A New Family of Antioxidant Proteins: Antioxidant Properties of the Chloride Intracellular Ion Channel Protein, CLIC1

The Chloride Intracellular Ion Channel (CLIC) family consists of six evolutionarily conserved protein members in vertebrates. These proteins are unusual, existing in cells as both soluble and membrane bound proteins. CLIC1 was first isolated from activated monocytes and extensively characterised as an ion channel protein, is expressed widely across human tissues (1). Its ion channel activity and membrane insertion are regulated by sterol and phospholipid membrane composition (2,3), as well as pH and redox. Given the bulk of cellular CLIC1 is in a soluble form, we recently demonstrated this soluble form possesses oxidoreductase enzymatic activity (4). Our current study demonstrates for the first time that CLIC1 possesses cellular antioxidant activity. We expressed recombinant CLIC1 in bacterial E. coli cells exposed to the oxidising agent H2O2. Expression of CLIC1 in the cells was sufficient to provide increased tolerance to H2O2, while a mutant form of the protein lacking the critical cysteine residue in the enzymatic active site did not. This demonstrates for the first time the direct cellular antioxidant activity by CLIC1 and confirms it is a moonlighting protein, with two distinct functions – a membrane spanning ion channel and an antioxidant oxidoreductase enzyme.