= Transducin
\b[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>.<Flow of information in the light-triggered cyclic nucleotide cascade of vision by B. K. Fung and others>[Abstract: subunit composition, nucleotide-binding site and downstream activation.]{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.<Flow of information in the light-triggered cyclic nucleotide cascade of vision by B. K. Fung and others>[Reconstitution, chromatography and isolated alpha-subunit activation experiments.]{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.<Flow of information in the light-triggered cyclic nucleotide cascade of vision by B. K. Fung and others>[Abstract: 71 transducin molecules per photolyzed rhodopsin under the reported conditions.]{ref}
Back to article page
