Opsin Classification
There are two groups of protein termed opsins; these groups are not homologous, but have convergently evolved a similar form and function. Type I opsins are employed by prokaryotes, whereas animals use type II opsins. No opsins have been found outside these groups (for instance in plants, fungi, or placozoans), although as-yet unconfirmed reports of opsins in sponges would suggest that opsins were present in the ancestral metazoan.
Based on the phylogeny of their sequences, type 2 opsins can be grouped into six families; these families are very distinct, with under 20% of their sequences shared with any other subfamily. The families consist of the vertebrate opsins/encephalopsins; Go opsins; Gs opsins; invertebrate Gq opsins; the photoisomerases and neuropsins. These subfamilies can be grouped according to their expression; the first three are found in ciliary-type photoreceptor cells; Gq opsins in rhabdomeric-type photoreceptor cells; and the latter two are found elsewhere but based on their shared intron positions can be bundled together into the photoisomerases.
Read more about this topic: Opsin