27/06/2026
Molecular basis of human daylight vision
Human colour vision is mediated by three types of cone opsins, each tuned to a different region of the visible spectrum. L cones are most sensitive to red light, M cones to green light, and S cones to blue light. Although there are only three cone types, we see the world in more than just three colours, as our colour perception arises from the interplay of their overlapping spectral sensitivities.
The 3D architecture of cone opsins before they are activated by light, and the reasons behind their exceptionally rapid responses, have been challenging to resolve. These receptors are highly dynamic and can undergo spontaneous activation even in darkness, which makes it extremely challenging to isolate them in a single, well-defined state.
To avoid accidental activation, the researchers worked exclusively under very dim red light in the lab, at wavelengths far outside the sensitivity of cone opsins.
The research team now presents previously unresolved structures of human cone opsins, in particular the blue- and green-sensitive variants in their inactive states under dark conditions. Although the red cone opsin was not studied directly, its close genetic similarity to the green cone opsin suggests that similar molecular principles are likely to apply.
To understand why cone opsins are able to convert light pulses into electrical signals in a flash, it’s worth taking a look at their structural organisation. “At the heart of every cone opsin is the so-called retinal, a light-sensitive molecule derived from vitamin A,” explains the first author of the study.
When light hits the eye, it transfers energy to the retinal, causing it to change its shape. This in turn triggers the activation of the photoreceptor and the generation of an electrical signal to the brain, where visual information is processed. “Our new structural and functional data indicate that cone opsins are optimized for rapid signal transmission,” says the senior author. Their molecular structure includes a network of internal “microswitches” that allow them to connect with their intracellular signalling partner, the transducing G protein. Because this interaction already happens in the resting state, signal transmission can proceed extremely rapidly once the light is absorbed. This molecular readiness helps to explain how cone opsins fulfil the needs of daylight vision.
Another factor contributing to the speed of cone opsins lies in the architecture of the retinal binding site. In the green cone opsin, for example, this retinal binding pocket is relatively open at the entrance and exit. This allows the retinal to be quickly displaced after a light pulse, thus preparing for the next pulse. Such a rapid turnover supports fast updating of visual information in the brain.
The researchers discovered something else: The retinal binding site of the blue-sensitive opsin is more confined, with “closed doors” that effectively restrict retinal movement. As a result, a higher-energy light stimulus is required to induce a shape change in the retinal ligand. Blue light naturally carries more energy than green or red light and is therefore well suited to trigger this transition. In contrast, the retinal in the green-sensitive opsin can move much more freely, allowing the receptor to respond to lower-energy green light and even to activate spontaneously in the absence of light.
The findings of this study may provide a new molecular framework for understanding eye diseases associated with the loss or dysfunction of photoreceptors in the cone cells.
https://sciencemission.com/daylight-vision
