Influence of salts on rhodopsin photoproduct equilibria and protein stability

Salts may influence properties of membrane proteins in many ways. The visual pigment rhodopsin is one of the best studied membrane proteins and a prototypical G protein-coupled receptor. We have examined the influence of salts on different properties of this protein, highlighting thereby different aspects of the Hofmeister effect. In particular, we studied the pKa value of a deeply buried titratable group, the position of a photoproduct conformational equilibrium, and the stability of the protein in dependence of salt species and salt concentration. We show that presence of salts strongly increases the pKa of rhodopsinʹs retinal Schiff base in the Meta II state by simultaneous binding of an anion as counterion. Further, the Meta I/Meta II conformational equilibrium is shifted to the active state Meta II in the presence of salts, presumably by binding of ions to charged residues being part of an ionic lock that otherwise keeps the receptor in an inactive conformation. Finally, the stability of the protein was found to be controlled by two counteracting effects, namely a salt-dependent unfolding of protein domains and a salt-dependent stabilization of the protein, putatively by an increase of the lateral pressure on the helix bundle.