Stoichiometry of the MexA-OprM binding, as investigated by blue native gel electrophoresis.

Multidrug resistance has become a serious concern in the treatment of bacterial infections. A prominent role is ascribed to the active efflux of xenobiotics out of the bacteria by a tripartite protein machinery. The mechanism of drug extrusion is rather well understood, thanks to the X-ray structures obtained for the Escherichia coli TolC/AcrA/AcrB model system and the related Pseudomonas aeruginosa OprM/MexA/MexB. However, many questions remain unresolved, in particular the stoichiometry of the efflux pump assembly. On the basis of blue native polyacrylamide gel electrophoresis (BN-PAGE) (Wittig et al., Nat. Protoc. 2006, 1, 418-428), we analyzed the binding stoichiometry of both palmitylated and non-palmitylated MexA with the cognate partner OprM trimer at different ratios and detergent conditions. We found that β-octyl glucopyranoside (β-OG) detergent was not suitable for this technique. Then we proved that MexA has to be palmitylated in order to stabilized the complex formation with OprM. Finally, we provided evidence for a two by two (2, 4, 6, or upper) binding of palmitylated MexA per trimer of OprM.