The rhodopsin-transducin complex houses two distinct rhodopsin molecules.

Upon illumination the visual receptor rhodopsin (Rho) transitions to the activated form Rho(∗), which binds the heterotrimeric G protein, transducin (Gt) causing GDP to GTP exchange and Gt dissociation. Using succinylated concanavalin A (sConA) as a probe, we visualized native Rho dimers solubilized in 1mM n-dodecyl-β-d-maltoside (DDM) and Rho monomers in 5mM DDM. By nucleotide depletion and affinity chromatography together with crosslinking and size exclusion chromatography, we trapped and purified nucleotide-free Rho(∗)·Gt and sConA-Rho(∗)·Gt complexes kept in solution by either DDM or lauryl-maltose-neopentyl-glycol (LMNG). The 3 D envelope calculated from projections of negatively stained Rho(∗)·Gt-LMNG complexes accommodated two Rho molecules, one Gt heterotrimer and a detergent belt. Visualization of triple sConA-Rho(∗)·Gt complexes unequivocally demonstrated a pentameric assembly of the Rho(∗)·Gt complex in which the photoactivated Rho(∗) dimer serves as a platform for binding the Gt heterotrimer. Importantly, individual monomers of the Rho(∗) dimer in the heteropentameric complex exhibited different capabilities for regeneration with either 11-cis or 9-cis-retinal.