Figure 2.

Helix packing model of lac permease viewed from the cytoplasmic surface. The positions of the six irreplaceable residues (Glu-126, Arg-144, Glu-269, Arg-302, His-322, and Glu-325) are indicated by filled, enlarged circles. The two interacting pairs of Asp-Lys residues (Asp-237/Lys-358 and Asp-240/Lys-319) are highlighted by open, enlarged circles. The gray circles represent residues where the NEM reactivity of Cys replacement mutants is blocked by substrate. The positions of NEM-sensitive Cys replacements are indicated with a small, black dot.