Roles of charged residues in the conserved motif, G-X-X-X-D/E-R/K-X-G- [X]-R/K-R/K, of the lactose permease of Escherichia coli

N. J. Pazdernik, E. A. Matzke, A. E. Jessen-Marshall, R. J. Brooker

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15 Scopus citations


The lactose permease is a polytopic membrane protein that has a duplicated conserved motif, GXXX(D/E)(R/K)XG[X](R/K)(R/K), located in cytoplasmic loops 2/3 and 8/9. In the current study, the roles of the basic residues and the acidic residue were investigated in greater detail. Neutral substitutions of two positive charges in loop 2/3 were tolerated, while a triple mutant resulted in a complete loss of expression. Neutral substitutions of a basic residue in loop 8/9 (i.e., K289I) also diminished protein stability. By comparison, neutral substitutions affecting the negative charge in loop 2/3 had normal levels of expression, but were defective in transport. A double mutant (D68T/N284D), in which the aspartate of loop 2/3 was moved to loop 8/9, did not have appreciable activity, indicating that the negative charge in the conserved motif could not be placed in loop 8/9 to recover lactose transport activity. An analysis of site-directed mutants in loop 7/8 and loop 8/9 indicated that an alteration in the charge distribution across transmembrane segment 8 was not sufficient to alleviate a defect caused by the loss of a negative charge in loop 2/3. To further explore this phenomenon, the double mutant, D68T/N284D, was used as a parental strain to isolate suppressor mutations which restored function. One mutant was obtained in which an acidic residue in loop 11/12 was changed to a basic residue (i.e, Glu374 → Lys). Overall, the results of this study suggest that the basic residues in the conserved motif play a role in protein insertion and/or stability, and that the negative charge plays a role in conformational changes.

Original languageEnglish (US)
Pages (from-to)31-40
Number of pages10
JournalJournal of Membrane Biology
Issue number1
StatePublished - 2000


  • Conserved motif
  • Cotransporter
  • Lactose permease
  • MFS
  • Sugar transporter
  • Symporter


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