NMR structure of the seven transmembrane helix protein proteorhodopsin

Green-absorbing proteorhodopsin (PR), a light-driven proton pump, shows a strong dependence of its function on the pH. The primary proton acceptor D97 has an unusually high pKa value around 7.5 and its protonation state affects the absorption characteristics of the retinal cofactor. Furthermore, the direction of proton pumping switches in response to pH between an outward directed transport at alkaline pH and an inward directed transport at acidic pH. The potential function of this pH-dependency including the changing vectoriality is, however, still debated and a possible regulatory activity cannot be excluded. For a further insight into the structure-function relationship, we have solved the solution NMR structure of detergent-solubilised PR at acidic pH. NOE data was obtained with the help of stereo-array isotope labelling (SAIL) as well as selective labelling and complemented with a large number of distance restraints derived from paramagnetic relaxation enhancement (PRE). Additionally, restraints from residual dipolar couplings (RDCs) served to improve the structural accuracy of this seven-helix-bundle. The three-dimensional structure of PR reveals differences from its homologues such as the absence of the extended β-sheet in the B-C loop and enables insight into the mechanisms of color-tuning.