Microbial rhodopsin is a photo-receptive membrane protein of micro-organisms. The most ubiquitous microbial rhodopsins are light-driven ion pumps which actively transport H
+ or Cl
- against membrane chemical potential. In 2013, we reported a new class of ion pump rhodopsin, sodium pump rhodopsin (KR2) which outwardly transports Na
+ ion by the use of light energy. The mechanism of Na
+ transport by KR2 was investigated in spectroscopic and crystallographic studies. The results showed that the H
+ transfer between photoisomerized retinal Schiff base and its counter ion, Asp116, is a critical process for the Na
+-transport function. After this H
+ transfer, the protonated Asp116 sequesters the H
+ from the ion-transport pathway, and then immediately Na
+ is taken up from the cytoplasmic side. The Na
+ binds to the site composed of Asn112 and Asp251, and simultaneously H
+ goes back to the retinal Schiff base. Then, positive charge of the reprotonated retinal Schiff-base prevents the back flow of Na
+ to the cytoplasmic side. Finally, the Na
+ is released to the extracellular side. Furthermore, on the basis of structural insights about KR2, we have succeeded to develop new artificial K
+ and Cs
+ pumping KR2 mutants, KR2
K+ and KR2
Cs+, respectively. Wildtype KR2 and these mutants are expected to provide new ways of the application to optogenetics.
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