Tracking G-protein-coupled receptor activation using genetically encoded infrared probes

Rhodopsin is a prototypical heptahelical family A G-protein-coupled receptor (GPCR) responsible for dim-light vision1,2. Light isomerizes rhodopsin's retinal chromophore and triggers concerted movements of transmembrane helices, including an outward tilting of helix 6 (H6) and a smaller movement of H5, to create a site for G-protein binding and activation3,4. However, the precise temporal sequence and mechanism underlying these helix rearrangements is unclear. We used site-directed non-natural amino acid mutagenesis to engineer rhodopsin with p-azido-l-phenylalanine residues incorporated at selected sites5, and monitored the azido vibrational signatures using infrared spectroscopy as rhodopsin proceeded along its activation pathway. Here we report significant changes in electrostatic environments of the azido probes even in the inactive photoproduct Meta I, well before the active receptor state was formed. These early changes suggest a significant rotation of H6 and movement of the cytoplasmic part of H5 away from H3. Subsequently, a large outward tilt of H6 leads to opening of the cytoplasmic surface to form the active receptor photoproduct Meta II3. Thus, our results reveal early conformational changes that precede larger rigid-body helix movements, and provide a basis to interpret recent GPCR crystal structure6,7 and to understand conformational sub-states observed during the activation of other GPCRs 8. © 2010 Macmillan Publishers Limited. All rights reserved.