ACTIVATION OF PHOSPHODIESTERASE BY TRANSDUCIN IN BOVINE ROD OUTER SEGMENTS - CHARACTERISTICS OF THE SUCCESSIVE BINDING OF 2 TRANSDUCINS

In bovine retinal rods, transducin loaded with GTP or GTP gamma S (T) activates a cGMP phosphodiesterase (PDE) by forming a tightly membran e-bound complex with it [Catty, P., et al. (1992) J. Biol. Chem. 267, 19489-19493]. Up to two Ts are able to bind to PDE [Clerc, A., and Be nnett, N. (1992) J. Biol. Chem. 267, 6620-6627]. We analyze here PDE a ctivation by two successive bindings of T In the mathematical model u sed, we took into account that the membrane concentration determines t he amount of PDE able to interact efficiently with T through the atta chment of PDE itself to the membrane. We therefore fitted the data obt ained over a wide range of membrane and PDE concentrations. We found t hat the binding of the first T to PDE elicits 80-100% of the maximal activity of PDE, whereas the binding of the second T to PDE elicits l ittle or no additional activation of PDE. This finding profoundly diff ers from previous conclusions. The carefully controlled conditions of our experiments permit one to understand these discrepancies. In the p hysiological situation, PDE would be nearly maximally activated throug h its interaction with only one T, The efficient binding of the secon d T to those complexes would then ensure a rapid deactivation of T* t hrough the enhancement of the rate of GTP hydrolysis in T bound to PD E [Pages, F., et al. (1999) J. Biol. Chem. 267, 22018-22021; Pages, F. , et al. (1993) J. Biol. Chem. 268, 26358-26364].