Class B G-protein coupled receptors (GPCRs) are peptide hormone-binding receptors that are implicated in the pathogenesis several human diseases which makes them attractive targets for drug therapy. To develop new compounds to target these receptors a detailed understanding of the molecular structure is required which has not been succeeded to date. To elucidate the structure of proteins by X-ray crystallography or NMR spectroscopy large quantities of protein are required. For GPCRs this prerequisite is difficult to achieve as the vast majority of GPCRs exhibits low endogenous expression and is very unstable in solution. Therefore, improved expression conditions are required for the efficient characterization of new GPCR structures. In the proposed project class B GPCRs will be optimized for improved heterologous expression and increased thermostability by means of directed evolution. Libraries of class B GPCRs will be obtained by random mutagenesis and will be heterologously expressed using a system in which functional GPCR is targeted to the inner membrane of E. coli. Mutants that display increased receptor expression levels and ligand binding will be selected by flow cytometry using fluorescently labeled ligands. Repetitive cycles of randomization and selection will allow to gradually increasing the level of protein expression and stability. With this evolutionary approach key residues within the receptor sequence can be rapidly identified that are responsible for improved biophysical properties without affecting the pharmacological properties of the receptor. Such GPCR mutants will be a valuable tool on the way to express high quantities of stable receptor protein for subsequent structural studies.
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