Characterization and Engineering of a Blue-Sensitive, Gi/o-Biased, and Bistable Ciliary Opsin from a Fan Worm

Ciliary opsins (c-opsin) have been identified not only in vertebrates but also in invertebrates. An invertebrate ciliary opsin was recently identified in the fan worm <i>Acromegalomma interruptum</i> (formerly named <i>Megalomma interrupta</i>); however, its spectral and signaling characteristics are unknown. In the present study, we characterized the spectral properties and light-induced cellular signaling properties of opsin (<i>Acromegalomma</i> invertebrate ciliary opsin (<i>Acr</i>InvC-opsin)). <i>Acr</i>InvC-opsin showed an absorption maximum at 464 nm, and upon blue light absorption, the spectrum was red-shifted by approximately 50 nm. The two states are interconvertible by illumination with blue and orange light. Blue light illumination of <i>Acr</i>InvC-opsin caused specific coupling with Gi, sustained Gi dissociation, decreased intracellular cAMP levels, and the activation of GIRK channels. The cellular responses by the activated opsin were partially terminated by orange light illumination. These light-dependent responses indicate that InvC-opsin is a typical bistable pigment wherein the resting and activated states can be interconverted by visible light illumination. We also attempted to modulate the spectral and functional properties of <i>Acr</i>InvC-opsin by using site-directed mutagenesis. Substitution of Ser-94 with Ala caused little spectral shift in the resting state but a further red shift of ∼10 nm in the activated state, indicating that the absorption spectra of the two states were tuned differently. In contrast, the substitution of S94A did not significantly affect the light-dependent signaling properties of <i>Acr</i>InvC-opsin. Because <i>Acr</i>InvC-opsin is a blue-sensitive, Gi/o-biased, and bistable pigment, it has the potential to serve as an optical control tool to specifically and reversibly regulate Gi/o-dependent signaling pathways by visible light.