Evolutionary tuning of rhodopsin-mediated light protection

Visual sensitivity introduces the inherent risk of blinding photodamage. Rhodopsin directly mediates this vulnerability, as activation drives excessive signaling via the Meta-II (MII) state, which subsequently releases the toxic, desensitizing agonist all-trans retinal (atRAL). In dim-light specialists, slow atRAL release is assumed to reflect extended MII signaling; however, this model ignores that arrestin quenches signaling in milliseconds and that apparent release reflects reversible atRAL rebinding. Here we reveal that extended post-bleach lifetimes, including an atRAL-driven shift toward non-signaling Meta-III (MIII), evolved independently of intrinsic G-protein signaling to dually protect retinal cells against photochemical toxicity and shield neighboring receptors from agonist noise. Dim-light specialists whose rod-dense retinas experience daytime solar flux, such as megabats, possess an extremely cytoprotective rhodopsin that retains nearly 60% of atRAL at equilibrium by partitioning into MIII, validating long-standing predictions that MIII buffers retinoid toxicity while explaining why bat rhodopsin signals more weakly than faster-releasing human or bovine orthologs. To definitively uncouple sequestration from intrinsic signaling, we engineered an active-state-locked, signaling-null rhodopsin that achieves 100% atRAL sequestration and confers extreme cytoprotection in retinal cells. Mimicking partial bleaches with heterogeneous active and inactive receptor populations, co-incubation reveals that this signaling-null sink acts in trans to boost wild-type rhodopsin regeneration and signaling by shielding neighboring dark-state receptors from agonist interference. Lastly, while consensus human rhodopsin is non-protective, alleles restoring cytoprotective sequestration emerged in high-irradiance geographies, conferring a 38% reduced risk of blindness in modern populations. Our findings redefine rhodopsin as a tunable light buffer shaping adaptation and human disease.