The molecular mechanisms used by biological systems to detect light are diverse, with at least 10 classes of photosensor proteins and additional photosensitive domains characterized. At least six of these protein classes-Type I microbial opsins, Type II animal opsins, cryptochromes, gustatory-related receptors (GRRs), transient receptor potential A1 ion channels, and euglenoid photoactivated adenylyl cylases-can be considered as playing a role in extraocular systems (e.g., expressed outside of the eye in organisms with a visual system). These six classes of extraocular photosensor proteins consist of four broad groups: (1) seven transmembrane proteins, (2) cryptochromes, (3) ion channels, and (4) adenylyl cyclases. The light-driven functions of these extraocular photoreceptors are diverse, ranging from circadian entrainment to phototactic behavior. There are surprising similarities in structural motifs, with at least three independent families-the GRRs and Types I and II opsins-evolving a seven transmembrane helical tertiary structure for light sensing. When considering all of the photosensitive proteins, particularly those in microbial lineages, an image of evolutionary flexibility is emerging, with examples of fusion proteins from multiple types of photosensors and photosensitive domains shared among diverse arrays of proteins. In general, large questions remain for most of these photosensor proteins about exactly how the protein evolved light sensitivity, how light interacts with the protein, and how the photosensitive protein is transducing the signal.
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