Chloride Intracellular Channels (CLICs) are a family of proteins that are somewhat unique as they can exist in both a soluble form as well as a membrane inserted form. The CLICs can also autonomously and reversibly switch between these two different forms and so belong to a new and growing class of proteins coined ‘metamorphic’ [1]. The members of the CLIC protein family are highly conserved across nature and have a wide cellular distribution. These properties suggest that they play a vital role in the cell. However, despite electrophysiological studies demonstrating that the membrane-inserted form of CLICs can function as an ion channel, their main physiological function is still not entirely understood.
The soluble forms of several CLIC family members have been solved by X-ray crystallography. However, efforts in solving the membrane-inserted structures have so far been unsuccessful due to the CLICs metamorphic nature. In this study, Fluorescence Resonance Energy Transfer (FRET) spectroscopy was used to measure distances between residues in the transmembrane region of CLIC1 and the lipid bilayer. The FRET assay was also used to examine the role of different environmental parameters for promoting insertion of the CLIC1 protein into the bilayer. Additionally, tryptophan scanning mutagenesis and acrylamide quenching experiments were performed to assess the degree of association of the CLIC1 protein with the lipid and to aid in the positioning of the CLIC1 transmembrane domain across the bilayer. The outcome of this research is the construction of a model for how CLIC1 sits within the lipid bilayer.
[1] Reference: Murzin (2008) Science, 320 (5884): p.1725