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To flip or not to flip: lipid–protein charge interactions are a determinant of final membrane protein topology
Correspondence to William Dowhan: William.Dowhan{at}uth.tmc.edu.
The molecular details of how lipids influence final topological organization of membrane proteins are not well understood. Here, we present evidence that final topology is influenced by lipid–protein interactions most likely outside of the translocon. The N-terminal half of Escherichia coli lactose permease (LacY) is inverted with respect to the C-terminal half and the membrane bilayer when assembled in mutants lacking phosphatidylethanolamine and containing only negatively charged phospholipids. We demonstrate that inversion is dependent on interactions between the net charge of the cytoplasmic surface of the N-terminal bundle and the negative charge density of the membrane bilayer surface. A transmembrane domain, acting as a molecular hinge between the two halves of the protein, must also exit from the membrane for inversion to occur. Phosphatidylethanolamine dampens the translocation potential of negative residues in favor of the cytoplasmic retention potential of positive residues, thus explaining the dominance of positive over negative amino acids as co- or post-translational topological determinants.
J. Xie's present address is Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75235.
Abbreviations used in this paper: aTC, anhydrotetracycline; IPTG, isopropyl-β-D-thiogalactoside; LacY, lactose permease; MPB, 3-(N-maleimidylpropionyl) biocytin; PE, phosphatidylethanolamine; SCAM; substituted cysteine accessibility method applied to transmembrane domains; TM, transmembrane domain, TMG, methyl-β-D-galactopyranoside.
© 2008 Bogdanov et al. This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.jcb.org/misc/terms.shtml). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 3.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/3.0/).
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