Developing a common structural and functional framework for G Protein coupled receptors by means of site-directed spin labeling and pressure modulation EPR spectroscopy

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G protein coupled receptors (GPCRs) represent a large group of highly homologous receptors which share a common seven transmembrane helical architecture. These receptors constitute the first step of various recognition and signaling pathways by accepting extracellular stimuli and activating intracellular G proteins or arrestins. Knowledge of GPCR structure and function is therefore of high pharmaceutical interest. The close structural similarity of GPCRs is manifested by a scaffold of highly-conserved amino acids and connecting hydrogen bond networks. These motifs are also indispensable for proper receptor function as their reorganization renders conformational changes which accompany receptor activation. This makes it is feasible to delineate general structure/function relations for GPCRs.One specific GPCR which is well suited for mechanistic studies is rhodopsin, the light receptor in the rod cell of the vertebrate eye. Rhodopsin has been subject of numerous biophysical and biochemical studies resolving its structure down to the level of hydrogen bonds, single amino acids or internal water molecules. However, conformational dynamics have been described to a very limited extent even for the rhodopsin model system, despite their important role for receptor function.In this study rhodopsin as well as other GPCRs will be investigated by recently developed site-directed spin labeling (SDSL) EPR techniques which are capable of providing detailed information on protein conformational equilibria as well as the dynamics of exchange between conformations. By using pressure modulation it is feasible to populate and describe functionally important conformations which are otherwise difficult to detect due to low populations. Using pressure-jump and time domain EPR spectroscopy, these data will be complemented by information on protein dynamics and how these dynamics are altered during receptor function. Based on the results and the detailed knowledge of the rhodopsin archetype, a common structural and functional framework of GPCR activation will be developed. Parameters of the attempted universal model provide the possibility of customization which is needed to describe each specific GPCR types.