Schematic model of the T-domain in the open-channel state, showing the approximate vertical alignment between the TH6–TH7 and TH8–TH9 segments. As in Fig. 9 B, residues colored red, orange, yellow, and green have f values of ∼0.0, 0.2–0.3, 0.6–0.8, and 1.0, respectively. In contrast to Fig. 9 B, here all side chains whose f value was determined are displayed as color-coded stick figures, regardless of whether the state of reaction was determined. (I306C was omitted because of the difference in f values for MTS-ET and MTS-glucose.) The position of TH9 relative to TH8 was left the same as in the crystal structure; we do not mean to imply that it is located away from the pore. Outside of the TH6–TH7 segment, the overall topology is based on the model of Senzel et al. (2000). Residues 275–288, 297–317, and 326–376 (which roughly correspond to segments TH5, TH6–TH7, and TH8–TH9, respectively) were excerpted from the crystal structure (references in the Fig. 9 B legend), reoriented as needed, and displayed using PyMOL. Solid black curves represent loops that connect the segments. The pair of horizontal dotted lines indicates the approximate boundaries of the membrane hydrocarbon region.