The crystal structure of human chloride intracellular channel protein 2: A disulfide bond with functional implications

Human chloride intracellular channel proteins (CLICs) are intracellular ion channels based on sequence homology. Human CLICs include six members (CLIC1-6), and all of them contain a conserved core about 240 amino acids. The most distinct feature of CLICs is that they can exist in two states: the soluble cytosolic state and the integral membrane ion channel state. Purified water-soluble CLICs adopt the glutathione S-transferase (GST) fold and can insert into a lipid membrane to form chloride channel.1,2 Among the human CLICs, CLIC1 and CLIC4 have been well studied with the crystal structures solved.1,3,4 The CLIC1 and CLIC4 have important functions in regulating cell cycle, apoptosis, and so forth, and have become potential drug targets for cancer therapy.5–8 Human CLIC2 has a molecular weight of 28.4 kDa and a calculated isoelectric point of 5.44. The CLIC2 gene locates in the telomeric region of Xq28 and is composed of six coding exons and five introns. Since this region of the X chromosome is closely associated with many hereditary diseases, CLIC2 has thus been proposed as a candidate gene for some genetic disorders linked to Xq28.9 Northern blot has demonstrated that the gene of CLIC2 could be expressed in lung, spleen, heart, and skeletal muscle.10 Compared with the CLIC1 or CLIC4, little functional knowledge is known about CLIC2 so far. The only documented function of CLIC2 is to regulate the cardiac calcium ion channel, ryanodine receptor 2 (RyR2).11 In this work, we present the crystal structure of the soluble form of human CLIC2 and tried to correlate some structural features to its potential function. MATERIALS AND METHODS