Functional analysis of cell-free-produced human endothelin B receptor reveals transmembrane segment 1 as an essential area for ET-1 binding and homodimer formation

The functional and structural characterization of G-protein-coupled receptors (GPCRs) still suffers from tremendous difficulties during sample preparation. Cell-free expression has recently emerged as a promising alternative approach for the synthesis of polytopic integral membrane proteins and, in particular, for the production of G-protein-coupled receptors. We have now analyzed the quality and functional folding of cell-free produced human endothelin type B receptor samples as an example of the rhodopsin-type family of G-protein-coupled receptors in correlation with different cell-free expression modes. Human endothelin B receptor was cell-free produced as a precipitate and subsequently solubilized in detergent, or was directly synthesized in micelles of various supplied mild detergents. Purified cell-free-produced human endothelin B receptor samples were evaluated by single-particle analysis and by ligand-binding assays. The soluble human endothelin B receptor produced is predominantly present as dimeric complexes without detectable aggregation, and the quality of the sample is very similar to that of the related rhodopsin isolated from natural sources. The binding of human endothelin B receptor to its natural peptide ligand endothelin-1 is demonstrated by coelution, pull-down assays, and surface plasmon resonance assays. Systematic functional analysis of truncated human endothelin B receptor derivatives confined two key receptor functions to the membrane-localized part of human endothelin B receptor. A 39 amino acid fragment spanning residues 93-131 and including the proposed transmembrane segment 1 was identified as a central area involved in endothelin-1 binding as well as in human endothelin B receptor homo-oligomer formation. Our approach represents an efficient expression technique for G-protein-coupled receptors such as human endothelin B receptor, and might provide a valuable tool for fast structural and functional characterizations.