Metamorphic Response of the CLICl Chloride Intracellular Ion Channel Protein upon Membrane Interaction

A striking feature of the CLIC (chloride intracellular channel) protein family is the ability of its members to convert between a soluble state and an integral membrane channel form. Direct evidence of the structural transition required for the CLIC protein to autonomously insert into the membrane is lacking, lamely because of the challenge of probing the conformation of the membrane-bound protein. However, insights into the CLIC transmembrane form can be gained by biophysical methods such as fluorescence resonance energy transfer (FRET) spectroscopy. This approach was used to measure distances from tryptophan 35, located within the CLICI putative N-domain transmembrane region, to three native cysteine residues within the C-terminal domain. These distances were computed both in aqueous solution and upon the addition of membrane vesicles. The FRET distances were used as constraints for modeling of a structure for the CLICI integral membrane form. The data are suggestive of a large conformational unfolding occurring between the N- and C-domains of CLICI upon interaction with the membrane. Consistent with previous findings, the N-terminal domain of CLICI is likely to insert into the lipid bilayer, while the C-domain remains in solution on the extravesicular side of the membrane.