Rhodopsin's active state is frozen like a DEER in the headlights

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Abstract
That G protein-coupled receptor (GPCR) signaling complexes are allosteric machines par excellence is not in dispute. Agonist receptor ligands outside the cell induce catalytic guanine–nucleotide exchange on a heterotrimeric G protein inside the cell, where the ligand binding site on the receptor and the nucleotide binding site on the G protein are on the order of 8–10 nm or more apart. Although the precise molecular pharmacology and chemical basis of ligand binding and specificity are becoming clearer—especially from recent reports of high-resolution crystal structures of engineered β2-adrenergic receptors (ARs) (1, 2) and earlier structures of rhodopsin (3, 4)—what we need to know is how the signaling complex works in space and time. What dynamic receptor conformational changes are induced by an agonist ligand? Or simply, how does the “active” state structure of a receptor differ from its “inactive” state structure? In this issue of PNAS, Altenbach et al. (5) have gone to extraordinary lengths to map the surface movement of rhodopsin upon photoactivation, using a newly emerging electron pair spin resonance (EPR) technology called “double electron–electron resonance” (DEER) spectroscopy, which interrogates pairs of nitroxide spin labels. The label pairs were introduced by site-directed mutagenesis and chemical modification, using the rhodopsin crystal structure as a guide. The authors present a quantitative triangulation of relative interhelical distance changes between transmembrane (TM) helices as rhodopsin converts to its active form. What is most significant here, beyond the sheer technical achievement, is that the work provides a solid foundation for the so-called “helix movement model” of receptor activation. Since the initial reports of substantial movement of certain TM helices of rhodopsin during photoactivation from EPR experiments of site-directed, spin-labeled (SDSL) mutants (6) and from engineered metal-ion binding sites that can block photoactivation …