Substrate-driven conformational changes in ClC-ec1 observed by fluorine NMR

The CLC 'Cl- channel' family consists of both Cl-/H+ antiporters and Cl- channels. Although CLC channels can undergo large, conformational changes involving cooperativity between the two protein subunits, it has been hypothesized that conformational changes in the antiporters may be limited to small movements localized near the Cl- permeation pathway. However, to date few studies have directly addressed this issue, and therefore little is known about the molecular movements that underlie CLC-mediated antiport. The crystal structure of the Escherichia coli antiporter ClC-ec1 provides an invaluable molecular framework, but this static picture alone cannot depict the protein movements that must occur during ion transport. In this study we use fluorine nuclear magnetic resonance (NMR) to monitor substrate-induced conformational changes in ClC-ec1. Using mutational analysis, we show that substrate-dependent 19F spectral changes reflect functionally relevant protein movement occurring at the ClC-ec1 dimer interface. Our results show that conformational change in CLC antiporters is not restricted to the Cl- permeation pathway and show the usefulness of 19F NMR for studying conformational changes in membrane proteins of known structure.