The Study on a Novel Light-driven Sodium Pump and Creation of New Functional Molecules

Abstract

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.