Dowhan focused on phosphatidylethanolamine (PE), which is the only zwitterionic lipid in Escherichia coli (the other lipids all have anionic head groups). He knew that PE was needed for LacY activity in vitro, and found that the same was true in vivo. Mutant E. coli lacking PE made a version of LacY that allowed facilitated but not active transport of lactose. This aberrant LacY had half of its 12 transmembrane domains in a configuration opposite to that of normal LacY. But when PE synthesis was induced in the absence of new LacY synthesis, the existing LacY flipped back into its native conformation, and could now do active transport.
The key to this change may lie in the weakly hydrophobic seventh transmembrane domain, which may flip out of the membrane or form a hairpin loop. Dowhan hopes to detect the direction of propagation of the change—where the flipping begins and ends—by blocking the process with a single large covalent modifier. He will also test whether the flipping requires reentry into the translocon machinery.
For LacY, the dependence on PE may be a structural accident. But other proteins function only in certain parts of the cell because of differing lipid environments. And for Dowhan, the result emphasizes the importance of lipids. “Everyone focuses on the proteins, and ignores the lipids, viewing them as a simple solvent,” he says. “But membrane protein sequences are written for a particular lipid environment.” ▪