OprM and OprN belong to the outer membrane factor family of multidrug efflux proteins from Pseudomonas aeruginosa, a bacterium responsible of nosocomial infections. We report here the two-dimensional (2D) crystallization of OprN and OprM into lipid bilayers and the determination of their 2D projected structure by cryo-electron crystallography, at 1 and 1.4nm, respectively. Both proteins present a dense ring of protein density, of similar to 7 nm diameter. An additional thin peripheral ring is resolved in OprN structure. Both proteins are assembled as trimers. The results presented here indicate a high structural homology between OprN (and OprM) and TolC, a multidrug efflux protein from Escherichia coli. (c) 2005 Elsevier Inc. All rights reserved.