G-protein activation of the dark-state conformation of the visual G protein-coupled receptor rhodopsin by releasing critical structural constraints.

G protein-coupled receptors (GPCRs) operate through the binding and activation of heterotrimeric G proteins. Ligand interaction drives the receptor's transition from an inactive to an active state, ultimately triggering G-protein activation and downstream signal transduction. The visual GPCR rhodopsin contains the chromophore 11-cis-retinal, which is covalently bound and functions as an inverse agonist, maintaining very low basal activity in the absence of light. Disruption of this basal receptor activity can lead to physiological consequences associated with retinal diseases such as congenital stationary night blindness and retinitis pigmentosa. Here, we describe a functional dark-state rhodopsin generated through engineered double and triple mutations at three well-defined structural microswitches that regulate the conformational stability of the dark ground-state: i) T942.61I, located near the protonated Schiff base linkage environment and linked to congenital stationary night blindness; ii) M2576.40Y positioned close to the tyrosine cluster (Y2235.58 and Y3067.53 of the NPxxY motif); and iii) E1343.49 within the conserved (D/E)RY motif, which participates in the so-called ionic lock involving R1353.50 and E2476.30. Characterization of these mutant rhodopsins provides additional insights into the structural basis of the inactive-to-active conformational transition in important functional domains and emphasizes rhodopsin conformational flexibility.