Discovery of a simplest mechanism for color detection

They published their results in Proceedings of the National Academy of Sciences.

"Human color vision involves three kinds of photosensitive molecules (opsins), red, green, and blue sensitive opsins, which are expressed in different photoreceptor cells. So far, it has been believed that multiple kinds of color opsins are required to achieve color detection. However, we discovered that a super simple mechanism based on a single kind of opsin in a single kind of photoreceptor cell achieves color detection, namely UV and visible light discrimination, in the fish pineal organ," said Akihisa Terakita, a professor at the Graduate School of Science at Osaka City University in Japan.

According to Terakita and his colleagues, Seiji Wada and Mitsumasa Koyanagi, assistant and associate professors at Osaka City University, the pineal organ of zebrafish employs a pineal UV-sensitive opsin, called parapinopsin, which has a unique molecular property different from visual opsins in eyes. Both the pineal and visual opsins convert to a signaling active photoproduct (light state) upon light absorption, whereas their light states have different molecular properties: the light state of parapinopsin is highly stable and reverts to the original dark state (inactive state) upon visible light absorption although the light state of visual opsin is unstable and rapidly decays. Because both of dark inactive and light active states of parapinopsin are stable, this feature is called "bistable nature."

"Two stable states of parapinopsin, which have different color sensitivity, UV- and visible light sensitivities for the dark and light active states, respectively is very important. It enables to detect color of light despite of one kind of opsin," Terakita said. "Environmental light contains all colors of light and its color composition varies depending on time and place. Because two states are photo-interconvertible, the mixture of the two states is formed under natural light. That is, parapinopsin alone behave like two kinds of light sensors."

The two states as two kinds of light sensors can act antagonistically one another. UV-biased light causes the enhancement of signal. On the other hands, visible light-biased light causes the inhibition of signal. Such biased color component appears in the late afternoon. In the late afternoon, a sunny location contains abundant visible light component, but a shady location contains abundant UV component. Parapinopsin alone possibly detects such color component changes, according to Terakita.

The evolution of opsins tells us very interesting possibility, according to Terakita, because ancestral opsins are considered to have bistable nature.

"Taken together with the process of evolution of opsins, it is imagined bistable opsin has evolved into the current eye opsin. That is, because eyes and pineal organs have a common origin, it can be speculated that the emergence of a mechanism such as color detection by bistable opsin of fish pineal organ had been an important first step for vertebrates to acquire the complex function of color vision," Terakita said.