A conserved cationic motif enhances membrane binding and insertion of the chloride intracellular channel protein 1 transmembrane domain

The chloride intracellular channel protein 1 (CLIC1) is unique among eukaryotic ion channels in that it can exist as either a soluble monomer or an integral membrane channel. CLIC1 contains no known membrane-targeting signal sequences and the environmental factors which promote membrane binding of the transmembrane domain (TMD) are poorly understood. Here we report a positively charged motif at the C-terminus of the TMD and show that it enhances membrane partitioning and insertion. A 30-mer TMD peptide was synthesized in which the positively charged motif was replaced by three glutamate residues. The peptide was examined in 2,2,2-trifluoroethanol (TFE), sodium dodecyl sulfate micelles and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine liposomes using size-exclusion chromatography, far-UV CD, and fluorescence spectroscopy. The motif appears to enhance membrane interaction via electrostatic contacts and functions as an electrostatic plug to anchor the TMD in membranes. In addition, the motif is also involved in orientating the TMD with respect to the cis and trans faces of the membrane. These findings shed light on the intrinsic and environmental factors that promote the spontaneous conversion of CLIC1 from a water-soluble to a membrane-bound protein.