rhodopsins

Rhodopsins are a family of light-sensitive proteins that use retinal as a chromophore. They occur in a variety of organisms, including animals and microbes, and play roles in vision and in light-driven processes.

In animals, rhodopsin refers to the visual pigment located in rod photoreceptor cells. This protein is a G protein–coupled receptor (GPCR) bound to 11-cis retinal. When a photon is absorbed, retinal isomerizes to all-trans, activating the opsin and the associated G protein transducin. This triggers a cascade that reduces cGMP levels, closes certain ion channels, and hyperpolarizes the cell, providing the neural signal that contributes to vision.

Microbial rhodopsins are a distinct class of retinal-binding proteins found in bacteria and archaea (and in some algae). They are not GPCRs. Many function as light-driven ion pumps or channels. Bacteriorhodopsin, from halophilic archaea, acts as a proton pump to build a proton motive force. Halorhodopsin pumps chloride ions into the cell. Channelrhodopsins form light-gated cation channels, enabling rapid depolarization of membranes. Channelrhodopsins have become foundational tools in optogenetics for controlling neuronal activity with light.

Structural and functional differences distinguish animal rhodopsins from microbial rhodopsins, though both families use retinal and are activated by photon absorption. The distribution of rhodopsins is broad, spanning vertebrate visual systems and diverse microbial lineages. Beyond vision, microbial rhodopsins have substantial applications in neuroscience, bioengineering, and research into light-driven biology.