby enhancing the folding and membrane insertion capability of the Bam complex.