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, KR2K+ and KR2Cs+, respectively. Wildtype KR2 and these mutants are expected to provide new ways of the application to optogenetics.
2016 Japan Society for Molecular Science