Structure of human green cone opsin yields insights into mechanisms underlying the rapid decay of its active, signaling state

Cone opsins enable daylight vision and color discrimination. Like their dim-light cousin rhodopsin (Rho) found in rod cells, they use a covalently attached retinal ligand to sense light and initiate visual phototransduction by activating G proteins. Unfortunately, we know less about their structural properties, in part because their activated state is unstable-cone opsins release their retinal agonist within seconds after light activation, ~100× faster than Rho. To determine what causes this rapid release and how it affects G protein activation, we solved the structure of active-state, wild-type human green cone opsin (GCO_WT) stabilized with a mini-G protein and then compared its structural and biophysical properties to Rho. Our results reveal unique features in the active-state GCO_WT structure. These include i) a larger water channel connected to a larger retinal binding cavity, ii) a larger "hole" near the retinal Schiff base that could facilitate both retinal escape and water access; and iii) a potential anionic residue, E102, that lies within ~3.6 Å of the Schiff base. Our biophysical assays show that neutralizing E102 (mutant GCO_E102Q) slows retinal release (~8×) from the receptor and increases G protein activation. Surprisingly, our kinetic studies suggest that entropic factors are the main cause for the faster retinal release from activated GCO_WT. These unique attributes in GCO_WT likely facilitate its function in bright daylight. These results support the proposal that rapid retinal release from an active-state cone opsin helps prevent signal saturation and enables rapid resetting of the receptor.