Mechanism for the regulation of mammalian cGMP phosphodiesterase6. 2: Isolation and characterization of the transducin-activated form

Rod photoreceptor cGMP phosphodiesterase (PDE6) consists of a catalytic subunit complex (Pαβ) and two inhibitory subunits (Pγ). In the accompanying article, using bovine photoreceptor outer segment homogenates, we show that Pγ as a complex with the GTP-bound transducin α subunit (GTP-Tα) dissociates from Pαβγγ on membranes, and the Pαβγγ becomes Pγ-depleted. Here, we identify and characterize the Pγ-depleted PDE. After incubation with or without guanosine 5′-O-(3-thiotriphosphate) (GTPγS), Pαβ complexes are extracted. When a hypotonic buffer is used, Pαβγγ, Pαβγ, and a negligible amount of a Pαβ complex containing Pγ are isolated with GTPγS, and only Pαβγγ is obtained without GTPγS. When an isotonic buffer containing Pδ, a prenyl-binding protein, is used, Pαβγγδ, Pαβγδδ, and a negligible amount of a Pαβ complex containing Pγ and Pδ are isolated with GTPγS, and Pαβγγδ is obtained without GTPγS. Neither Pαβ nor Pαβγγ complexed with GTPγS-Tα is found under any condition we examined. Pαβγ has ~12 times higher PDE activity and ~30 times higher Pγ sensitivity than those of Pαβγγ. These results indicate that the Pγ-depleted PDE is Pαβγ. Isolation of Pαβγγδ and Pαβγδδ suggests that one C-terminus of Pαβ is involved in the Pαβγγ interaction with membranes, and that Pγ dissociation opens another C-terminus for Pδ binding, which may lead to the expression of high PDE activity. Cone PDE behaves similarly to rod PDE in the anion exchange column chromatography. We conclude that the mechanisms for PDE activation are similar in mammalian and amphibian photoreceptors as well as in rods and cones.