FIGURE 1.

Revised topology model of XanQ permease. The model is derived from homology threading using the UraA structure (3QE7) as a template (see supplemental Fig. S1) and initial topology predictions for XanQ based on program TMHMM and the accessibility of loops to hydrophilic reagents (8). The α-helical segments are indicated in rectangles with continuous (core domain) or broken-line perimeters (gate domain). Irreplaceable residues of XanQ (as well as Asp-276 where the carboxyl group is essential) are numbered and bolded and shown in blue. NEM-sensitive positions are shown in dark background. Ala-323 and Gly-275 where the NEM sensitivity is enhanced by substrate binding are shown in red. Residues analyzed with site-directed mutagenesis are circled. Residues of TM3 analyzed in this study are shown in green.