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Rhodopsin is the light-sensitive G protein-coupled receptor of vertebrate rod photoreceptors. It consists of the protein opsin bound to the light-absorbing molecule 11-cis-retinal. Absorbing a photon changes the retinal configuration toward all-trans-retinal, initiating structural changes that activate the signaling partner transducin. The visual cycle supplies fresh 11-cis-retinal so the pigment can be regenerated.^[ref]

The 2000 bovine rhodopsin structure, obtained by X-ray crystallography at $2.8$ angstrom resolution, showed seven membrane-spanning alpha helices surrounding the bound chromophore. The dark-state retinal helps stabilize an inactive receptor configuration. The structure made a molecular question visible: how can a small rearrangement of a bound light absorber alter the intracellular surface that engages another protein?^[ref]

The distinction between opsin and functional pigment matters experimentally. In mice lacking RPE65, investigators detected rod opsin but no normal rhodopsin, together with loss of rod responses on electroretinography. Having the receptor chain is insufficient when the required retinal supply is disrupted.^[ref]

A photoreceptor class important for low-light vision. Greater sensitivity and downstream pooling support detection under different conditions from cone-dominated vision. When only one spectral receptor class supplies usable information, the principle of univariance prevents independent wavelength–intensity identification.

Transducin is the heterotrimeric G protein that couples light-activated rhodopsin to an intracellular enzyme in vertebrate rod photoreceptors. Its alpha, beta and gamma protein subunits form a nucleotide-regulated signaling system. Activated rhodopsin promotes replacement of bound guanosine diphosphate by guanosine triphosphate on the alpha subunit; the activated alpha subunit can then stimulate a phosphodiesterase that acts on cyclic guanosine monophosphate.^[ref]

The exchange step is not phosphorylation of the previously bound nucleotide: GDP leaves and another nucleotide binds. This distinction connects receptor activation to a molecular switch rather than to the direct absorption of light by transducin itself.

The 1981 experiments separated components and put them back together. Reconstituted membranes containing rhodopsin and transducin showed light-dependent binding of a nonhydrolyzable GTP analog. Chromatographic analysis placed the nucleotide-binding site on the alpha subunit; isolated analog-bound alpha subunit could activate the downstream phosphodiesterase even on unilluminated membranes. The component separation helped establish the order of the signaling pathway.^[ref]

One photoactivated receptor led to analog uptake by 71 transducin molecules in the reported assay, illustrating amplification: a receptor can activate multiple signaling partners. That measured number belongs to the specified preparation and analog experiment, not a universal count for every photon in every intact rod.^[ref]

The visual cycle is the set of biochemical transformations that regenerates the light-sensitive retinal supply of visual pigments after illumination. In the classical rod pathway, a photon absorbed by rhodopsin converts its bound 11-cis-retinal toward all-trans-retinal. Subsequent processing involving the photoreceptor and the retinal pigment epithelium returns retinal to a form that can rebuild light-sensitive pigment. This recycling connects a rapid light-triggered molecular event to slower recovery of sensitivity.^[ref]

A useful experiment interrupted this supply rather than removing the receptor itself. T. M. Redmond and collaborators disrupted RPE65 in mice and measured retinal compounds and visual responses. The animals lacked normal rhodopsin and rod electroretinography responses even though rod opsin remained; all-trans retinyl esters accumulated, while 11-cis retinoids were absent. These results established that RPE65 is necessary for the normal production of 11-cis vitamin A derivatives in this pathway.^[ref]

The contrast supplies an illuminating example of biochemical dependence: a cell may retain a receptor protein yet lose the response because it cannot regenerate the small molecule required by that receptor. The visual cycle is the chemical recycling pathway; electroretinography measures an electrical consequence of its success or failure. The knockout establishes necessity in the tested pathway and conditions, rather than proving that every photoreceptor follows an identical regeneration route.

Conversion of absorbed light into a photoreceptor response. In vertebrate rods and cone cells, activation of visual pigment changes a signaling cascade and light-sensitive membrane current, typically hyperpolarizing the cell and changing transmitter release. A receptor voltage, ganglion-cell spikes and a reported color are distinct measurement levels.