The role of the N-capping box in the x1 helix on the structure and stability of CLIC1
Date
2011-06-24
Authors
Karatas, Fidan
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Abstract
CLIC1 is an intracellular channel protein that either exists in a soluble state in the cytoplasm or is bound to the membrane. The mechanism by which CLIC1 traverses the membrane is not currently known although it is likely to depend on oxidation and pH. The α1 helix of CLIC1, located in the putative transmembrane region, is proposed to span the membrane with the N-terminal end protruding into the extracellular matrix. Recent studies show that this helix contains an N-capping box residue (Ser27), which will act as a lock on the α1 helix (Stoychev et al., 2009). In order to find out the contribution of this residue to the stability of the protein, Ser27 was replaced with proline, a helix breaker. A comparative study was conducted that focused on the characterisation of the mutant in terms of secondary and tertiary structure and determination of its conformational stability at equilibrium in comparison with the wild-type. The study was performed at both pH7 and 5.5 which takes into account the environment in the cytoplasm (pH7) and at the membrane surface (pH5.5), respectively. The secondary structure of the mutant was found to be less helical as compared to the wild-type. In terms of tertiary structure, the unique tryptophan residue, Trp35 was found to be more buried in the mutant compared to the wild-type. The wild-type does not have exposed hydrophobic patches and does not bind to 8-anilino-1-naphthalene sulfonic acid (ANS) in the native state at pH7 and pH5.5. However, ANS binds to the mutant in the native state indicating that more hydrophobic patches in the protein are exposed to the solvent. At both values, the mutant displays much lower stability at both pH7 and 5.5 as compared to the wild-type. As a conclusion, it was found that the N-capping box residue, Ser27, plays a significant role as a good helix initiating residue. Moreover, this residue has a significant effect in stabilising the α1 helix and may play a role in the cooperativity between the N- and C-domains and thus in stabilising the whole protein.