Retinoid-binding proteins are indispensable for visual cycles in both vertebrate and invertebrate retinas. These proteins stabilize and transport hydrophobic retinoids in the hydro-philic environment of plasma and cytoplasm, and allow regeneration of visual pigments. Here, we identified a novel retinol-binding protein in the eye of a butterfly, Papilio xuthus. The protein that we term Papilioretinol-binding protein (PapilioRBP) is a major component of retinal soluble proteins and exclusively binds 3-hydroxyretinol, and emits fluorescencepeakingat 480 nmunder ultraviolet (UV) illumination. .
