Location of helix III in the lactose permease of Escherichia coli as determined by site-directed thiol cross-linking

The six N-terminal transmembrane helices (N-6) and the six C-terminal transmembrane helices (C-6) in the lactose permease of Escherichia coli, each containing a single Cys residue, were coexpressed, and cross-linking was studied. The proximity of paired Cys residues in helices III (position 78, 81, 84, 86, 87, 88, 90, 93, or 96) and VII (position 227, 228, 231, 232, 235, 238, 239, 241, 243, 245, or 246) was examined by using iodine or two rigid homobifunctional thiol-specific cross-linking reagents with different lengths [N,N'-o-phenylenedimaleimide (o-PDM; 6 Angstrom) and N,N'-p-phenylenedimaleimide (p-PDM; 10 Angstrom)]. Cys residues in the periplasmic half of helix III (position 87, 93, or 96) cross-link to Cys residues in the periplasmic half of helix Vn (position 235, 238, 239, 241, or 245). In contrast, no cross-linking is evident with paired Cys residues near the cytoplasmic ends of helices III (position 78 or 81) and VII (position 227, 228, 213, 232, or 235). Therefore, the periplasmic halves of helices III and VII are in close proximity, and the helices tilt away from each other toward the cytoplasmic face of the membrane. On the basis of the findings, a modified helix packing model for the permease is presented.