Structure and mechanisms of sodium-pumping KR2 rhodopsin

Rhodopsins are the most universal biological light-energy transducers and abundant phototrophic mechanisms that evolved on Earth and have a remarkable diversity and potential for biotechnological applications. Recently, the first sodium-pumping rhodopsin KR2 from Krokinobacter eikastus was discovered and characterized. However, the existing structures of KR2 are contradictory, and the mechanism of Na⁺ pumping is not yet understood. Here, we present a structure of the cationic (non H⁺) light-driven pump at physiological pH in its pentameric form. We also present 13 atomic structures and functional data on the KR2 and its mutants, including potassium pumps, which show that oligomerization of the microbial rhodopsin is obligatory for its biological function. The studies reveal the structure of KR2 at nonphysiological low pH where it acts as a proton pump. The structure provides new insights into the mechanisms of microbial rhodopsins and opens the way to a rational design of novel cation pumps for optogenetics.