Unfolding of a CIC chloride transporter retains memory of its evolutionary history

CIC chloride channels and transporters are important for chloride homeostasis in species from bacteria to human. Mutations in CIC proteins cause genetically inherited diseases, some of which are likely to involve folding defects. The CIC proteins present a challenging and unusual biological folding problem because they are large membrane proteins possessing a complex architecture, with many reentrant helices that go only partway through membrane and loop back out. Here we were able to examine the unfolding of the Escherichia coli CIC transporter, CIC-ec1, using single-molecule forced unfolding methods. We found that the protein could be separated into two stable halves that unfolded independently. The independence of the two domains is consistent with an evolutionary model in which the two halves arose from independently folding subunits that later fused together. Maintaining smaller folding domains of lesser complexity within large membrane proteins may be an advantageous strategy to avoid misfolding traps