LIGHT-DEPENDENT ACTIVATION OF ROD TRANSDUCIN BY PINEAL OPSIN

The pinneal gland expresses a unique member of the opsin family (P-ops in; Max, M., McKinnon, P, J., Seidenman, K. J, Barrett, R. K., Applebu ry, M. L., Takahaslai, J. S., and Margolskee, R. F. (1995) Science 267 , 1502-1506) that may play a role in circadian entrainment and photo-r egulation of melatonin synthesis. To study the function of this protei n, are epitope-tagged P-opsin was stably expressed in anembryonic chic ken pineal cell line. When incubated with 11-cis-retinal, a light-sens itive pigment was formed with a lambda(max) at 462 +/- 2 nm, P-opsin b leached slowly in the dark (t(1/2) = 2 h) in the presence of 50 mM hyd roxylamine. Purified P-opsin in dodecyl maltoside activated rod transd ucin in a light-dependent manner, catalyzing the exchange of more than 300 mol of GTP gamma S (guanosine 5'-O-(3-thiotriphosphate))/mol of P -opsin. The initial rate for activation (75 mol of GTP gamma S bound/m ol of P-opsin/min at 7 mu M) increased with increasing concentrations of transducin. The addition of egg phosphatidylcholine to P-opsin had little effect on the activation kinetics; however, the intrinsic rate of decay in the absence of transducin was accelerated. These results d emonstrate that P-opsin is ale efficient catalyst for activation of ro d transducin and suggest that the pineal gland may contain a rodlike p hototransduction cascade.