The Conformation of Membrane-Bound and Detergent-Solubilised Bovine Rhodopsin. A Comparative Hydrogen-Isotope Exchange Study

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Abstract
The conformations of the intrinsic membrane protein rhodopsin in its membrane-bound and detergent-solubilized states are compared by hydrogen isotope exchange measurements. The IR peptide exchange data show that the highly hydrophobic nature of rhodopsin is conserved in the presence of the 2 detergents used: Cemulsol LA 90 and Ammonyx LO. About 50% of the peptide hydrogens exchange under these conditions; about 80% would exchange in most soluble proteins. The conformational stability of rhodopsin in these 2 detergents is also demonstrated by the similarity of the tritium exchange-out kinetics and the IR amide I band frequencies for both membrane-bound and detergent-solubilized rhodopsin. Upon illumination of rhodopsin (bleaching) in the presence of detergents, the H 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. The conformation of rhodopsin apparently is not altered by solubilization in non-ionic detergents. Bleached rhodopsin is stabilized by the membrane but does not retain a native conformation in these detergents.