Structure of Escherichia coli BamD and its functional implications in outer membrane protein assembly.

The outer membrane protein complex (BAM complex) plays an important role in outer membrane protein (OMP) assembly in Escherichia coli. The BAM complex includes the integral β-barrel protein BamA as well as four lipoproteins: BamB, BamC, BamD and BamE. One of these lipoproteins, BamD, is essential for the survival of Escherichia coli. The structure of BamD at 2.6 Å resolution shows that this lipoprotein is composed of ten α-helices that form five tetratricopeptide-repeat (TPR) motifs. The arrangement of the BamD motifs is similar to that in the periplasmic part of BamA. One of the ten α-helices, α10, which has been shown to be important for the assembly of the BAM complex, is located in the very C-terminal region of BamD. A deep groove between TPR domains 4 and 5 is also observed. This groove, as well as the surface around α10, may provide binding sites for other components of the BAM complex. The C-terminal region of BamD serves as a platform for interactions with other components of the BAM complex. The N-terminal region shares structural similarity to other proteins whose functions are related to assistance in or regulation of secretion. Therefore, this region is likely to play an important role in the insertion of other outer membrane proteins.