Binding of light harvesting salinixanthin antenna to Donghaeana dokdonensis rhodopsin

Rhodopsin is a light sensitive receptor protein which is used in visual photo-transduction. Microbial rhodopsins is a family of membrane proteins comprise of seven trans-membrane alpha helixes with a retinal chromophore covalently bound to the protein via a Lysine amino acid residue. Xanthorhodopsin (XR) is a retinal based proton pump membrane protein isolated from Salinibacter ruber. In addition to the retinal chromophore it contains also a salinixanthin carotenoid. Salinixanthin acts as a light-harvesting antenna and transfers approximately 40% absorbed quanta to the retinal. Therefore, salinixanthin plays an important role in the maximization of energy transfer efficiency. Here we report a new functional class of a microbial rhodopsin named rhodopsin DDR2, derived from Donghaeana dokdonensis which occupies hot springs. This rhodopsin functions as a light-driven sodium ion pump as well as proton pump . DDR2 is a retinal based membrane protein, which use retinal molecule to harvest the solar energy. Reconstitution of DDR2 with salinixanthin is accompanied by characteristic changes in absorption spectra and the appearance of CD bands similar to those observed for XR, which is an indication of binding of the carotenoid to the protein. The results indicate that salinixanthin binds to DDR2 in a conformation similar to that in XR.