Light-Powered Transport of Organic Anions by Microbial Rhodopsins

Microbial rhodopsins are photoactive membrane proteins known for transporting small inorganic ions such as H⁺, Cl^-, and Na⁺. Their compact structure─comprising seven transmembrane helices─has long been thought to limit their substrate range to such ions. Here, we report that several anion-pumping rhodopsins can also transport organic anions. In particular, a rhodopsin from cyanobacteria transports bulky organic anions, including those with a benzene ring, with volumes up to ∼120 ų─five times larger than Cl^-. These anions bind in the dark state and are translocated upon photoactivation, via a mechanism similar to Cl^-. Notably, only anions with pK_a values below 2 are transported, suggesting that negative charge is essential for binding. This study provides the first evidence that naturally occurring proteins can use light to transport organic compounds across membranes. These findings broaden the functional scope of microbial rhodopsins and open new possibilities for light-driven transport of organic ions.