Low-temperature Raman spectroscopy of sodium-pump rhodopsin from Indibacter alkaliphilus: insight of Na+ binding for active Na+ transport.

We carried out the low-temperature Raman measurement of a sodium pump rhodopsin from Indibacter alkaliphilus (IaNaR) and examined the primary structural change for the light-driven Na+ pump. We observed that photoexcitation of IaNaR produced the distorted 13-cis retinal chromophore in the presence of Na+, while the structural distortion was significantly relaxed in the absence of Na+. This structural difference of the chromophore with/without Na+ was attributed to the Na+ binding to the protein, which alters the active site. Using the spectral sensitivity to the ion binding, we found that IaNaR had a second Na+ binding site in addition to the one already specified on the extracellular surface. To date, the Na+ binding has not been considered as a prerequisite for Na+ transport. However, this study provides insight that the protein structural change induced by the ion binding involved the formation of an R108-D250 salt bridge, which has critical importance in the active transport of Na+.