A conformational change in the lactose permease of Escherichia coli is induced by ligand binding or membrane potential

Abstract

Lactose transport in membrane vesicles containing lactose permease with a single Cys residue in place of Val 315 is inactivated by N‐ethylmaleimide in a manner that is stimulated by substrate or by a H+ electrochemical gradient (δμ H +, Sahin‐Tóth M, Kaback HR, 1993, Protein Sci 2:1024–1033). The findings are confirmed and extended in this communication. Purified, reconstituted Val 315Ψ Cys permease reacts with N‐ethylmaleimide or hydrophobic fluorescent maleimides but not with a membrane impermeant thiol reagent, and β‐galactosides specifically stimulate the rate of labeling. Furthermore, the reactivity of purified Val 315 Ψ Cys permease is enhanced by imposition of a membrane potential (δΨ, interior negative). The results indicate that either ligand binding or ΔΨ induces a conformational change in the permease that brings the N‐terminus of helix X into an environment that is more accessible from the lipid phase. Copyright © 1994 The Protein Society