Genetic and Molecular Basis of Cone Opsin Phototransduction

The vertebrate visual system contains a sophisticated physiological assembly of highly-specialized proteins that mediate light stimuli to be processed by the brain. Two types of cells, the rod and cone cells, are involved in detecting dim and bright light, respectively. The rod cell contains rhodopsin whose structure and function has been extensively studied and the molecular interactions involving rhodopsin and the other proteins of the phototransduction cascade have been dissected in great detail, although several mechanistic features remain to be determined. The cone phototransduction process has been less studied, particularly at the cone opsin structural level, and some conformational and mechanistic information has been inferred from analogy to the rod system. In spite of this, current efforts of investigation are focused on the study of the structure and function of cone opsins and other proteins of the cone phototransduction system. In this review we cover the current knowledge on the genetic and molecular aspects of cone opsins and the consequences of cone opsin mutations on the structure and function of these photoreceptor proteins causing vision disorders ranging from mild color vision abnormalities to severe cone-mediated retinal degeneration. A second part of the review is dedicated to the analysis of the molecular interactions involving calcium-binding proteins (such as recoverin and guanylate cyclase-activating proteins) in zebrafish which has emerged as a very useful model organism for the study of cone opsin phototransduction particularly in the deactivation steps of the visual phototransduction cascade.