Temperature dependence of G-protein activation in photoreceptor membranes. Transient extra metarhodopsin II on bovine disk membranes.

The thermal activation barrier of guanosine triphosphate dependent dissociation of the light-induced rhodopsin-G-protein complex has been determined using a spectroscopic technique (enhanced formation of metarhodopsin II). The dissociation rate has been measured in the range - 2 degrees C less than or equal to t less than or equal to 12 degrees C. The Arrhenius plot yields apparent activation energies: 166 +/- 10 kJmol-1 with 5'-guanylylimidodiphosphate (GMPPNP) and 175 +/- 15 kJmol-1 with GTP. The rhodopsin-G-protein dissociation rate is linearly related to the concentration of GMPPNP in the measurable range (less than or equal to 200 microM). The data show that, at low temperature (1 degree C), the rate limiting step of G-protein activation is the bimolecular reaction between the protein and the nucleotide. This also seems to hold true for more physiological conditions as suggested by extrapolation and comparison with nucleotide exchange rates in the literature. The high activation barrier of the nucleotide exchange reaction is explained in terms of rapid endothermic preequilibrium between an inactive and an exchanging state of the rhodopsin-G-protein complex.