Absorption of a
photon by
visual pigments induces isomerization of 11-cis-retinaldehyde (RAL) chromophore to all-trans-RAL. Since the
opsins lacking 11-cis-RAL lose
light sensitivity, sustained vision requires continuous regeneration of 11-cis-RAL via the process called '
visual cycle'. Protostomes and vertebrates use essentially different machinery of
visual pigment regeneration, and the origin and early evolution of the vertebrate
visual cycle is an unsolved mystery. Here we compare
visual retinoid cycles between different photoreceptors of vertebrates, including rods, cones and non-visual photoreceptors, as well as between vertebrates and invertebrates. The
visual cycle systems in ascidians, the closest living relatives of vertebrates, show an intermediate state between vertebrates and non-chordate invertebrates. The ascidian
larva may use retinochrome-like
opsin as the major
isomerase. The entire process of the
visual cycle can occur inside the
photoreceptor cells with distinct
subcellular compartmentalization, although the
visual cycle components are also present in surrounding non-photoreceptor cells. The adult ascidian probably uses
RPE65 isomerase, and trans-to-cis isomerization may occur in distinct
cellular compartments, which is similar to the vertebrate situation. The complete transition to the sophisticated
retinoid cycle of vertebrates may have required acquisition of new genes, such as interphotoreceptor retinoid-binding protein, and functional evolution of the
visual cycle genes.
