Photoactivation of rhodopsin and interaction with transducin in detergent micelles. Effect of 'doping' with steroid...

On detergent-solubilised bovine rhodopsin, we have studied the formation of the active photoproduct, metarhodopsin II (MII), and its interaction with the rod G-protein, transducin (G1). The measured rate of flash-induced MII formation decreases by a factor of 300 from n-dodecyl-beta-D-maltoside (k = 4 x 10(3) at 18 degrees C, pH 6.5), over (3-(lauroyloxy)propyl)-phosphorylcholine (deoxylysolecithin), n-octyl-beta-D-glucopyranoside, sodium cholate to Chapso. For the two last agents, MII formation is similarly slow as in the native disc membrane; however, the micellar system does not display the very large decrease of the rate with lowering temperature, as is characteristic for the membrane. This points to entropic factors determining the rate in the micellar systems. An admixture of rigid steroid molecules (11-deoxycorticosterone) to lysolecithin micelles ('doped micelles') slows MII formation and shifts the MI/MII equilibrium to values typical for detergents of rigid structure. The observation gives further support to the surface free energy concept of MII formation outlined in previous studies. The free adjustment of the MI/MII equilibrium in these doped micelles allows Gt-induced formation of extra-MII to be measured, providing a convenient monitor of rhodopsin-Gt interaction in solution.