Mutations in the eukaryotic integral membrane protein Cystic Fibrosis Transmembrane conductance Regulator (CFTR) cause the hereditary disease cystic fibrosis (CF). CFTR functions as an ion channel at the surface of epithelial cells and regulates the movement of chloride ions and water across the plasma membrane. CFTR is difficult to express and purify in heterologous systems due to its propensity to form insoluble aggregates and its susceptibility to degradation. Obtaining good yields of highly purified CFTR has proven problematic and contributes to our limited understanding of the structure and function of the protein. The most prevalent disease causing mutation, F508del, results in misfolded CFTR which is particularly unstable and is quickly targeted for degradation by the host system and is prevented from being trafficked to the plasma membrane. There are limited treatment options for patients with the F508del mutation and it is therefore of significant interest within CF research. New methods and assays are required to identify potential compounds which could correct the F508del mutation. This thesis investigates the use of Saccharomyces cerevisiae to express and purify codon optimised recombinant CFTR. The use of a green fluorescent protein (GFP) tag enabled quick and simple detection of CFTR in whole cells and after extraction from the plasma membrane. By optimising the culture conditions for CFTR expression and detergent solubilisation conditions, relatively high yields of full-length protein were obtained. When used as a chemical chaperone at the time of inducing CFTR expression, glycerol increased yields of full-length protein. Degradation of CFTR could be limited by inducing expression at an optimal cell density and by harvesting cells within a specific time window. CFTR was extracted by solubilisation in the mild detergent dodecyl-β-D-maltopyranoside (DDM) in the presence of up to 1 M NaCl with up to ~87% efficiency in some cases. Using a gene optimisation strategy in which additional purification tags and a yeast Kozak-like sequence were added, the human CFTR (hCFTR) protein was expressed and purified. Fluorescence microscopy revealed CFTR localisation at the periphery of yeast cells. Immunoaffinity chromatography facilitated by the GFP tag at the C terminus of CFTR produced protein of up to 95% purity. An assessment of the thermal stability of this highly purified CFTR using a fluorescent probe binding assay revealed a denaturation midpoint (Tm) of ~43 degC. The ability of this assay to determine the stability of CFTR is encouraging and there is the potential to further develop it in a high-throughput manner to identify compounds which stabilise the F508del protein and which may hold the key to developing new treatments for CF.
|Date of Award||31 Dec 2014|
- The University of Manchester
- CFTR, ABCC7, cystic fibrosis, membrane protein, GFP, ion channel, ABC-transporter