The Conformation of Membrane-bound and Detergent-solubilised Bovine Rhodopsin. A Comparative Hydrogen-isotope Exchange Study

The conformations of the intrinsic membrane protein, rhodopsin, in its membrane-bound and detergent-solubilised states have been compared by hydrogen isotope exchange measurements. The infrared peptide exchange data show that the highly hydrophobic nature of rhodopsin is conserved in the presence of the two detergents used: Cemulsol LA 90 and Ammonyx LO. Only about 50% of the peptide hydrogens exchange under conditions where about 80% would exchange in most soluble proteins. The conformational stability of rhodopsin in these two detergents is also demonstrated by the similarity of the tritium exchange-out kinetics and the infrared amide I band frequencies for both membrane-bound and detergent-solubilised rhodopsin. Upon illumination of rhodopsin (bleaching) in the presence of detergents, the hydrogen exchange rates are greatly increased and shifts in the amide I band frequencies are observed, indicative of a large conformation change. No such change occurs upon bleaching membrane-bound rhodopsin. We conclude that the conformation of rhodopsin is not altered by solubilisation in non-ionic detergents. However, in agreement with previously published results, bleached rhodopsin is stabilised by the membrane but does not retain a native conformation in these detergents.