ACTIVATION OF TRANSDUCIN BY A XENOPUS SHORT-WAVELENGTH VISUAL PIGMENT

Phototransduction in cones differs significantly from that in rods in sensitivity, kinetics, and recovery following exposure to light. The c ontribution that the visual pigment makes in determining the cone resp onse was investigated biochemically by expressing a Xenopus violet con e opsin (VCOP) cDNA in COS1 cells and assaying the light dependent act ivation of transducin. Light exposed VCOP stimulated [S-35]guanosine 5 '-(gamma-thio)triphosphate nucleotide exchange on bovine rod transduci n in a time dependent manner with a half-time for activation of 0.75 m in, similar to that of bovine rhodopsin. In exhaustive binding assays, VCOP and rhodopsin activity showed similar concentration dependence w ith half-maximal activation occurring at 0.02 mol of pigment/mol of tr ansducin. Although VCOP was able to activate as many as 12 transducins per photoisomerization, rhodopsin catalyzed significantly more. When assays were performed with lambda > 420 nm illumination, VCOP exhibite d rapid regeneration and high affinity for the photoregenerated 11-cis -retinal. Recycling of the chromophore and reactivation of the pigment resulted in multiple activations of transducin, whereas a maximum of 1 transducin per VCOP was activated under brief illumination. The deca y of the active species formed following photobleaching was complete i n <5 min, similar to 10-fold faster than that of rhodopsin. In vitro, VCOP activated rod transducin with kinetics and affinity similar to th ose of rhodopsin, but the active conformation decayed more rapidly and the apoprotein regenerated more efficiently with VCOP than with rhodo psin. These properties of the violet pigment may account for much of t he difference in response kinetics between rods and cones.