Researchers led by Polina Isaikina from the Paul Scherrer Institute have determined for the first time the three-dimensional molecular structure of human cone opsins in their dark state, before activation by light, according to a June 25 announcement. The study, published in Science and conducted with collaborators at PSI, the Extreme Light Infrastructure in the Czech Republic, and the University of Tokyo, provides new insights into human vision and its evolution.
Cone opsins are photoreceptor proteins found in cone cells within the retina. These proteins enable detailed color vision during daylight and allow humans to perceive thousands of colors as well as track fast-moving objects. Impairment or dysfunction of these receptors due to genetic mutations or degenerative processes can lead to conditions such as color blindness and age-related macular degeneration.
The research team succeeded in determining how the molecular architecture of cone opsins enables rapid activation by light. "In order to determine the three-dimensional structure of these receptors in their dark state and understand their rapid activation, we had to overcome major technical hurdles," said Isaikina. She added that advanced approaches including cryo-electron microscopy, ultrafast laser spectroscopy, biochemical assays, cellular assays, and computational tools were combined for this study.
The findings include previously unresolved structures for blue- and green-sensitive variants of human cone opsins under dark conditions. Although red cone opsin was not directly studied, researchers suggest similar molecular principles apply due to its close genetic similarity with green cone opsin. Sarah L. Schmidt explained that at the center of each cone opsin is retinal—a molecule derived from vitamin A—which changes shape when exposed to light: "Our new structural and functional data indicate that cone opsins are optimized for rapid signal transmission." Schmidt described a network of internal microswitches enabling quick interaction with intracellular signaling partners even before light absorption.
Differences were observed between blue- and green-sensitive variants regarding how retinal is bound within each protein; blue-sensitive sites restrict movement more than green-sensitive ones do. This results in blue cones requiring higher-energy stimuli while green cones can respond more readily—even spontaneously—in low-light conditions.
Isaikina said that understanding these mechanisms may help identify targets for future therapies addressing diseases caused by loss or dysfunction of photoreceptors: "A detailed structural understanding of these mechanisms helps us identify where things go wrong in such diseases and where targeted therapies might be possible." The researchers hope their work will aid drug development aimed at stabilizing cone function or slowing vision loss.
