Identification by genetic suppression of Escherichia coli TolB residues important for TolB-Pal interaction

The Tol-Pal system of Escherichia coli is involved in maintaining outer mem brane stability. Mutations in tolQ, tolR, tolA, tolB, or pal genes result i n sensitivity to bile salts and the leakage of periplasmic proteins. Moreov er, some of the tol genes are necessary for the entry of group A colicins a nd the DNA of filamentous bacteriophages. TolQ, TolR, and TolA are located in the cytoplasmic membrane where they interact with each other via their t ransmembrane domains. TolB and Pal form a periplasmic complex near the oute r membrane. We used suppressor genetics to identify the regions important f or the interaction between TolB and Pal. Intragenic suppressor mutations we re characterized in a domain of Pal that was shown to be involved in intera ctions with TolB and peptidoglycan. Extragenic suppressor mutations were lo cated in tolB gene. The C-terminal region of TolB predicted to adopt a beta -propeller structure was shown to be responsible for the interaction of the protein with Pal. Unexpectedly, none of the suppressor mutations was able to restore a correct association between Pal and peptidoglycan, suggesting that interactions between Pal and other components such as TolB may also be important for outer membrane stability.