Figure 4. Schematic representation of differing complexes formed between Rho* and G_t.

Light illumination triggers structural changes in Rho and allows the binding of G_t to the Rho*–Rho dimer. In the pathway on the right-hand side, activation of one Rho subunit in the dimer suffices to induce G_t association, which stimulates GDP release from the G_t nucleotide-binding pocket. This results in formation of the Rho*–G_te complex with a free nucleotide-binding pocket. However, loading of GTP causes complex dissociation. In the pathway on left-hand side, Rho*–G_te is accessible to hydroxylamine that promotes all-trans-retinylidene chromophore hydrolysis and dissociation from the chromophore-binding pocket. This results in formation of the complex with the empty chromophore-binding pocket and an empty nucleotide-binding pocket in Rho_e*–G_te. The Rho_e*–G_te complex can be regenerated with 11-cis-retinylidene resulting in a Rho–G_te complex that can be dissociated in the presence of GTP. Red ovals represent Rho in the dark state, yellow ovals represent photoactivated Rho* (Meta II), grey ovals represent opsin (Rho_e) and blue ovals represent heterotrimeric G_tαβγ. An animated version of this Figure is available at http://www.BiochemJ.org/bj/428/0001/bj4280001add.htm.