FIG. 1.

Amino acid sequence of the gating loop of FhuA in the outer membrane of E. coli, localized at the cell surface by determination of the proteolysis within genetically inserted peptides in cells and spheroplasts (21). The model illustrates the sites of the natural cysteines present at positions 318 and 329 in FhuA (○) and those of the newly inserted cysteines at sites 336 and 347 (□). The model does not imply that the surface loops (top) and periplasmic turns (bottom) extend from the outer membrane as drawn; rather, they are predicted to be highly folded (Fig. 8).