Skip to main content
. 1999 Sep 14;96(19):10695–10698. doi: 10.1073/pnas.96.19.10695

Figure 3.

Figure 3

NEM modification and substrate protection by d-galactopyranosyl β-d-thio-galactopyranoside (TDG). ESI-MS is used to quantitate the addition of NEM groups to Cys residues of the solubilized protein. Each NEM addition increases the mass of the protein by 125 Da. (A) Unmodified permease. (B) Ten-minute labeling with NEM on ice. (C) Ten-minute labeling with NEM at room temperature. (D) same as C after preincubation with a saturating concentration (50 mM) of the substrate analog TDG. Of eight thiol groups in lactose permease only two are modified under the conditions used. Preincubation with substrate reveals differential protection of these Cys residues against alkylation with NEM (see text).