Convergent evolution of animal and microbial rhodopsins

Rhodopsins, a family of photoreceptive membrane proteins, contain retinal as a chromophore and were firstly identified as reddish pigments from frog retina in 1876. Since then, rhodopsin-like proteins have been identified mainly from animal eyes. In 1971, a rhodopsin-like pigment was discovered from the archaeon Halobacterium salinarum and named bacteriorhodopsin. While it was believed that rhodopsin- and bacteriorhodopsin-like proteins were expressed only in animal eyes and archaea, respectively, before the 1990s, a variety of rhodopsin-like proteins (called animal rhodopsins or opsins) and bacteriorhodopsin-like proteins (called microbial rhodopsins) have been progressively identified from various tissues of animals and microorganisms, respectively. Here, we comprehensively introduce the research conducted on animal and microbial rhodopsins. Recent analysis has revealed that the two rhodopsin families have common molecular properties, such as the protein structure (i.e., 7-transmembrane structure), retinal structure (i.e., binding ability to cis- and trans-retinal), color sensitivity (i.e., UV- and visible-light sensitivities), and photoreaction (i.e., triggering structural changes by light and heat), more than what was expected at the early stages of rhodopsin research. Contrastingly, their molecular functions are distinctively different (e.g., G protein-coupled receptors and photoisomerases for animal rhodopsins and ion transporters and phototaxis sensors for microbial rhodopsins). Therefore, based on their similarities and dissimilarities, we propose that animal and microbial rhodopsins have convergently evolved from their distinctive origins as multi-colored retinal-binding membrane proteins whose activities are regulated by light and heat but independently evolved for different molecular and physiological functions in the cognate organism.