A GTP-protein activator of phosphodiesterase which forms in response to bleached rhodopsin.

A specific protein associated with rod-outer-segment disc membranes binds GTP only in the presence of bleached rhodopsin. Once formed the protein-GTP complex becomes a soluble activator of cGMP phosphodiesterase. It is shown that this activator complex can be completely separated from rhodopsin and retain its ability to activate phosphodiesterase when added to a pool of totally dark (unilluminated) disc membranes. The photoreactive GTP analogue p3-(4-azidoanilido)-5' GTP (AAGTP) is shown to be a more effective substrate than GTP, Gpp(NH)p or 8-azido GTP. [8, 5' 3H] AAGTP was used to specifically covalently label the GTP-binding protein. The protein labeled exhibits a mass of 40,000 daltons when analyzed by SDS-PAGE.