A light-activated GTPase in vertebrate photoreceptors: Regulation of light-activated cyclic GMP phosphodiesterase

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Abstract
The mechanism by which light and nucleoside triphosphates activate the disc-membrane phosphodiesterase (oligonucleate 5''-nucleotidohydrolase; EC 3.1.4.1) was studied in frog [Rana catesbeiana] rod outer segments. GTP was orders of magnitude more effective than ATP as a cofactor in the light-dependent activation step. GTP and the analogue guanylyl-imidodiphosphate functioned equally as allosteric activators of photoreceptor phosphodiesterase rather than participating in the formation of a phosphorylated activator. A light-activated (5-fold) GTPase which participates in the modulation of photoreceptor phosphodiesterase was found. This GTPase activity appeared necessary for the reversal of phosphodiesterase activation in vitro and may play a critical role in the in vivo regulation of light-sensitive phosphodiesterase. The Km for GTP in the light-activated GTPase reaction was < 1 .mu.M. The light sensitivity of this GTPase (number of photons required for half-maximal activation) was identical to that of light-activated phosphodiesterase. The GTPase action spectrum corresponded to the absorption spectrum of rhodopsin. There was, in addition, a light-insensitive GTPase activity with a Km for GRP of 90 .mu.M. At GTP concentrations above 5 .mu.M, there was no appreciable activation of GTPase activity by light. The substrate Km values for guanylate cyclase, light-activated GTPase and light-activated phosphodiesterase ordered an enzyme array that might permit light to simultaneously cause the hydrolysis of both the substrate and product of guanylate cyclase. These findings revealed yet another facet of light regulation of photoreceptor/cyclic GMP levels and also provide a striking analogy to the GTP regulation of nonphotoreceptor, hormone-sensitive adenylate cyclase.