Structural insights into the soluble form of CLIC1 and its mechanism of membrane insertion

The chloride intracellular channel (CLIC) family has a unique characteristic which enables this human protein to be expressed as a soluble protein, however with an increase in the levels of intracellular zinc ions it can insert into the membrane. In the membrane, CLIC proteins have chloride ion channel activity. To date, the exact mechanism of CLIC1 insertion into the membrane is unknown.

Utilising biophysical techniques such as SAXS and NMR, we have identified that CLIC can exist in an ensemble of structures within solution that likely aid its insertion into the membrane. Further studies demonstrate that zinc can bind to CLIC1 and alters the relaxation properties of regions in the N-terminal domain that surround the predicted transmembrane region. We demonstrate that zinc is able to further promote higher order oligomeric states which would aid membrane insertion as previous literature suggests chloride ion channel conductance occurs in a tetrameric arrangement.

Mutations to residues impacted by zinc binding are able to disrupt the equilibrium of oligomeric states in solution, further pointing to the importance of these regions and residues for the metamorphic activity of CLIC1. A more detailed understanding of the mechanism of insertion of CLIC1, will aid pharmacological development of inhibitors for CLIC1. CLIC1 is upregulated in the membrane bound form in cancers such as glioblastoma. Inhibition of this mechanistic switch will provide a novel drug target for many cancers with poor prognosis.