Activating an invertebrate bistable opsin with the all-trans 6.11 retinal analogue

Animal vision depends on opsins, a category of G protein-coupled receptor (GPCR) that achieves light sensitivity by covalent attachment to retinal. Typically binding as an inverse agonist in the 11-cis form, retinal photoisomerizes to the all-trans isomer and activates the receptor, initiating downstream signaling cascades. Retinal bound to bistable opsins isomerizes back to the 11-cis state after absorption of a second photon, inactivating the receptor. Bistable opsins are essential for invertebrate vision and non-visual light perception across the animal kingdom. While crystal structures are available for bistable opsins in the inactive state, it has proven difficult to form homogeneous populations of activated bistable opsins either via illumination or reconstitution with all-trans retinal. Here we show that a non-natural retinal analogue, all-trans retinal 6.11 (ATR6.11), can be reconstituted with the invertebrate bistable opsin, Jumping Spider Rhodopsin-1 (JSR1). Biochemical activity assays demonstrate that ATR6.11 functions as an agonist of JSR1. ATR6.11 binding also enables complex formation between JSR1 and downstream signaling partners. Our findings demonstrate the utility of retinal analogues for biophysical characterization of bistable opsins, which will deepen our understanding of light perception in animals. ### Competing Interest Statement The authors have declared no competing interest.