Conformation and dynamics of [3-C-13]Ala-labeled bacteriorhodopsin and bacterioopsin, induced by interaction with retinal and its analogs, as studied by C-13 nuclear magnetic resonance

C-13 nuclear magnetic resonance (NMR) spectra of [3-C-13]Ala-labeled bacteriorhodopsin (bR), bacterioopsin (bO), and regenerated bR with retinal or bO complex with retinal analogs were recorded in order to gain insights into how the conformation and dynamics of apoprotein (bO) vary with or without retinal or its analogs. First, we assigned the C-13 NMR peak resonating at 16.3 ppm to Ala 53 of both bR and bO, which appears to contact the side chain of Lys 216 at the site of the Schiff base in the former, utilizing the C-13 NMR peaks of A53V and A53G proteins in comparison with those of wild-type bR and bO. Characteristic spectral differences between the apoprotein and bR were observed upon removal of the retinal: the changes of the peak intensities at 16.4, 15.9, and 16.9 ppm are notable. We found that the loops (17.4 ppm) and transmembrane alpha(II) helical region (15.9 ppm) acquired motional freedom with a correlation time of 10(-5) s when the retinal was removed, as detected by proton spin-lattice relaxation times in the rotating frame. A C-13 NMR spectrum very similar to that of native bR was recorded when bR was regenerated by addition of retinal to bO. On the other hand, the addition of the retinal analogs retinol or beta-ionone, which are bound in the retinal binding site but are incapable of forming a Schiff base to the apoprotein, caused distinct spectral changes different from those of bR, as manifested from the displacements of C-13 chemical shifts. These spectral changes must be ascribed to significant conformational changes of apoprotein at various locations in the protein, including the site of Ala 53 induced by modified interaction between the apoprotein and chromophore.