Irreversible conformational change of bacterio-opsin induced by binding of retinal during its reconstitution to bacteriorhodopsin, as studied by (13)C NMR

We compared (13)C NMR spectra of [3-(13)C]Ala- and [1-(13)C]Val-labeled bacterio-opsin (bO), produced either by bleaching bR with hydroxylamine or from a retinal-deficient strain, with those of bacteriorhodopsin (bR), in order to gain insight into the conformational changes of the protein backbone that lead to correct folding after retinal is added to bO. The observed (13)C NMR spectrum of bO produced by bleaching is not greatly different from that of bR, except for the presence of suppressed or decreased peak-intensities. From careful evaluation of the intensity differences between cross polarization magic angle spinning (CP-MAS) and dipolar decoupled-magic angle spinning (DD-MAS) spectra, it appears that the reduced peak-intensities arise from reduced efficiency of cross polarization or interference of internal motions with proton decoupling frequencies. In particular, the E-F and F-G loops and some transmembrane helices of the bleached bO have acquired internal motions whose frequencies interfere with proton decoupling frequencies. In contrast, the protein backbone of the bO from the retinal-negative cells is incompletely folded. Although it contains mainly a-helices, its very broad (13)C NMR signals indicate that its tertiary structure is different from bR. Importantly, this changed structure is identical in form to that of bleached bO from wild-type bR after it was regenerated with retinal in vitro, and bleached with hydroxylamine. We conclude that the binding of retinal is essential for the correct folding of bR after it is inserted in vitro into the lipid bilayer, and the final folded state does not revert to the partially folded form upon removal of the retinal.