Novel Hydra opsin aligned with rag-worm ciliary opsin. As expected, the sequence conservation is highest in the seven transmembrane regions (underlined). The signature residue perfectly conserved in all opsins is the lysine that retinal forms a Schiff base with. This conserved position is shown in red. The position of the well-characterized glutamate counterion of some ciliary opsins, is instead a tyrosine in several ciliary opsins and all rhabdomeric opsins. Hydra ciliary opsin has a serine at that position. The cytoplasmic regions shown by mutagenesis to be critical for G protein interaction are underlined by gray boxes. The first of those cytoplasmic regions contains the highly conserved DRY element, which contains an arginine critical for G protein activation. That sequence is more similar in Hydra (YRY) than in rag-worm (VRC), shown by gray highlighting.
Alvarez BMC Evolutionary Biology 2008 8:222 doi:10.1186/1471-2148-8-222