Transmembrane helix tilting and ligand-induced conformational changes in the lactose permease determined by site-directed chemical crosslinking in situ.
The N-terminal six transmenbrane helices (N6) and the C-terminal six transmembrane helices (C6) of the lactose permease of Escherichia coli, each with a Cys residue, were co-expressed independently, and crosslinking was studied. Proximity of paired Cys residues in helices II (position 49, 52, 53, 56, 57, 60, 63 or 67) and VII (position 227, 230, 231, 234, 238, 241, 242 or 245) or XI (position 350, 353, 354, 357, 361 or 364) was examined by using two homobifunctional thiol-specific crosslinking agents of different lengths (6 or 10 A). The results demonstrate that a Cys residue placed in the periplasmic half of helix II (position 49, 52, 53 or 57) crosslinks to Cys residues in the periplasmic half of helix VII (position 241, 242 or 245). In contrast, no crosslinking is evident with paired-Cys residues in the cytoplasmic halves of helices II (position 60, 63 or 67) and VII (position 227, 230, 231, 234 or 238). Remarkably, a Cys residue in the cytoplasmic half of helix II (position 60, 63 or 67) crosslinks with a Cys residue in the cytoplasmic half of helix XI (position 350, 353 or 354), while paired-Cys residues at positions in the periplasmic halves of the two helices do not crosslink. Therefore, helix II is tilted in such a manner that the periplasmic end is close to helix VII, and the cytoplasmic end is close to helix XI. Furthermore, ligand-binding alters the crosslinking efficiency of paired-Cys residues in helices II and VII or XI, indicating that both interfaces are conformationally active. The results are consistent with the conclusion that ligand-binding induces a scissors-like movement of helices II and VII that increases interhelical distance by 3 to 4 A at the periplasmic ends and decreases the distance by 3 to 4 A at the approximate middle of the two transmembrane helices.