Fig. 2.

Trp replacements in two pairs of Gly-Gly residues that connect the N- and C-terminal six-helix domains on the periplasmic side of LacY. The 12 transmembrane helices that make up LacY are colored light yellow [N-terminal (N-term) bundle] and tan [C-terminal (C-term) bundle]. Gly residues G159 and G370 in helices V and XI, respectively, and Trp replacements G46W (helix II) and G262W (helix VIII) are indicated. The putative outward-open structure is viewed from the side (A) or from the periplasm (B). The crystal structure of the almost occluded, narrow outward-open conformer of LacY with Gly→Trp replacements at positions 46 and 262 and bound galactoside (dark gray) is viewed from the side (C) or the periplasm (D), respectively.