ENHANCEMENT BY PHOSPHODIESTERASE SUBUNITS OF THE RATE OF GTP HYDROLYSIS BY TRANSDUCIN IN BOVINE RETINAL RODS - ESSENTIAL ROLE OF THE PHOSPHODIESTERASE CATALYTIC CORE

Phosphodiesterase (PDE) in bovine retinal rod outer segments is activa ted when it forms a membrane-bound complex with the alpha-subunit of t ransducin loaded with GTP (Talpha). At maximal activation, this compl ex contains two Talpha and all the subunits of native PDE (PDEalpha, PDEbeta, and two inhibitory PDEgamma). We observed previously (Pages, F., Deterre, P., and Pfister, C. (1992) J. Biol. Chem. 267,22018-22021 ) that the rate of GTP hydrolysis by transducin in a rod outer segment suspension is enhanced when Talpha is bound to native PDE (PDEalphab etagamma2). In this article, we compare the effects of PDE species wit h different PDEgamma contents. We show that Talpha hydrolyzes its GTP faster not only when bound to PDEalphabetagamma2, but also when bound to PDEalphabetagamma or PDEalphabeta. Moreover, trypsin-treated PDE ( PDEgamma-deprived soluble PDE) also induces an acceleration of GTP hyd rolysis. On the contrary, addition of isolated PDEgamma alone does not accelerate GTP hydrolysis. The interaction between Talpha and PDEgam ma, which is essential for the activation of PDE by Talpha, is appare ntly not responsible of the feedback of PDE on Talpha. The interactio n of primary importance for the acceleration of GTP hydrolysis would b e that existing between Talpha and PDEalphabeta.