The retinas of primates, including humans, contain three types of cone visual pigments that respond preferentially to red, green, and blue light. By absorbing different wavelengths of light, these pigments enable us to perceive color. Although red- and green-sensitive cone visual pigments share approximately 96% of their amino acid sequences, their peak absorption wavelengths differ by about 30 nanometers (nm), allowing us to distinguish red from green. However, the structural basis underlying this difference in absorption wavelength had remained unclear.
A research team led by Associate Professor Kota Katayama of Nagoya Institute of Technology determined the three-dimensional structures of red- and green-sensitive cone visual pigments from macaques in their dark, light-unactivated state at near-atomic resolution using cryo-electron microscopy. The researchers found that retinal, the light-absorbing molecule within cone visual pigment, adopts nearly identical structures in the red and green pigments. Instead, differences in the arrangement of three hydrophilic amino acid residues specific to the red pigment, and in the electrostatic environment they create around retinal, generate the approximately 30 nm difference in absorption wavelength. The researchers also identified lateral openings in the membrane-facing region of cone visual pigments and showed that retinal release and uptake occur through different pathways, potentially supporting the rapid visual cycle characteristic of cone photoreceptor cells.
These findings provide the first structural explanation of how primates distinguish colors at the molecular level. By advancing our molecular understanding of human color vision diversity, including congenital red-green color vision deficiency, these findings may contribute to future studies of visual disorders and ultimately to improved diagnostic and therapeutic strategies.
(Article: Masanori Nakajo)
Together, these mechanisms support the distinctive functions of cone visual pigments in color vision.