Transmembrane extension and oligomerization of the CLIC1 chloride intracellular channel protein upon membrane interaction

Chloride intracellular channel proteins (CLICs) differ from most ion channels as they can exist in both soluble and integral membrane forms. The CLICs are expressed as soluble proteins but can reversibly autoinsert into the membrane to form active ion channels. For CLIC1, the interaction with the lipid bilayer is enhanced under oxidative conditions. At present, little evidence is available characterizing the structure of the putative oligomeric CLIC integral membrane form. Previously, fluorescence resonance energy transfer (FRET) was used to monitor and model the conformational transition within CLIC1 as it interacts with the membrane bilayer. These results revealed a large-scale unfolding between the C- and N-domains of CLIC1 as it interacts with the membrane. In the present study, FRET was used to probe lipid-induced structural changes arising in the vicinity of the putative transmembrane region of CLIC1 (residues 24-46) under oxidative conditions. Intramolecular FRET distances are consistent with the model in which the N-terminal domain inserts into the bilayer as an extended α-helix. Further, intermolecular FRET was performed between fluorescently labeled CLIC1 monomers within membranes. The intermolecular FRET shows that CLIC1 forms oligomers upon oxidation in the presence of the membranes. Fitting the data to symmetric oligomer models of the CLIC1 transmembrane form indicates that the structure is large and most consistent with a model comprising approximately six to eight subunits.