Red-shifting mutation of light-driven sodium-pump rhodopsin.

Microbial rhodopsins are photoreceptive membrane proteins that transport various ions using light energy. While they are widely used in optogenetics to optically control neuronal activity， rhodopsins that function with longer-wavelength light are highly demanded because of their low phototoxicity and high tissue penetration. Here， we achieve a 40-nm red-shift in the absorption wavelength of a sodium-pump rhodopsin (KR2) by altering dipole moment of residues around the retinal chromophore (KR2 P219T/S254A) without impairing its ion-transport activity. Structural differences in the chromophore of the red-shifted protein from that of the wildtype are observed by Fourier transform infrared spectroscopy. QM/MM models generated with an automated protocol show that the changes in the electrostatic interaction between protein and chromophore induced by the amino-acid replacements， lowered the energy gap between the ground and the first electronically excited state. Based on these insights， a natural sodium pump with red-shifted absorption is identified from Jannaschia seosinensis.