Fig. 3. ESI-MS of CNBr peptides from LacY. CNBr peptides from 100 µg of purified single-Cys148 LacY were separated as described in the Materials and methods. (A) Schematic representation of single-Cys148 LacY. The 12 transmembrane α-helices (rectangles), six irreplaceable residues and CNBr cleavage sites (arrows) are shown. Peptides resolved by either RP- or SE-HPLC are indicated by lines. (B) Identification of fragments. The total ion current following ESI-MS is shown. Individual CNBr fragments, numbered as in (A), were assigned by comparing the expected molecular masses obtained from the program Peptide Mass in the Expasy Molecular Biology Server (http://www.expasy.org) with the measured molecular masses.