FIGURE 4.

Detection of dual topologies for LacY as a function of membrane PE content. Radiolabeled ([³²P]PO₄) and unlabeled cells expressing LacY with a H205C replacement in C6 were grown in parallel at the indicated aTc levels to result in negligible, intermediate, and fully induced PE levels as determined by analysis of the labeled culture. The nonradiolabeled culture was analyzed for LacY topology using a two-step labeling protocol. Intact cells were labeled with MPB (A) or pretreated with AMS followed by treatment of intact and sonicated cell with MPB (B) as described under “Experimental Procedures.” The diagrams indicate the resulting biotinylation (+) of LacY.