Crystal Structure of OprM and MexA of the Multidrug Efflux pump from Pseudomonas aeruginosa

Atsushi Nakagawa1, Hiroyuki Akama2, Takanori Matsuura1, Tomitake Tsukihara1, and Taiji Nakae2

1Insititute for Protein Research, Osaka University,
2Tokai University School of Medicine

BSR2004 (the 8th International Conference on Biology and Synchrotron Radiation), (Himeji, Japan 2004.9.7-11)

The MexAB-OprM efflux pump of Pseudomonas aeruginosa exports structurally and functionally diverse xenobiotics and confers a broad spectrum antibiotic resistance. This is a large problem in the therapy of hospital acquired infections. MexA and MexB are the membrane fusion protein and the transporter, respectively. OprM is assumed to form the xenobiotic exit channel across the outer membrane that collaborates with many other transporters. To investigate structure and function of the MexAB-OprM complex, we carried out X-ray crystal structure analysis of the MexA and the OprM proteins.


Trimeric OprM mainly formed a short β-barrel and a long α-helical barrel. The β-barrel structure, which is assumed to anchor the outer membrane, formed very small pore opening with a narrowest diameter of about less than 5 angstroms. A bundle of α-helices formed a long water-filled empty space along longitudinal axis of which periplasmic end was totally occluded.


In the current model of MexA, 68% of the total residues were visible. Residues of N- and C-terminals provided little structural information. The best ordered monomer structure shows 252 residues (from 23 to 274) among 369 amino acid residues of Azu-MexA-(his)6. The overall structure showed that the MexA monomer consisted mainly of 3 domains, and possibly one additional unseen domain due to a disorder as follows. (1) At one end, two α-helices (from Ala74 to Ala102 and from Lys108 to Arg133) formed a long left-hand twisted α-helical hairpin structure designated as the α-domain. (2) The globular domain adjacent to the α-helical domain consisted of 8 short β-sheets designated as the β-domain. This domain shows a topology similar to the biotinyl/lipoyl-carrier proteins and domains family was defined in the SCOP database. (3) Another globular domain adjacent to the central β-domain, distal to the α-domain, was composed of 7 short β-strands and one short α-helix designated as the α+β-domain. (4) The poorly solved disordered domain contained the N- and C-terminal regions There was a turn of about +60° against α-helix hairpin at the loops between the α-domain and the β-domain forming a sickle-shape. The global crystal structure of MexA appeared as a spiral assembly of the 13 protomer by contiguous joining of the hexamer and heptamer forming a rod at the middle and a funnel-top structure at both ends. The 13-mer had an internal space with widely opened ends.

Using the atomic structures of MexA and OprM together with AcrB as a model of MexB, we proposed an assembly model of MexAB-OprM multidrug efflux pump of Pseudomonas aeruginosa.

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