Crystal structure of the cell wall anchor domain of MotB, a stator component of the bacterial flagellar motor: implications for peptidoglycan recognition.

The stator ring of the bacterial flagellar motor is composed of the MotA and MotB proteins that act together to generate a turning force (torque) acting on the FliG ring of the rotor. The C-terminal domain of MotB (MotB-C) is believed to anchor the MotA/MotB complex to peptidoglycan (PG) of the cell wall. The first crystal structures of MotB-C and its complex with N-acetylmuramic acid (NAM) have been determined to 1.6- and 2.3-A resolution, respectively. MotB-C is a dimer, both in solution and in the crystal. The two glycan chains of the PG ligand can be modeled as semirigid helices and docked into the grooves harboring the NAM molecules on the opposite faces of the dimer. The model suggests that a concave hydrophilic surface created upon edge-to-edge beta-sheet dimerization and centered around the 2-fold axis of the dimer can accommodate the peptide cross-bridge linking the two sugar chains. Significant structural similarities were found between MotB-C and the PG-binding domains of reduction-modifiable protein M and peptidoglycan-associated lipoprotein exclude, suggesting that PG recognition by different outer membrane protein A-like proteins may be governed by very similar molecular mechanisms that evidently involve protein dimerization.