Beyond the Eye: Molecular Evolution of Extraocular Photoreception

Area of Science:

• Biochemistry
• Molecular Biology
• Evolutionary Biology

Background:

• Biological light detection involves at least 10 classes of photosensor proteins.
• Extraocular systems utilize six protein classes: microbial opsins, animal opsins, cryptochromes, gustatory-related receptors (GRRs), transient receptor potential A1 ion channels, and adenylyl cyclases.
• These photosensors are broadly categorized into seven transmembrane proteins, cryptochromes, ion channels, and adenylyl cyclases.

Purpose of the Study:

• To review the diversity of extraocular photosensor proteins.
• To explore the structural similarities and evolutionary flexibility of these proteins.
• To identify outstanding questions regarding the evolution and function of photosensor proteins.

Main Methods:

• Literature review and comparative analysis of known photosensor protein classes.
• Examination of structural motifs and evolutionary relationships.
• Identification of functional roles in extraocular systems.

Main Results:

• Six classes of photosensor proteins function in extraocular systems, involved in processes like circadian entrainment and phototaxis.
• Convergent evolution is evident, with three families (GRRs, Type I and II opsins) independently developing seven transmembrane helical structures.
• Photosensitive domains are shared across diverse protein families, indicating significant evolutionary flexibility, including fusion proteins.

Conclusions:

• Extraocular photoreceptors display remarkable diversity in structure and function, with significant evolutionary convergence.
• Despite shared features, fundamental questions persist regarding the evolution of light sensitivity, light-protein interactions, and signal transduction mechanisms.
• Further research is needed to fully elucidate the molecular mechanisms underlying light detection by these diverse photosensor proteins.