Labeling of single-Cys mutants with thiol reagents by blockade of TMRM reactivity. As described in Materials and Methods, RSO membrane vesicles containing Cys-less LacY (A) or single-Cys replacements at positions 241 (B), 242 (C) or 245 (D) were treated with 1 mM MMTS, NEM, MTSES, MIANS, or TMRM, washed and labeled with 1 mM TMRM for 30 min at 25 °C in the presence of 10 mM TDG. Purified proteins were then subjected to SDS-PAGE. LacY bands labeled with TMRM (upper panels) or avidin-conjugated horse radish peroxidase (lower panels) were imaged.