Molecular Changes of The Membrane Embedded Carboxyl Group Glu122 of Bovine Rhodopsin During The Transition to the Active State Metarhodopsin-II: an Investigation on the Glu122→ASP Mutant Using FT-IR Difference Spectroscopy

Abstract

The visual pigment rhodopsin belongs to the class of G-protein coupled receptors. Its light sensitive part, the chromophore 11-cis-retinal, is bound to a lysine of the protein via a protonated Schiff base. Photoisomerization of the chromophore to the all-trans geometry transforms rhodopsin into an active state, called metarhodopsin-II (MII), in which the Schiff base is deprotonated. This state binds the visual G-protein transducin and thereby triggers the signal transduction cascade1. It was shown that this deprotonation step is essential for rhodopsin activation2. Biochemical studies (e.g. ref.3) on recombinant rhodopsins have shown that Glu113 acts as a counterion for the positive charge of the protonated Schiff base. Membrane embedded carboxyls may well serve as groups participating in proton exchange with the Schiff base. This was especially demonstrated for another retinal protein, the light-driven proton pump bacteriorhodopsin4. Therefore, it is of special interest to investigate the molecular changes of internal carboxyl groups by FT-IR difference spectroscopy. This method, which allows the detection of molecular changes of the chromophore and the protein, was applied to the photoreaction of rhodopsin5. The difference spectra obtained for the formation of MII showed strong bands which could be assigned to internal carboxyl groups.