Transducin subunit stoichiometry and cellular distribution in rod outer segments

Transducin is a heterotrimeric GTP-binding protein found in the outer segment of vertebrate retinas that links the photoactivation of rhodopsin (R∗) with activation of a robust type VI cGMP phosphodiesterase (PDE6). Association of the alpha subunit of Transducin (Gαt) with the beta-gamma complex (Gβγ) is necessary for interaction of the holoprotein with R∗ and exchange of a GTP for a previously bound GDP. We have investigated the abundances of the three Transducin subunits by eluting them from bovine rod outer segment membranes by centrifugation under various conditions in vitro. We find that a substantial amount of Gβγ is eluted from ROS under conditions that do not elute Gαt and that there is an overall three to fourfold molar excess of Gβγ to Gαt in rod outer segments. These results suggest that the production and/or turnover of Gαt, Gβ, and Gγ in the rod outer segment are controlled independently.