Conformational Changes Of Rhodopsin

11.2.1 Early Magnetic Resonance Studies of Chromophore AND PROTEIN-LlPID INTERACTIONS

The earliest documented experiments applying nuclear magnetic resonance (NMR) to the analysis of rhodopsin were limited to studying the isolated solubilized visual pigment chromophores. Conformations of the polyene chains of all-trans, 9-cis, 11-cis and 13-cis retinal, as well as model forms of Schiff Base all-trans analogs, were solved by 1H and natural abundance 13C solution NMR providing details about the planarity, twisted irregularities, and motional freedom of various regions of the chro-mophore.10,11 The production of the first isotopically enriched chromophore, designed to include 13C at carbon 14 (C14), was reconstituted into rhodopsin and analyzed in a detergent-solubilized system.12 Eventually a perturbed, protonated Schiff Base was confirmed, linking the 11-cis retinal to the protein in ground state rhodopsin.11

Some of the earliest studies probing interactions of rhodopsin with lipids and detergents were carried out by application of spin-label electron paramagnetic resonance (EPR) spectroscopy. Conformation and stability of rhodopsin in solubilized detergent-mixed micelles as well as the effect of rhodopsin on lipid bilayer mobility were probed by application of various spin-labeled system components.13-15

Other early magnetic resonance experiments focused on analysis of sonicated aqueous suspensions of rhodopsin in lipids and membranes for which rigid or motionally averaged NMR powder pattern spectra were collected.16 Detailed studies of the dynamic properties of the rhodopsin-membrane system by molecular fluctuation sensitive spin-lattice (Tj) relaxation measurements as well as 13C-1H NOE showed small but significant decreases in T1 relaxation times of the rod outer segment (ROS) membranes compared to ROS lipid vesicles.16-18 These results, which were in agreement with EPR results obtained with spin-labeled lipids,19-21 indicated that the average rate of phospholipid segmental motion was reduced four-fold by the presence of rhodopsin for lipids immediately adjacent to the receptor.

Moment analysis and spectral subtraction of deuterium (2H) NMR data collected on rhodopsin in deuterated vesicles at low temperatures indicated two discrete lipid pools — one comprised of bulk gel lipids and the second comprised of phospholipids presumably in two-dimensional protein aggregates with rhodopsin.22-24 Although probing head group mobility and lipid phases of rhodopsin membranes and extracted phospholipids by solid-state 31P NMR25-27 further supported a role for rhodopsin in organizing ROS phospholipids; subsequent measurement of 31P T1 relaxation indicated conformational head group changes in non-native high protein content membranes (probably arising from lipids trapped between adjacent proteins), but not when protein:lipid ratios were maintained at natural or lower levels.28-30

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