Protein glycosylation and transport to the cell membrane were unaffected. Although there was some diminution in the accumulation of the opsin, this was insufficient to account for the observed effect. Like rhodopsin, the formation of the cone opsins visual pigments is dependent on the binding of retinal into a hydrophobic pocket that is formed by the second and fourth transmembranous loops. Disruption of a conserved proline near the retinal binding site represents a cause of color vision deficiency that is unrelated to spectral shifts of the photopigment. Rhodopsin provides monochromatic vision under low intensity light.
Cone opsins provide vision under higher intensity light.
The genes for red and green cone opsins occur as a tandem array on the distal long arm of the X chromosome. This conformational change in the visual pigment molecule causes activation of the G-binding protein, transducin. The structure and function of the rhodopsin molecule have been extensively studied by peptide mapping and site-directed mutagenesis, and is predicted to be a protein with seven transmembrane segments. Disulfide cross-linking between the third transmembrane helix and the second extracellular loop occurs at cysteines 126 and 203. Based on its conservation in rhodopsin, this residue is predicted to alter the structure of the retinal binding pocket.