FOURIER-TRANSFORM INFRARED-SPECTROSCOPY INDICATES A MAJOR CONFORMATIONAL REARRANGEMENT IN THE ACTIVATION OF RHODOPSIN

The study of the structural differences between rhodopsin and its acti ve form (metarhodopsin II) has been carried out by means of deconvolut ion analysis of infrared spectra. Deconvolution techniques allow the d irect identification of the spectral changes that have occurred, which results in a significantly different view of the conformational chang es occurring after activation of the receptor as compared with previou s difference spectroscopy analysis. Thus, a number of changes in the b ands assigned to solvent-exposed domains of the receptor are detected, indicating significant decreases in extended (beta) sequences and in reverse turns, and increases in irregular/aperiodic sequences and in h elices with a non-alpha geometry, whereas there is no decrease in alph a-helices. In addition to secondary structure conversions, qualitative alterations within a given secondary structure type are detected. The se are seen to occur in both reverse turns and helices. The nature of this spectral change is of great importance, since a clear alteration in the helices bundle core is detected. All these changes indicate tha t the rhodopsin --> metarhodopsin II transition involves not a minor b ut a major conformational rearrangement, reconciling the infrared data with the energetics of the activation process.