Activation of cGMP phosphodiesterase in retinal rods: mechanism of interaction with the GTP-binding protein (transducin).
The mechanism of activation of cGMP phosphodiesterase by the GTP-binding protein in the disc membrane of retinal rods has been investigated by measuring the light-induced phosphodiesterase activity in reconstituted systems where the concentration of either the GTP-binding protein or the phosphodiesterase is varied. The results are consistent with the existence of two activator sites per phosphodiesterase functional unit: binding of one G alpha GTP (alpha subunit of the G-protein with GTP bound) with high affinity (100 +/- 50 nM) partially activates the enzyme (Vmax1 approxmately 0.05 Vmax to 0.10V max to trypsin-activated phosphodiesterase); binding of a second G alpha GTP with lower affinity (600 +/- 100 nM) induces maximal activation (Vmax2 approximately Vmax of trypsin-activated phosphodiesterase). The two different states of activated phosphodiesterase have the same Km for cGMP and the same pH dependence; they differ in their sensitivity to GMP. Micromolar concentration of protamines increases the affinity of the two activator sites and slightly increases Vmax1. When G-protein is activated with GTP-gamma S instead of GTP, the affinities of the two activator sites are not significantly modified, while Vmax1 appears to be increased.
Bennett
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Laboratoire de Biophysique Moléculaire et Cellulaire, Centre d'Etudes Nucléaires de Grenoble, France.
Clerc
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A
D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D002417:Cattle; D004789:Enzyme Activation; D019204:GTP-Binding Proteins / Q000378:metabolism* / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010786:Photoreceptor Cells / Q000201:enzymology*; D015293:Transducin / Q000378:metabolism* / Q000494:pharmacology; D014357:Trypsin / Q000378:metabolism
D015106:3',5'-Cyclic-GMP Phosphodiesterases; D014357:Trypsin; D019204:GTP-Binding Proteins; D015293:Transducin