Ligand-induced changes in periplasmic loops in the lactose permease of Escherichia coli

N- and C-terminal halves of lactose permease, each with a single-Cys residue in a periplasmic loop, were coexpressed, and cross-linking was studied in the presence or absence of ligand. A Cys residue at position 36 between transmembrane helices I and II (loop I/II) forms a disulfide spontaneously with a Cys residue at position 253, 254, 255, or 256 in loop VII/VIII. Moreover, in the presence of o-phenanthroline-copper, a Cys residue at position 42 in loop I/II forms a disulfide with a Cys residue at position 253, 254, 257, or 258 in loop VII/VIII. Changes in the rate of cross-linking are also observed in the presence of substrate, suggesting that ligand binding induces movement between loops I/II and VII/VIII such that positions 253-256 are brought closer to position 36 or 42, while positions 257 and 258 move away from position 42.