Molecular studies of the structure-function relationships of Pseudomonas aeruginosa OprM: An outer membrane protein associated with efflux

Pseudomonas aeruginosa demonstrates high intrinsic resistance to multiple classes of structurally unrelated antimicrobial agents. This broad-spectrum resistance is primarily due to a combination of low outer membrane permeability coupled to secondary resistance mechanisms such as the resistance-nodulation-division (RND) efflux systems. The outer membrane protein OprM is involved in intrinsic and mutational multiple-antibiotic resistance as part of two RND efflux systems in P. aeruginosa. To study structure-function relationships for OprM, optimal conditions for oprM overexpression were determined; it appeared that excessive production of the protein was lethal to cells. In addition, overexpression of oprM alone in wild-type P. aeruginosa did not increase the resistance of the cells, suggesting OprM could not function independent of the pump and linker proteins of the efflux systems.; OprM was demonstrated for the first time to have channel-forming activities like porins, and was shown to be cation selective like the related TolC protein of Escherichia coli. Based on the functional and sequence similarity between OprM and TolC, and their similar content of α-helical and β-sheet structures determined by circular dichroism spectroscopy, a three-dimensional model of OprM was constructed by threading its sequence to the TolC crystal structure. This suggested that, like TolC, OprM has a distinctive architecture comprising outer membrane β-barrel and periplasmic helical-barrel structures. Analyses of OprM insertion and deletion mutants in the context of this model indicated that the helical barrel is critical for both function and integrity of OprM, while a C-terminal domain localized around the equatorial plane of this helical barrel is dispensable. Unlike the classic porins, extracellular loops appear to play a minimal role in substrate specificity. There appears to be a correlation between the change in antimicrobial activity for certain OprM mutants and the channel size determined by planar bilayer analysis, supporting the “iris” mechanism of action first suggested for TolC. This study provides information on the structure-function relationships of OprM in a three-dimensional context, and will allow more focused hypotheses and studies about the functional domains of OprM and its related family of efflux proteins.