Tyrosine structural changes detected during the photoactivation of rhodopsin

We present the first Fourier transform infrared (FTIR) analysis of an isotope-labeled eukaryotic membrane protein. A combination of isotope labeling and FTIR difference spectroscopy was used to investigate the possible involvement of tyrosines in the photoactivation of rhodopsin (Rho). Rho → MII difference spectra were obtained at 10 °C for unlabeled recombinant Rho and isotope-labeled L-[ring-²H₄]Tyr-Rho expressed in Spodoptera frugiperda cells grown on a stringent culture medium containing enriched L-[ring-²H₄]Tyr and isolated using a His₆ tag. A comparison of these difference spectra revealed reproducible changes in bands that correspond to tyrosine and tyrosinate vibrational modes. A similar pattern of tyrosine/tyrosinate bands has also been observed in the bR → M transition in bacteriorhodopsin, although the sign of the bands is reversed. In bacteriorhodopsin, these bands were assigned to Tyr-185, which along with Pro-186 in the F-helix, may form a hinge that facilitates α-helix movement.