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Journal of Bacteriology, March 2001, p. 1734-1739, Vol. 183, No. 5
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.5.1734-1739.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Identification of Essential Charged Residues in Transmembrane Segments of the Multidrug Transporter MexB of Pseudomonas aeruginosa

Lan Guandagger and Taiji Nakae*

Department of Molecular Life Science, Tokai University School of Medicine, Isehara 259-1193, Japan

Received 20 October 2000/Accepted 13 December 2000

The MexABM efflux pump exports structurally diverse xenobiotics, utilizing the proton electrochemical gradient to confer drug resistance on Pseudomonas aeruginosa. The MexB subunit traverses the inner membrane 12 times and has two, two, and one charged residues in putative transmembrane segments 2 (TMS-2), TMS-4, and TMS-10, respectively. All five residues were mutated, and MexB function was evaluated by determining the MICs of antibiotics and fluorescent dye efflux. Replacement of Lys342 with Ala, Arg, or Glu and Glu346 with Ala, Gln, or Asp in TMS-2 did not have a discernible effect. Ala, Asn, or Lys substitution for Asp407 in TMS-4, which is well conserved, led to loss of activity. Moreover, a mutant with Glu in place of Asp407 exhibited only marginal function, suggesting that the length of the side chain at this position is important. The only replacements for Asp408 in TMS-4 or Lys939 in TMS-10 that exhibited significant function were Glu and Arg, respectively, suggesting that the native charge at these positions is required. In addition, double neutral mutants or mutants in which the charged residues Asp407 and Lys939 or Asp408 and Lys939 were interchanged completely lost function. An Asp408right-arrowGlu/Lys939right-arrowArg mutant retained significant activity, while an Asp407right-arrowGlu/Lys939right-arrowArg mutant exhibited only marginal function. An Asp407right-arrowGlu/Asp408right-arrowGlu double mutant also lost activity, but significant function was restored by replacing Lys939 with Arg (Asp407right-arrowGlu/Asp408right-arrowGlu/Lys939right-arrowArg). Taken as a whole, the findings indicate that Asp407, Asp408, and Lys939 are functionally important and raise the possibility that Asp407, Asp408, and Lys939 may form a charge network between TMS-4 and TMS-10 that is important for proton translocation and/or energy coupling.

* Corresponding author. Mailing address: Department of Molecular Life Science, Tokai University School of Medicine, Isehara 259-1193, Japan. Phone: 81-463-93-5436. Fax: 81-463-93-5437. E-mail: nakae{at}is.icc.u-tokai.ac.jp.

dagger Present address: HHMI/UCLA, Los Angeles, CA 90095-1662.

Journal of Bacteriology, March 2001, p. 1734-1739, Vol. 183, No. 5
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.5.1734-1739.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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