Relocating the active-site lysine in rhodopsin and implications for evolution of retinylidene proteins

Type I and type II rhodopsins share several structural features including a G protein-coupled receptor fold and a highly conserved active-site Lys residue in the seventh transmembrane segment of the protein. However, the two families lack significant sequence similarity that would indicate common ancestry. Consequently, the rhodopsin fold and conserved Lys are widely thought to have arisen from functional constraints during convergent evolution. To test for the existence of such a constraint, we asked whether it were possible to relocate the highly conserved Lys296 in the visual pigment bovine rhodopsin. We show here that the Lys can be moved to three other locations in the protein while maintaining the ability to form a pigment with 11-cis-retinal and activate the G protein transducin in a light-dependent manner. These results contradict the convergent hypothesis and support the homology of type I and type II rhodopsins by divergent evolution from a common ancestral protein.