Structural insights into spectral tuning and retinal exchange in cone visual pigments

Color vision in catarrhine primates relies on red-, green-, and blue-sensitive cone pigments that share an 11-cis-retinal chromophore but differ in absorption maxima. Red and green pigments arose by recent gene duplication and differ at only a few residues. Here, we report cryo-electron microscopy structures of red and green cone pigments from the cynomolgus macaque (Macaca fascicularis) integrated with low-temperature vibrational spectroscopy and quantum mechanical and molecular mechanical modeling. The red-green spectral shift is dominated by threonine 285, the hydroxyl dipole of which modulates chromophore electrostatics, whereas steric effects appear modest. We also identified membrane-facing lateral openings in cone pigments but not in inactive rhodopsin. Comparisons with active-state structures suggest activation-dependent gating, and mutational and spectroscopic analyses support a role for this opening in retinal uptake and rapid pigment regeneration.